EFFECT OF PHYSICAL AND CHEMICAL AGENTS 59 



The coliphage T5 belongs to the same class, and its response to 

 heating has been studied in detail by Lark and Adams (1953). 



T5 is maximally resistant to heat at cation concentrations 

 exceeding Af/ 1,000 for magnesium or M/X for sodium, and 

 maximally sensitive at concentrations less than M/10^ for magne- 

 sium or M/10 for sodium. The span of inactivation rates at 50 ° 

 C. (measured indirectly) between these two extremes is about 

 1 X 10'''. These and other characteristics of the dependence on 

 cations presumably indicate complex formation between cations 

 and some heat-labile phage protein. In its stable form, the 

 complex resists heating to 70 ° C. ; in its least stable form the phage 

 is rapidly inactivated at 30 ° C, or even below in the presence of 

 chelating agents. Inactivation of the phage by heat is accom- 

 panied by release of nucleic acid into the solution. The result- 

 ing nucleic acid-free ghosts do not adsorb to bacteria. Further 

 details of the physical and biological characteristics of heat in- 

 activation are given by Lark and Adams (1 953). 



Coliphage T5 mutates to a form, T5st, that is about 1,000 

 times more heat resistant in 0.1 iVNaCl than is the parental wild 

 type (Lark and Adams, 1953). The two forms do not differ in 

 heat sensitivity at high salt concentrations. They differ, there- 

 fore, in the manner of interaction with cations. The mutant 

 occurs with a frequency of about 1 0~^ in wild type stocks initiated 

 from single plaques, but on repeated subculture accumulates 

 until it may amount to 10 per cent of the population. Wild 

 type stocks of phage T5 also contain a phenotypically heat-resist- 

 ant form composing about 0.1 per cent of the population. The 

 phenotypically heat-resistant particles have the same stability 

 as the heat-resistant mutants but differ in that the property of 

 heat resistance is lost on a single growth cycle in the host bac- 

 terium. Similar phenotypically heat-resistant particles have 

 been found in stocks of phages BG3, 29 alpha, and PB which are 

 serologically related to phage T5 (Adams, 1953a). No satis- 

 factory explanation for the origin of these phenotypically heat- 

 resistant particles is available at present. 



Another example of nonheritable heat resistance was observed 



