Microbiology

MICROBIOLOGY. - Microbiology studies living beings that are extremely small, invisible to the naked eye and visible only by means of special optical devices (various microscopes), which go under the name of microorganisms or microbes (μικρός, small). These do not constitute a determinate zoological or botanical group, but comprise beings belonging both to the animal and to the plant kingdom, of a nature not always well defined and difficult to classify.

The microbiological era began at the start of the sixteenth century with the new tendency expressed by Girolamo Fracastoro (1478-1553), who with admirable intuition asserted that the transmission of infectious diseases takes place by means of animate corpuscles capable of multiplying rapidly, which he called "seminaria", that is, germs. These ideas of Fracastoro were taken up in the following century and developed by others, such as Pier Giovanni Faber, a physician of Montpellier, Father Athanasius Kircher and August Hauptman of Frankfurt, who in his Tractatus de viva mortis imagine even affirmed that the cause of infectious diseases resides in the presence of worm-like animalcules in the blood of the sick. But only in the second half of the seventeenth century (1675) was the existence of these very small beings ascertained by the Dutch naturalist Van Leewenhoeck (1632-1723). This scholar, who was particularly versed in the construction of microscopes, using a simple microscope of his own making, consisting of a small biconvex lens that allowed magnifications of about 270 times, was able to observe and describe in vegetable infusions, in putrid waters, in the tartar of teeth, in saliva, in the feces of man and of various animals, certain very small elements having the form of straight or curved rods, of filaments, of spirals and of small spheres. Van Leewenhoeck also succeeded in determining the size of these elements, which he called "animalcules", comparing them with grains of dust having a diameter of a quarter of a millimeter. Unfortunately neither this researcher, nor others who made analogous observations, understood their true significance, so that much time passed before the importance due to microbes was recognized as agents of infectious diseases and as producers of fermentations and of other natural phenomena. Nevertheless the idea that these very small beings were capable of multiple actions made its way into the minds of many scholars even in the absence of experimental proofs, which were not long in coming, thanks above all to Italian biologists.

First among all must be remembered the memorable experiments of Lazzaro Spallanzani (1729-99), who, in order to demonstrate the groundlessness of the theory of spontaneous generation, which was based on the principle of "corruptio unius, generatio alterius", succeeded in demonstrating that the animalcules were not born spontaneously in decomposing liquids but were themselves the cause of the putrefactive processes. Subsequently (1824) the Italian Vincenzo Sette succeeded in demonstrating that the reddening observed on certain microbiological studies by Agostino Bassi of Lodi (1773-1856), to whose genius the first clear demonstration of the pathogenic properties of microbes and of the modes of transmission of infectious diseases is owed. In fact this acute researcher devoted himself for about thirty years, following a rigorous experimental method, to the study of a disease of silkworms called "mal calcino" [muscardine], because the diseased silkworms appear wrapped in a thick whitish layer that looks like lime, and he succeeded in demonstrating that this disease was not to be attributed to particular atmospheric conditions or to food or to other aspects of silkworm rearing, as was then thought, but was caused by a specific microorganism. Bassi also established that this microbe was a cryptogam and that the whitish matter, the cause of the contagion, was constituted by the dense vegetation of the parasite. Moreover, he expressed with precision the concept that even human diseases, such as plague, cholera and smallpox, were determined by "vegetable or animal parasites" that "feed on the blood of the individual" and "whose extreme fineness does not permit us to be able to see them." Subsequently Pasteur ascertained the precise relationships existing between microorganisms and various processes of fermentation as well as between microorganisms and the diseases caused by them, at the same time establishing the fundamental canons of microbiological technique. Almost contemporaneously another great man, Robert Koch, brought further and decisive contributions to microbiological studies, introducing new techniques and discovering the bacterium of tuberculosis. Today microbiology, although still a young science, having little more than a century of life, has assumed very great importance. In fact, the knowledge in our possession concerning the morphological and functional characteristics of the various microorganisms has led to a satisfactory clarification of their complex biology, as well as of their intimate relationships with the life of man and of other living beings. On the other hand, the precise knowledge of the mechanisms by which these microscopic beings carry out imposing transformations of organic matter has permitted notable applications of a practical order in various sectors of industry, particularly agrarian and chemical industry. Thus microbiology comprises various fields of study — the purely biological, the medical, the veterinary and the industrial — representing therefore a fundamental part of biology, of human and veterinary medicine, of agriculture and of chemistry.

Microorganisms, although all sharing the characteristic of extreme smallness that renders them invisible to the naked eye, differ among themselves by their different size, in that some, the largest, are of the order of magnitude of the micron, that is, of a thousandth of a millimeter, while others, the smallest, are of the order of magnitude of the millimicron, that is, of a millionth of a millimeter. The largest microorganisms are easily observable with ordinary microscopes, also called optical microscopes; the smallest ones, which have dimensions a thousand times smaller, cannot be observed, in the great food substances
(polenta and bread)
in the province of
Padua was of microbial
origin and that
the germ responsible
for the phenomenon
was a small microorganism that was
called "zoogalactina inetrofa." The
same conclusions
were reached by another Italian, Bartolomeo Bizio, who in
the same period of
time had studied
in Venice an analogous
phenomenon.

Much more important is the contribution brought to the majority of cases, even with the aid of the most powerful optical microscopes, and they have a structure so elementary as to render difficult any attempt at definition and classification and any investigations into their nature. These beings are called ultramicrobes, infraviruses, ultraviruses, or simply viruses, and the science that deals with them is at present called virology (v. VIRUS).
Microbes, christened by van Leeuwenhoek, who first observed them, as little animals, were for a very long time still considered as belonging to the animal kingdom because they appeared, in the great majority of cases, endowed with movement. Only when, toward the middle of the last century, it was shown that many microorganisms, for example that of anthrax, were immobile, and investigations into their nature were deepened, was it possible to establish that they belong for the most part to the vegetable kingdom and that there are also some belonging to the animal kingdom: the former are called protophytes and the latter protozoa. There is no lack, however, of microbes, such as, for example, the spirochetes, regarding which many uncertainties still reign as to their belonging to one or the other kingdom. In consideration of these difficulties Haeckel proposed to unite all unicellular beings into a single group, calling them protists (from the Greek πρότστος) precisely to indicate that they are the first among all organisms, from which the pluricellular animal and vegetable beings would originate. Among the protophytes, the myxomycetes and the unicellular algae have little importance for microbiological studies, at least as far as the present state of our knowledge is concerned; more important are the bacteria and the fungi; indeed, the importance of bacteria in comparison with other microorganisms is such that for a long time, under this denomination, all microbes were included, even the animal ones.

Bacteria are unicellular vegetable organisms devoid of chlorophyll, of extremely small dimensions, being of the order of magnitude of the micron and the smallest measuring even less than half a micron. From a systematic point of view bacteria occupy an intermediate place between unicellular algae and fungi.

The morphological study of bacteria is done by means of the optical microscope, which is an apparatus formed in its main part by two systems of lenses arranged so as to obtain a double magnification of the object one wishes to observe. Today one has at one's disposal perfected microscopes that allow one to obtain an image of bacteria magnified even 2500 times. For particular investigations one can also have recourse to the electronic supermicroscope, which permits magnifications of up to more than 50,000 times. Bacteria present themselves in the form of small spheres or of straight or curved rods: the spherical forms are called cocci and, in turn, are called micrococci if they present themselves isolated, diplococci if united in two elements, tetrads if
united in four
elements, sarcinae if
united in eight in a cubic
arrangement,
streptococci if ar-ranged in a row like
the links of a
chain, staphylococci if
united in irregular
masses like
bunches of grapes. The bacteria in the
form of straight rods
represent the bacteria properly
so called (from the Greek
βακτήριον, little
rod) or the bacilli and can present themselves isolated or
variously grouped
and arranged;
sometimes they form short

(courtesy of Prof. Stefanelli)
MICROBIOLOGY - Portrait of Louis Pasteur (1822-95).

chains, taking the name of streptobacilli. Rod-shaped bacteria that are curved are called vibrios if their body has a single curvature like a comma, and spirilla if the curvatures are greater in number. Close to the spirilla in shape are the spirochetes. Bacterial cells, whatever their shape, have a very simple structure, being made up of a small cytoplasmic mass, very rich in various nuclear substances, bounded by a thin wall. Many bacteria are provided on their surface with very slender, very long filaments, even two or three times the length of the bacterium itself, called flagella, which are, depending on the bacterial species, in numbers varying from one to a hundred and variously arranged, around the entire bacterial body, at a single end or at both ends. The flagella represent the organs of locomotion of bacteria, imparting to them true movements of translation. Bacterial cells feed by taking from the environment the substances necessary for metabolic processes; these cross the cell wall by means of complex osmotic and enzymatic mechanisms. As regards respiratory processes, bacteria are to be distinguished into three groups: aerobes, which live only in the presence of atmospheric oxygen; anaerobes, which live in the absence of air, oxygen being harmful to them; and finally facultative aerobes, capable of living both in the presence and in the absence of air.

Bacteria reproduce by simple division or scissiparity, which takes place in the following manner: the bacterial cell increases in volume by elongating, then constricts at the center and a dividing septum forms perpendicular to the major axis, which leads to the division of the mother cell into two daughter cells. These may detach completely, giving rise to two isolated bacteria, or they may remain united to form the various cell aggregates. If environmental conditions are favorable to the life of the bacteria — through the presence of sufficient quantities of nutrients, the degree of humidity and temperature, and other factors — the entire process of scissiparity of the mother cell into two daughter cells takes place rapidly in twenty to thirty minutes; and since the daughter cells immediately enter, in their turn, into the reproductive phase, each of them giving rise to two other cells, it follows that the rhythm of bacterial multiplication proceeds according to a geometric progression, reaching very high values in a short time. This makes it possible for these tiny beings to accomplish truly colossal works, such as the various transformations of organic substances observed in nature.

The conditions that favor bacterial multiplication are not the same for all species; indeed, they vary for each of them and, sometimes, the extremely interesting fact occurs that a factor which facilitates the multiplication of one species clearly hinders that of another. A very characteristic example in this regard is found in the case of aerobic and anaerobic bacteria, the former multiplying in the presence of air and the latter in its absence, so that when aerobic and anaerobic germs come to be together in the same material, the former develop preferentially if there is an abundance of air, or the latter in its absence: under certain conditions it is precisely the aerobic bacteria that, by consuming atmospheric oxygen, create the conditions favorable to the development of the anaerobes.

The degree of temperature also notably influences bacterial multiplication, inasmuch as each species requires specific thermal conditions for its development. The majority of bacteria develop well between 15° and 40° C, but there are some, such as the psychrophiles, that prefer low temperatures near zero, and others that are capable of developing even at high temperatures (up to 70°-80° C).

(courtesy of prof. Stejanelli)
MICROBIOLOGY - Bacteria observed under the electron supermicroscope.

These latter bacteria, which are therefore called thermophiles, represent an exception to biological laws, managing to live and multiply at temperatures higher than that of the coagulation of the protein substances of living organisms. Almost all bacteria succumb at temperatures near 60° C, whereas they tolerate low temperatures well, at which they are able to resist even for several months, remaining in a condition of latent life until the temperature returns to higher values.

From what precedes it is clear that bacterial multiplication can be hindered by multiple causes, the best known of which are the scarcity of nutritive substances, an unsuitable degree of temperature and humidity, and the presence of substances eliminated by the bacteria and harmful to these same bacteria. Such unfavorable conditions create an environment unsuited to the life of the bacteria, leading the less resistant ones to death and hindering the multiplication of the others. Often certain bacterial species give rise to the production of spherical or oval little bodies, called endospores, which form within the interior of the bacterial body, one for each cell. These spores, by reason of a thick membrane with which they are covered, prove very resistant to external agents and in particular to desiccation, to sunlight and to the action of high temperatures. Moreover they have great biological importance, because they ensure the preservation of many bacterial species.

Among the biological manifestations to which many bacteria give rise, one should recall the production of colored substances or pigments, which diffuse into the material in which they multiply. This occurs frequently in various foods contaminated by germs: thus milk may appear colored blue or yellow as a result of the luxuriant growth in it of special chromogenic bacteria, and bread and various sweets may take on a characteristic blood-red coloration through the active proliferation of a small bacterium known as the prodigious bacterium.

Another extremely interesting phenomenon is that of the luminescence produced by photogenic bacteria, which are found especially on codfish and on other fish and which impart to them a particular luminosity, visible during the night. These photogenic bacteria are also found on meat when putrefactive processes begin in it and, sometimes, in sweat and in urine.

Bacteria are diffused in extraordinarily great number in the environment: they are found in the soil, where very often the degree of temperature and humidity and the presence of organic substances create conditions extremely favorable to their life; in waters, to which they arrive from the soil; in the air, in which they live adhering to particles of dust or to minute droplets of various liquids; in our foods, in which they often produce serious alterations; on the skin and on the mucous membranes of man and of animals; on plants, on clothing and on the outside on non-living organic matter. This distinction is not absolute but relative, inasmuch as sometimes it may happen that bacteria that are ordinarily pathogenic turn out to be devoid of any capacity, and vice versa that bacteria accustomed to saprophytic life can become pathogenic. A very demonstrative example in this regard is provided by the bacterium coli, which is normally a saprophytic bacterium, the habitual guest of the intestine of man and animals, in which it lives in very great numbers without causing any harm, indeed contributing effectively to secondary digestion, but which, under certain conditions, can acquire a very high pathogenic power, causing appendicitis, peritonitis, cystitis and other serious infections.

Among the pathogenic bacteria, particular importance attaches to the staphylococcus, a germ that is very widespread in the external environment and that can cause abscesses, phlegmons, boils, urinary infections; the streptococcus, likewise an agent of suppurative processes as well as of erysipelas, puerperal fever and malignant endocarditis, which is a very serious infection of the heart valves; the pneumococcus, which causes pneumonia; the meningococcus, which is the cause of epidemic cerebrospinal meningitis. A bacterium sadly notorious for the bereavements and damage it causes is that of tuberculosis, of which four types exist: the human type, widespread throughout the world, which can establish itself in every organ and tissue, giving rise to tuberculosis of the lung, intestine, bones, joints, etc.; the bovine type, which chiefly affects cattle and sometimes also man with extrapulmonary forms; the avian type, which only exceptionally develops in man, while it normally affects chickens, pigeons and other birds; finally there is the type of cold-blooded animals, which can take hold in the frog, the turtle and certain fish.

Very similar to the tubercle bacteria is the one discovered by Hansen, which causes in man the terrible disease of leprosy, almost absent in Italy but by which, even today, several million people throughout the world are affected. Two other bacteria sadly famous for the imposing epidemics they caused in the past are those of plague and cholera, which still represent a not negligible danger, since foci of plague still exist in Tibet, Mongolia and Arabia, and of cholera in India, Indochina, Malaysia and other regions of Asia. Other pathogenic bacteria are those of typhus and of the paratyphoid organisms that cause the various clinical forms of typhoid fever; those of dysentery, which cause the disease of the same name, especially among small children and troops operating in hot periods; as well as the bacteria of diphtheria, tetanus, anthrax, whooping cough, etc. Among the spirochetes too, pathogenic ones are not lacking, and among these the primacy belongs to the spirochete of syphilis which, in all likelihood, was introduced into Europe from America when Christopher Columbus and his men returned from their famous voyage. This germ, which was named by its discoverer, Schaudinn, treponema pallidum, spares no part of the body, causing nephritis, hepatitis, various heart diseases, serious affections of the nervous system, such as progressive paralysis furnishings. It can be stated that bacteria are present everywhere; they are absent only in the high layers of the atmospheric air, far from any manifestation of animal or plant life.

Bacteria must be distinguished into pathogenic and saprophytic: the former exert an aggressive and harmful action on the organism of man and animals, the latter live in the environment and tabes, and also produces very serious alterations in the unborn. A fatal prerogative of pathogenic bacteria is that of passing easily, sometimes with a mere cough, from a sick host to a healthy individual, making him fall ill, so that bacterial diseases prove to be contagious. And since many bacteria are common to man and animals, the case frequently occurs of diseases transmitted from an animal to man. Thus, for example, milk coming from a sick milk-producing animal (cow, goat) that contains pathogenic bacteria, such as that of bovine tuberculosis or of Malta fever, can cause the respective diseases in man. The penetration of pathogenic bacteria into the human or animal organism takes place through the skin and the various mucous membranes, more easily if lesions of continuity, even minimal ones, exist. Thus the bacterium of plague and that of anthrax penetrate through intact skin or skin presenting minimal erosions from scratching. Sometimes the penetration of pathogenic bacteria can take place through the skin by means of the bite of various insects. The bacterium of tuberculosis, on the other hand, penetrates as a rule through the mucosa of the bronchi, of the lung or of the intestine. Another gateway for bacteria is constituted by the tonsils, which by their particular anatomical structure facilitate their penetration. Once they have penetrated the organism, the bacteria begin to multiply in the tissues, organs and humors and to carry out their aggressive actions, which consist above all in the production of particular strongly toxic substances, which are called bacterial toxins and must be distinguished into exotoxins and endotoxins: the former are secretion products of the bacteria, of a protein nature, and act independently of the bacteria themselves that produced them, spreading through the organism and producing serious lesions affecting the various tissues; the endotoxins are likewise endowed with high toxic capacity but are released only following the breakdown of the bacterial body. Some germs act chiefly through exotoxins, others through endotoxins, and others through both exo-and endotoxins.

Among the bacteria that exert their pathogenic power by means of exotoxins, particular importance attaches to the diphtheria and tetanus ones. These two bacteria, once they have penetrated the human organism, show no invasive tendency but remain localized at the point of entry, where they secrete their exotoxins, which spread through the organism producing very serious lesions of the elements of the nervous system. A strongly toxic exotoxin is the botulinum one, which produces botulism if ingested by man. It is interesting that the botulinum bacillus, producer of this so active a toxin, is not capable of leading a parasitic life, that is, of living in the human organism; it is found instead in the soil or in foods, vegetable or animal, preserved in cans, in which it vegetates, forming in abundance its powerful toxin, which makes those foods harmful to man. Unfortunately, cases of botulism, even fatal ones, occur not infrequently.

To combat in humans the harmful effects of bacterial exotoxins, antidiphtheritic, antitetanic, antibotulinic sera, etc., are used with advantage (v. below). Among the best-studied endotoxins, also as regards chemical constitution, is that of the typhoid bacterium, which has a great electivity for the intestine, in which it produces characteristic ulcerations. The endotoxin of the plague bacterium, on the other hand, produces severe alterations of the circulatory system and of nervous tissue. The dysentery bacterium of Shiga Kruse, instead, exerts its pathogenic power by means of an exotoxin with elective action on nervous tissue and of an endotoxin that produces severe alterations in the intestine. The study of bacteria is practiced today by means of appropriate techniques, which make it possible to know their fine structure and biological properties, to isolate them from the environment in which they are found, by means of artificial cultures in suitable nutrient media where they can develop and multiply, and to test their pathogenic power in laboratory animals. Among the many classifications proposed for bacteria, the simplest is the one based on morphological characters, which divides them into coccaceae, which have a spherical or slightly oval shape, into bacteriaceae, of rod-like shape, and into spirillaceae, which appear as variously curved rods.

The fungi (Mycophytae of Engler's classification) are microorganisms of the plant kingdom, like bacteria devoid of chlorophyll, consisting of a single cell or of an aggregate of several cells identical or little different from one another; they have a well-defined cellular structure and exhibit reproductive phenomena that are sometimes very complex, for which reason they are to be considered among the most evolved microbial forms. They present two fundamental vegetative types with distinct morphological characteristics, one with a globose cellular structure and one with a filamentous structure. The fungi of the first type, which are also called blastomycetes and saccharomycetes, appear as large cells 6-8 microns in diameter and of rounded or oval shape, which present themselves isolated or united together in conglomerates of several elements variously arranged. Each single cell is constituted by the wall or cell membrane, formed predominantly of chitin, cellulose and hemicellulose, by the cytoplasm, with vacuoles and granules of various reserve material, and by nuclear bodies often having all the characters of true nuclei. Sometimes these cells elongate, assuming the filamentous type. The fungi with filamentous structure, also known by the name of molds, appear as long filaments, called hyphae, more or less variously branched and intertwined, constituting characteristic formations with the appearance of tubercles, of compact tissues or of velvety felts. The individual filaments may prove to be formed of several identical cells united together or of a single formation of a syncytial character.

The fungi reproduce by division of one cell into two or more daughter cells or else by means of the production by the mother cell of one or more buds of rounded or oval shape (budding or blastogony) or also by means of the process of sporification or propagation, which consists in the production of small globose or elliptical bodies, very resistant to external agents, which are formed either inside the cells (endospores) or outside them (exospores), assuming various arrangements in clusters, chains or brushes, as occurs, for example, in fungi of the genus Penicillium, which are in fact characterized by filamentous cells, the hyphae, which bear at their extremity chains of exospores that give the whole the appearance of a brush. Many fungi exhibit characteristic processes of sexual reproduction, which range from simple autogenous fertilization, consisting in the union of two identical cells, to complex processes of heterogamy, in which a true copulation can take place between two sexually differentiated elements, one male and one female. Fungi have a chemical composition analogous to that of bacteria, proving to be constituted by a large quantity of water, by protein substances, fats, carbohydrates and by minimal quantities of mineral salts.

Fungi are, like bacteria, very widespread in nature: they are found in very great numbers in water, in air, in soil, on foods, on clothing, almost everywhere and, as soon as environmental conditions favorable to their development occur, they enter into very active multiplication, covering with a dense vegetation characteristically colored white, black, green, red, yellow the materials on which they have developed, thus giving rise to the well-known phenomenon of molding. This frequently appears on the walls of damp environments and sometimes the vegetation of particular molds on walls painted with colors based on arsenic can provoke the formation of volatile arsenical products very poisonous for the persons who breathe them. Damp timber too undergoes the invasion of molds, so that the wooden parts of a construction, such as beams and shutters, can under particular conditions of humidity assume a yellowish or reddish-brown color, become covered with very tiny droplets of water and lose their normal consistency, becoming friable.

Generally fungi are devoid of pathogenic power; nevertheless some of them are capable of determining, in humans and animals, particular morbid states sometimes even severe, known by the generic name of mycoses. One of these is thrush in infants, which is an affection of the oral and pharyngeal mucosae, which present themselves covered by characteristic whitish deposits due to the proliferation of various fungi. Other mycoses very important for their diffusion and contagiousness are the tineas, diseases of the skin, of the hair and of the nails, produced by fungi belonging to the genera Ctenomyces, Sabouradites, Trichophyton and Epidermophyton. Pneumoconioses too and the various forms of actinomycosis are of particular importance in human pathology. In recent years studies and research on fungi have undergone a further impulse owing to the fact that they produce substances (penicillin, streptomycin, etc.) endowed with energetic antibiotic action toward many pathogenic microbes.

Among the many classifications proposed for fungi, the most accepted is the one that divides them into two great classes: that of the eumycetes, which comprises all the fungi that exhibit processes of sexual reproduction, and that of the deuteromycetes (δεύτερος, secondary), which comprises all the others for which these processes of higher reproduction are not known. The class of the eumycetes is in turn subdivided into three subclasses (basidiomycetes, ascomycetes and phycomycetes) according to the type of sexual reproduction that gives rise, respectively, to the formation of basidia, of asci and of particular zygotic formations. These three subclasses are in turn subdivided into orders, suborders, families, etc. The class of the deuteromycetes comprises three orders, characterized by different sporal formations.

The protozoa are animal organisms formed by a single cell that provides for both vegetative functions and those of relational life. They constitute a very heterogeneous group of great biological importance, which some authors even consider as a subkingdom of the animal kingdom. Their size ranges from a few microns, for some species, to several microns, up to 200 and even more for others, so that almost all of them fall among the microorganisms. Their shape is also highly variable; and the same can be said of their structure, which is very simple, almost elementary, for some protozoa and notably complex for others. Unlike the other microbes, in protozoa there is always a clear distinction between nucleus and cytoplasm, and the latter is almost always differentiated into endoplasm and ectoplasm. The endoplasm, which is found internally, is more fluid, often presents vacuoles and various granules, and generally carries out the vegetative functions; the ectoplasm is found externally, is denser, and provides for nutritive exchanges and the functions of relational life. The nucleus is never absent; indeed, many protozoa have two, a smaller one, called the micronucleus, which presides over reproductive phenomena, and a larger one, the macronucleus, with vegetative functions; some species have a greater number. Many protozoa are also provided with a centrosome and other organelles. The body of protozoa, exceptionally devoid of any covering, is as a rule delimited all around by a more or less thick membrane, which in some classes appears as a very resistant cuticle.

The type of nutrition differs in the various species: for some protozoa it takes place, as for bacteria, by absorption through the cytoplasm of nutritive substances found in the surrounding liquid; for the more evolved forms there is a true holozoic type of nutrition, which consists in the taking in of very tiny food particles, which are carried into the interior of the cell, then digested and subsequently utilized. Movement is a very frequent phenomenon in protozoa and is produced either by small contractile fibrils or by short and slender prolongations of the cytoplasm, which go under the name of cilia and are of varying number and variously arranged within the cell body, or by large filaments called flagella or, finally, by small outpushings of the ectoplasm having the shape of a club, stubby and slender, which the protozoon itself emits and retracts; the latter are called pseudopodia and serve, besides the motor function, also for the taking in of food particles. In protozoa the most disparate reproductive processes are found: simple or multiple division, in which the nucleus first and the cytoplasm afterwards divide into two or more parts, endogenous or exogenous budding, various phenomena of sexual reproduction, including those of the parthenogenetic type. Moreover, the reproductive phenomena by division or by budding alternate very frequently with the sexual ones, giving rise to interesting biological cycles, which prove of great practical importance when it is a question of protozoa parasitic on man and animals. Protozoa resist external agents poorly; however, many of them, when environmental conditions become unfavorable, transform into cysts, covering themselves with a particular very resistant capsule that protects them from harmful stimuli. Protozoa are very widespread in nature, being found in the saprophytic state in soil and water or as parasites in man and various animals.

Protozoa are divided into four classes: flagellates, rhizopods, ciliates or infusorians, and sporozoa.

The Flagellates are characterized by one or more flagella. To this class belong many species parasitic on man, such as the trypanosome gambiense observed for the first time in the blood of the negroes of Gambia in Africa, which produces sleeping sickness, and the leishmanias, which cause oriental sore, an ulcerous affection of the skin and mucous membranes, and an acute febrile anemia that is accompanied by large tumors of the spleen and liver. The Rhizopods are protozoa provided with pseudopodia that serve, as has been said, for locomotion and for the taking of food. To them belong the amoebas, which owe their name to the fact that they continually change shape. Some of them live saprophytically in waters and in the intestine of man and other animals; other amoebas are instead pathogenic: thus the histolytic amoeba, which is capable of producing a serious contagious intestinal disease, called amoebic dysentery or amoebiasis, which is characterized by intestinal lesions with diarrhea, often bloody, and by hepatic disturbances. The Ciliates among protozoa are the most differentiated, being very large and provided even with a rudiment of a buccal and anal opening. Only one species, Bolantidium coli, which lives as a commensal in the intestine of the pig, is capable of producing intestinal disturbances in man, sometimes serious. All the other ciliates live in fresh or salt waters in the saprophytic state. The class of Sporozoa, which comprises the telosporidia and the neosporidia, is characterized by the fact that phenomena of sexual reproduction alternate with phenomena of sexual reproduction. These protozoa are very small and are all parasites of vertebrates and invertebrates. To them belong the various plasmodia or hemosporidia of malaria, which complete their life cycle in the mosquito and in man, in whom they produce the various forms of malarial fever.

Microbes carry out in nature a series of complex transformations of organic matter, so important for the development of the life of higher beings as to authorize the assertion that, if there were no microorganisms on earth, life would come to a halt. Microbial actions are made easy by the fact that microbes are very widespread and have a tendency to multiply rapidly, reaching in a short time an infinitely great number. Among the many and interesting manifestations of microbial life, particularly important is the one connected with the cycle of organic matter that takes place between the animal and the plant kingdom through the action of numerous microorganisms and that consists in a series of processes of decomposition and reconstruction. In fact, organic substances of an animal nature, such as corpses, carrion, feces, urine, etc. etc., once they have reached the soil are decomposed by the various microbes found there in a very great number and transformed through successive reductions into bodies of simpler structure. Of these, some, such as water and carbon dioxide, are rapidly and directly utilized by plants; others instead, such as, for example, nitrogen and ammonia, are further elaborated through the intervention of particularly specialized microbes, transformed into mineralized compounds (nitrites, nitrates, etc.) and utilized by plants. It is interesting that gaseous nitrogen, resulting from the decomposition of organic matter, could not be utilized by plants without the providential intervention of particular nitrogen-fixing microbes, some of which, called azotobacteria, are capable by themselves of fixing nitrogen, transforming it into usable products; other microbes, instead, carry out these fixations and transformations of nitrogen in symbiosis with legumes, in whose roots they live. The remarkable activity of these microorganisms has as a consequence that soils become enriched with many nitrogenous substances and explains the fact, known since ancient times, that legumes increase the fertility of soils.

Other manifestations of microbial life of very great importance are fermentations, which are impressive transformations of organic matter, with or without the development of gas, carried out by various microbes.

A fermentation known since the most ancient times is alcoholic fermentation, which consists in the transformation of sugars into alcohol. There exists in fact a whole series of alcoholic beverages, such as wine, beer, etc., which are obtained through fermentative processes of different substances such as grapes, hops, etc., and the advanced knowledge of the intimate mechanism by which microbes carry out alcoholic fermentation has allowed the implementation of highly specialized industrial processes by which typical wines with characteristic aromas are obtained. Other fermentations of notable practical importance are the bread-making one, in which the dough swells because of the gases produced by the microbes, increasing in volume, and becomes soft and spongy; and the acetic one, in which acetic bacteria transform the alcohol of wine, beer or cider into vinegar, which respectively takes the names of wine vinegar, beer vinegar, etc. Closely connected with many microbiological phenomena are other phenomena which in their complex constitute immunity.

IMMUNITY

In the biological sense, it is the property possessed by certain organisms of opposing, by means of reactive processes, the offensive actions of pathogenic microbes, in order to defend their own biological integrity. These defensive processes are set in motion by the organism to prevent the penetration of pathogenic microbes through the skin and the mucous membranes that line the mouth, the nose, the conjunctivae and the other natural openings, and to oppose the spread of the microbes themselves in the tissues and organs, should they have overcome the barriers of cutaneous or mucous covering. When the defensive powers of the organism succeed in preserving it from the microbes of a given disease, the organism is said to be immune and therefore refractory to it. If, on the other hand, the microbes, having overcome the defensive barriers, invade the organism causing damage to the tissues and organs and serious disturbances to its functions, the organism is said not to be immune, that is, it is receptive. Between the two extreme limits of a maximum immunity, that is, of a complete refractoriness, and of an absolute lack of immunity, that is, of a complete receptivity, there exists a whole range of intermediate degrees of immunity.

Immunity must be distinguished as natural, acquired and artificial. Natural immunity consists in the state of refractoriness possessed from birth by an organism toward a given infection, being possibly related to the hereditary characteristics of an entire species, of a race or of single individuals. Therefore natural immunity must be distinguished, in turn, into natural immunity of species, of race and individual. Man is provided with natural immunity against many infections of other mammals and of birds, while he does not possess it toward others. In fact, he proves strongly receptive to measles infection, which is contracted sooner or later by all men, and to other exanthematous diseases such as smallpox and chickenpox, as well as to whooping cough, typhoid fever, the dysenteries, cholera, diphtheria, pneumonia and other serious infections. No animal species is naturally immune to tubercular infection, mammals, birds and even cold-blooded animals being receptive to it. Great analogies as regards immunity exist between zoologically close species, for example, between man and the higher apes, which permits the use of these animals in the experimental study of many infectious diseases of man. A classic example of natural immunity of race is that of the Algerian sheep, which are almost immune to anthrax infection, contrary to other sheep. Among human races there exist only small differences as regards natural immunity: negroes, for example, resist malaria and yellow fever better than whites and resist tuberculosis less.

Notable differences are observed, on the other hand, in the natural immunity of various individuals, so that among persons equally exposed to the same infection, some contract it at once, others later and others remain unscathed. Interesting is the fact that in the first months of life children rarely fall ill with measles, scarlet fever, diphtheria, smallpox and other infectious diseases because of the natural immunity possessed by them as a consequence of the passage of immunizing substances from the mother to the fetus during intrauterine life.

Natural immunity can weaken or disappear entirely through the effect of various causes, such as insufficient nutrition qualitatively or quantitatively, excessive work, certain intoxications (alcoholism, lead poisoning), the state of pregnancy, the puerperium, other intercurrent diseases and, finally, unfavorable climatic conditions with excessive cold or heat. Acquired immunity, or naturally acquired immunity, is that which appears in an organism after it has overcome an infectious disease: an individual, for example, who has been ill once with smallpox, even in a mild form, becomes almost certainly refractory to this infection for the rest of his life. The same occurs for chickenpox, scarlet fever, measles, yellow fever, petechial typhus and other diseases. With respect to other infections, e.g., diphtheria and pneumonia, acquired immunity is anything but substantial.

The mechanism of action of natural and of acquired immunity is related to complex functions carried out by certain cell groups (cellular or histogenous immunity) and by the various humors of the organism (humoral immunity). The most important manifestation of cellular immunity is phagocytosis, which consists in the capacity possessed by many cells, especially by the white blood cells, of engulfing microbes and digesting them. On the other hand, the humors are endowed in varying measure with the properties of immobilizing microbes, of killing them, of dissolving them and of neutralizing their toxic products, which go under the name of bacterial toxins. These properties possessed by the humors of the organism are attributed to the presence of substances, not yet fully identified, called antibodies or immune bodies. These, which are above all an expression of acquired immunity, appear in the humors of an organism following the introduction of microbes or of their toxins, to which the name antigens is given because they have generated the corresponding antibodies. Immunity must also be distinguished into antimicrobial, which is deputed to fight against microbes, and antitoxic, which consists in the capacity to neutralize microbic toxins.

Artificial immunity, or artificially acquired immunity, is that which is conferred on an organism against a given infection by means of appropriate treatments. Artificial immunity can be active or passive. The former is obtained by introducing into the organism that one wishes to immunize against a given infection the microbes of this disease killed or deprived of pathogenic power; in some cases, instead of the microbes, their completely detoxified toxins are used. To the introduction of the microbes or of the toxins, which behave as antigens, the organism reacts actively with a set of modifications, of a defensive character, both of the cells and of the humors, analogous to those that occur in natural infection and that lead to naturally acquired immunity.

The microbes and toxins introduced into organisms to confer active artificial immunity are called vaccines, by reason of the fact that the material that was used the first time to provoke an artificial immunity consisted of lymph taken from the pustules of a disease of cattle called "vaccine" (Jenner 1796). Vaccines are introduced into the organism by means of scarifications of the skin, of injections, or by the oral route. They are as a rule used in man for preventive purposes and represent a most important prophylactic means in the fight against many infectious diseases, such as smallpox, diphtheria, typhoid fever, cholera, rabies and others; the efficacy of these vaccinations is by now so well established that in some countries, among which Italy, the obligation of the anti-smallpox and of the anti-diphtheria vaccination is prescribed for all children within the second year of life and of the anti-typhoid vaccination in particular contingencies. Vaccinations are widely used in armies, which are particularly exposed to the dangers of infections, especially in wartime. They also find a useful application in the veterinary field to preserve economically important animals. Besides for preventive purposes, vaccines can be employed in the treatment of certain infectious diseases (typhoid fever, for example) to stimulate the defense powers of the organism.

Artificial passive immunity is obtained by inoculating parenterally (generally subcutaneously) into the organism that one wishes to immunize against a given infection the blood serum of an animal (e.g., the horse) that has previously been actively immunized and that therefore already contains ready in its blood the antimicrobial and antitoxic principles. These sera are called immune sera and are used in practice both for preventive and for curative purposes. The antidiphtheria and antitetanus sera are in wide use, as they prove very effective in preventing and combating the respective infections. Immune sera sometimes cause drawbacks and disturbances, especially when they are inoculated into individuals who have previously received other serum injections.

Phenomena of an immune nature have also been demonstrated in invertebrates, in protozoa (animal organisms consisting of a single cell) and in plants. In the latter too it is possible, by various technical methods, to bring about an increase in resistance against certain diseases.

Besides the immune phenomena that are an expression of the defense of animal and plant organisms against microbes, other biological phenomena of a certainly immune nature have been observed, the study of which has led to results of the greatest theoretical and practical interest. Among these phenomena, very important are those that are related to the structure of the red blood cells of individuals of the same species and of different species, which have made possible the spread of the practice of blood transfusion and notable medico-legal applications.

No less interesting and important is that other group of immune phenomena that go by the name of anaphylactic phenomena, because they represent a state contrary to that of immunity and manifest themselves with a maximum sensitivity toward substances that are in themselves harmless. The classic anaphylactic phenomenon is the one that occurs when normal or immune serum is injected into an individual who in the past has had one or more serum injections. It manifests itself almost suddenly with local disturbances, at the point where the injection was given, and general disturbances, which are very severe and can, very rarely, culminate in death.

Anaphylactic phenomena, characterized by extensive urticaria, can arise in particularly predisposed individuals following the ingestion of eggs, mollusks, strawberries and other foods (food anaphylaxis). Another form of anaphylaxis is the respiratory one, which presents with attacks of asthma, of coryza, with conjunctival disturbances and sometimes also with urticaria, and which is to be related to the inhalation of the pollen of certain plants or of other substances of various origin.

Aldo Cimmino