Bacteriophage

By definition "bacteriophage" (or phage) connotes a large group of sub-microscopic agents which induce transmissible lysis of bacteria and are capable of passing bacteriaretaining filters. They are widely distributed in nature, abound particularly in the intestinal tracts of man and animals and possess particulate diameters within the limits of 8 μμ to 100 μμ. Like the mosaic viruses, purified phage preparations have been found to be nucleoproteins. Molecular weight determinations by means of centrifugation analysis and diffusion have given figures of 400,000 to 300,000,000.

The first observations on bacteriophage were made in 1915 by Twort. He noted in colonies of Staphylococci isolated from calf-lymph curious, glassy-looking areas within which there were masses of granular debris. He found that it was possible to form vitreous zones in normal cultures by the simple expedient of placing on the surface of a young agar culture a drop of filtrate from a suspension of bacterial growth in which this degenerative change already had taken place.

In 1917 d'Herelle called attention to the same phenomenon observed while he was studying cultures of dysentery bacilli recently isolated from the stools of patients suffering from bacillary dysentery. For no apparent reason ordinary turbid broth cultures of dysentery bacilli would suddenly become crystal clear and this spectacular lysis could be induced in a fresh culture of dysentery bacilli by adding a drop of the filtered, cleared culture. d'Herelle coined the term "bacteriophagy" to denote all phases of the reaction between bacteriophage and the susceptible bacterial host and for several years he studied the phenomenon intensively. He developed the view that phage is a single living organism, "Protobios bacteriophagum" which may adapt itself to live upon a wide variety of bacterial substrates. Others, notably Bordet, believed that phage is a bacterial enzyme of unspecified origin but possessing such properties that its introduction into the bacterial cell disturbs the normal metabolic processes with the result that the cell initiates production of the same agent. The essential difference between the two hypotheses is simply that in d'Herelle's view phages are small living units stemming from a single parent "race" and acquiring special characteristics by adaptation while Bordet and his followers believe they are modified bacterial proteins. The consensus of opinion among modern workers is that phages are bacterial viruses, protein in nature and altogether analagous to the animal and plant viruses.