Bacterial viruses or bacteriophages
- M. Delbrueck
- Biological Reviews
- Biol Rev Camb Phil Soc 21 (1): 30-40
(i) The following factors are held to be chiefly responsible for recent progress in the field of bacterial viruses: (a) the introduction of 'one-step growth experiments', (b) the concentration of attention on a small group of bacterial viruses, (c) the proof that virus-resistant mutants originate independently of the presence of virus, (d) the application of the electron microscope. (2) Types of virus morphology, as revealed by the electron microscope, are described. The virus particles of any one strain are of uniform size and shape. Many strains show a 'head-and-tail' structure. Sizes between 45 and 100 mμ have been found. (3) Classification of a group of seven viruses by serological cross reactions agrees perfectly with classification by morphology. (4) A method for determining the origin of mutations of bacteria from virus sensitivity to virus resistance is outlined. There is evidence that the classification of viruses by cross-resistance tests is of little value in establishing natural relationships between viruses. (5) The phenomenon of 'lysogenesis' is discussed, and certain ambiguities in its definition are pointed out. (6) The physiological basis of resistance of a bacterial strain to the attack by a specific virus is discussed. Such resistance is generally coupled with failure on the part of the bacteria to fix the virus in question by adsorption. Evidence shedding a new light on the process of adsorption is reported. (7) Bacterial viruses can mutate in such a way that their host range is altered. These mutations occur during the multiplication of the virus in its host. (8) The technique and terminology of 'one-step growth experiments' are briefly described. (9) When a bacterium is attacked simultaneously by several virus particles of the same strain or of different strains the bacterium reacts as if it had been attacked by only one particle (mutual exclusion effect). A detailed study of this phenomenon has led to the assumption that the membrane of the bacterium becomes impermeable to other virus particles after one particle has entered the cell (penetration hypothesis). The excluded virus particles may affect the yield of virus of the successful type (depressor effect). (10) The adaptation of the technique of the 'one-step growth experiment' to biochemical studies of virus multiplication is described. (n) The controversies regarding the nature of bacterial viruses are briefly touched upon, particularly with reference to the evolutionary standing of these viruses. (12) It is pointed out that a thorough study of bacterial viruses may lead to a better understanding of all types of viruses.