VIRULENCE AND BACTERIAL INFECTION 



come of salmonellosis only when a mixed population of virulent and 

 avirulent pathogens was inoculated into the mice. As an alternative 

 possible explanation, the results of Muschenheim etal., (1943) should 

 be mentioned. They found that in rabbits infected with a virulent 

 Type I strain of pneumococcus the only effect of reduced body tem- 

 perature was a decrease in the local inflammatory response. How- 

 ever, with a relatively avirulent Type III strain, overwhelming 

 bacteremia and death resulted as well. These investigators con- 

 cluded that reduction in body temperature can alter resistance to 

 infection with an otherwise relatively avirulent pathogen. Despite 

 differences in experimental approach, the results reported in this 

 paper tend to support such a concept. 



If, as is reported by Berry and Smythe (1960), endotoxins con- 

 tribute to the toxic manifestations of salmonellosis, then the close 

 similarity of survivorship and survival time in animals infected 

 with virulent SR-U, regardless of subsequent temperature exposure, 

 might be theresultof the formation of a smaller amount of endotoxin 

 at 5° C than at 25° C. This argument would follow, since cold ex- 

 posed mice are killed with less endotoxin than those maintained at 

 normal temperatures. In line with this hypothesis, animals under 

 optimal environmental conditions might support more rapid growth 

 of certain virulent pathogens than animals in the cold. Perhaps in- 

 fection with the avirulent SR-U- A results in more deaths in a shorter 

 time at 5° C than at 25 C because the lethal level of endotoxin 

 accumulates sooner at the former temperature. The defenses of the 

 mouse might be sufficiently impaired by cold to permit a steady 

 in vivo proliferation of the bacteria which ultimately leads to death, 

 in contrast with the events in animals under more normal environ- 

 mental temperatures. 



The results with Staphylococcus aureus infections seem to fit into 

 the interpretations suggested above. The relationship between viru- 

 lent and avirulent S, aureus parallels closely the findings with 

 salmonellosis. Additional studies will be required, however, before 

 the role of cold in active infections can be clearly interpreted. 



From our results it appears that the host is able to overcome 

 certain effects of endotoxin within the first 6 to 12 hours of admini- 

 stration. Reports available in the literature suggest too that host 



229 



