BACTERIOPHAGE. - The discovery of the b. is due to the French bacteriologist d'Hérelle, who in 1917 described in the fecal filtrate of convalescents from bacillary dysentery a something capable of dissolving the cultures of the dysenteric germs.
If a culture thus lysed is filtered through a porcelain filter capable of retaining the bacteria, the lysing principle passes into the filtrate, and if a small quantity of the latter is added to a new culture of the same germ, the lysis is reproduced in it. 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, besides in the feces of man and of many animals, in water and in soil.
On the nature of this principle, called "b.", the opinions of scholars were for a long time discordant. D'Hérelle strenuously maintained that it was a true living being, capable of reproducing and of parasitizing bacteria, endowed with the characteristics of filterable viruses. Other scholars, especially Bordet and his school, considered the b. instead as an enzyme, and therefore devoid of the characteristics of living matter. This opinion, which until a few years ago was the one that enjoyed the greatest credit, was recently demolished when Ruska turned the electron microscope to the study of the b. With the aid of this powerful instrument of investigation he was able to establish that d'Hérelle's hypothesis is the correct one and that the b. is truly an organized, living being, endowed with the properties of filterable viruses. It presents itself for the most part in the form of very tiny isolated granules, with a diameter of 8-75 mµ, more rarely as rods or short small cylinders, or else as two paired granules provided with one or two minuscule cilia. These minute little bodies crowd around the attacked bacterium, which rapidly undergoes phenomena of lysis and disintegration. The modes of reproduction of the b. are still unknown.
The b., as indeed the great majority of filterable viruses, is not cultivable on artificial media, but develops only in the presence of living bacteria. While some b. are scarcely specific and succeed in lysing germs of several species, the majority of them are instead strictly specific, and at times prove capable of lysing only one species or even a single bacterial variety. On this property are based some recent methods that aim to classify germs very similar to one another (e.g., the salmonellae) by distinguishing them precisely on the basis of their different sensitivity to the different b.
The b. have also been employed in therapy, but generally with results not entirely satisfactory, both because of their frequent excessive specificity and because of the fact that they rarely succeed in lysing all the bacterial individuals, and that those which survive give rise to a bacteriophage-resistant progeny. The significance that the b. have in the spontaneous recovery from infectious diseases and in the elimination of pathogenic germs from waters and from the soil is variously assessed by different scholars.
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