SECONDARY BACTERIAL INFECTION IN MICE 



7 70 



INOCULUM 



700 7000 

 ( SR-II ) 



Figure 2. The per cent of liver cultures positive 

 for staphylococci in natural salmonella carrier mice 

 at 5° C and at 25° C as related to the infecting dose 

 of _S. typhimurium , strain SR-II. All liver cultures 

 were made 14 days postinfection. 



an incidence of livers positive for staphylococci in the same way as 

 do carrier mice in the cold infected with virulent SR-U. Staphylo- 

 cocci were not present in livers cultured from carrier mice main- 

 tained at 25° C except those infected with the highest dose of strain 

 SR-U, and even here only 10 per cent of the mice had a positive 

 culture. The high degree of immunity of the carrier mice is evi- 

 denced by the fact that all animals at room temperature were able 

 to withstant 7000 LD^qq of strain SR- 11, while only four of 10 animals 

 died from this dose at S^CTheSR-U strain is usually fatal to mice 

 when only a single cell is injected (Schneider and Zinder, 1956), 

 Even with enhanced resistance, some carrier animals succumbed at 

 5° C, The death of these mice shows that exposure to low environ- 

 mental temperature increases susceptibility to infection even in 

 highly immune mice. 



It might be suggested that as increasing numbers of salmonellae 

 are injected, progressively mounting stress is applied to a host al- 

 ready stressed by cold so that staphylococci already in the host or 

 its environment now become established in its tissues. Noteworthy 



281 



