BACTERIOPHAGE. - The discovery of the bacteriophage is due to the French bacteriologist d'Hérelle, who in 1917 described, in the fecal filtrate of convalescents from bacillary dysentery, a quid capable of dissolving cultures of dysentery bacilli.
If a culture thus lysed is filtered through a porcelain filter capable of retaining bacteria, the lytic principle passes into the filtrate, and if a small quantity of the latter is added to a new culture of the same germ, lysis is reproduced in II. By thus alternating filtrations and passages, it is possible to transmit the lytic action from culture to culture indefinitely.
Subsequently, lytic principles of this nature, active against numerous bacterial species, were discovered not only in the feces of man and of many animals, but also in water and soil.
Opinions among scholars long differed as to the nature of this principle, called the “bacteriophage.” D'Hérelle vigorously maintained that it was a true living being, capable of reproducing itself and parasitizing bacteria, endowed with the characteristics of filterable viruses. Other scholars, especially Bordet and his school, instead regarded the bacteriophage as an enzyme, and therefore devoid of the characteristics of living matter. This opinion, which until a few years ago enjoyed the greatest credit, was recently overturned when Ruska turned the electron microscope to the study of the bacteriophage. With the aid of this powerful instrument of investigation, he was able to ascertain that d'Hérelle's hypothesis was correct and that the bacteriophage is indeed an organized, living being, endowed with the properties of filterable viruses. It appears most often in the form of extremely minute isolated granules, 8–75 mµ in diameter; more rarely as rods or short cylinders, or as two paired granules provided with one or two tiny cilia. These minute corpuscles crowd around the attacked bacterium, which rapidly undergoes lysis and disintegration. The modes of reproduction of the bacteriophage are still unknown.
The bacteriophage, like most filterable viruses, cannot be cultivated on artificial media, but develops only in the presence of living bacteria. Whereas some bacteriophages are only slightly specific and are able to lyse germs of several species, most are instead strictly specific, and sometimes prove capable of lysing only one species or even a single bacterial variety. Some recent methods intended to classify germs very similar to one another (for example, salmonellae) are based on this property, distinguishing them precisely according to their differing sensitivity to different bacteriophages.
Bacteriophages have also been employed therapeutically, but generally with results not altogether satisfactory, both because of their frequent excessive specificity and because they rarely succeed in lysing all bacterial individuals, while those that survive give rise to a bacteriophage-resistant progeny. The significance of bacteriophages in the spontaneous cure of infectious diseases and in the elimination of pathogenic germs from water and soil is assessed differently by different scholars.