APPENDIX 449 



habitat of its host. For instance, viruses lysing the various en- 

 teric bacteria are found in pooled sewage and animal feces. Vi- 

 ruses lysing staphylococci have been found in the nasal passages 

 and on the skin. The soil is often a good place to look for bac- 

 terial viruses. Other sources are the so-called lysogenic strains 

 of bacteria. Such strains are contaminated with latent viruses 

 capable of lysing other strains of the same bacterial species. 

 For instance, Rountree (1947a) isolated 5 different lysogenic 

 strains and the appropriate susceptible propagating strains 

 from a series of 40 cultures of pathogenic Staph, aureus. 



Assay of Host Organisms 



For much of the quantitative work with bacterial viruses an 

 accurate estimate of the bacterial concentration is essential. 

 The standard method for determining bacterial concentrations 

 is by viable count. Aliquots (0.1 ml.) of serial dilutions of the 

 bacterial culture are spread over the surface of agar plates with 

 a glass rod, and after incubation the number of colonies are 

 counted. From this count the number of viable organisms/ml. 

 of the original culture may be calculated. It is usually neces- 

 sary to work with cultures of the host organism which are ac- 

 tively growing at an exponential rate, since in such cultures the 

 percentage of nonviable organisms is small. Presence in the 

 culture of any considerable percentage of dead bacteria will 

 lead to difficulties, since virus particles which become adsorbed 

 to such bacteria do not reproduce themselves or form plaques 

 on agar plates. In the case of bacteria that grow in clumps 

 or chains, such as streptococci, the viable count does not enum- 

 erate single cells, but rather the chain is the unit counted. 

 In such cases the clumps or chains must be broken up so that 

 most of the cells are seen singly under the microscope. If 

 this is not possible, some of the statistical methods discussed 

 later cannot be applied to such systems; e.g., estimates of the 

 yield of virus/bacterial cell are likely to be misleading if the 

 calculations are based on chains of cells. 



The viable cell count can be used to calibrate a photoelectric 



