SECONDARY BACTERIAL INFECTION IN MICE 



NOCULUM (RIA) 



Figure 1. The per cent of liver cultures positive 

 for staphylococci in mice at 5° C and at 25° C as 

 related to the infecting dose of S. typhimurium, 

 strain RIA. All liver cultures were made 14 days 

 postinfection. 



In order to test the possibility that a cold environment enhanced 

 the virulence of the organism (strain RIA), isolates were recovered 

 from the liver and spleen of infected mice sacrificed for the purpose 

 after being held at 5° C for various periods up to 14 days. These 

 isolates were injected into normal mice and the LD5Q and mean 

 survival times weredeterminedatboth temperatures. Nodifferences 

 from the parent strain were noted, nor were there any detectable 

 changes in colonial morphology or growth rate in vitro when com- 

 pared to the original strain. Using these criteria, it would appear 

 that a cold environment does not alter under in vivo conditions the 

 virulence of the invading organism. 



Table III shows the mortality rates of mice at room temperature 

 and in the cold when given graded doses of strain RIA. It is apparent 

 that animals maintained in the cold are not only more susceptible to 

 infection, as judged by the increased mortality rate, but the initial 

 deaths occur sooner than those at 25° C. When mice are injected 

 with an LD50 dose of strain RIA (4.8 x 10^ cells per mouse at 25° C, 

 as determined directly and not by calculation, as mentioned above) , 



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