428 BACTERIOPHAGES 



terion although it has been used in a numl:)er of the classification 

 studies listed in Section 2 above. 



It is possible that osmotic shock may be in a different category 

 with respect to taxonomic usefulness. The work of T. F. Ander- 

 son (1950) indicated that certain groups of phages were not sus- 

 ceptible to osmotic shock whereas other groups, notably T2, T4, 

 and T6, were highly susceptible. Further study is needed to 

 determine the possible usefulness of this technique in the tax- 

 onomy of phages. The same may be said for sensitivity to ultra- 

 violet light (Stent, 1958). 



/. Distinctive Physiological Properties 



A few tests are known which tend to separate phages into two 

 classes exhibiting qualitatively different responses. The all-or- 

 none properties detected by three tests furnish very useful tax- 

 onomic criteria, but unfortunately too few such properties are 

 known, and even the well-known properties have not been looked 

 for in many groups of phages. 



The calcium requirement of the T5-PB species is an example of 

 such a property. These phages require calcium ion for penetra- 

 tion, and phage reproduction cannot occur without this ion. 

 Many phages have no calcium requirement, and for many other 

 phages in which a calcium requirement has been demonstrated, 

 the precise role of the calcium ion has not been investigated. 

 The literature on this point is surveyed in Chapter XIV. Bur- 

 net (1933e) first suggested that the calcium requirement (citrate 

 inhibition) was a useful taxonomic criterion. There is no evi- 

 dence that the calcium requirement for penetration can be al- 

 tered by mutation, although the requirement of calcium as an 

 adsorption cofactor can (Delbriick, 1948). 



The phenomenon of multiplicity reactivation (Luria, 1947) of 

 ultraviolet-inactivated phage is another example of a useful tax- 

 onomic criterion. This property is demonstrated by all the 

 strains in the T2 and T5 species but not, or at a much lower 

 level, by strains in the T3 or Tl species. The occurrence of this 

 property outside of the enteric group of phages has not yet been 



