MIRAGLIA AND BERRY 



a highly significant difference is noted in the mortality ratios be- 

 tween the two temperatures. This difference is significant at the 

 0.8 per cent level by rank test (White, 1952), 



Influence of temperature on the per cent of livers positive for 

 bacteria . At the termination of each experiment, all animals that 

 survived the 14 day period of observation were killed by cervical 

 dislocation. The livers were immediately excised and cultured on 

 nutrient agar, MacConkey's agar, SS agar, and Staphylococcus HO 

 agar by the print method. The results are shown in Figure 1 for the 

 experiments conducted with strain RIA. Not indicated on this figure 

 was the observation that the per cent of mice from which salmon- 

 ellae could be isolated was greater in animals kept in the cold than 

 in those at 25° C. Moreover, as might be expected, the per cent of 

 positive livers increased in proportion to the size of the infectious 

 dose. Even more important was the number of mice which gave 

 staphylococci from liver imprint cultures. This secondary invader, 

 as can be seen from Figure l,isto be found in animals kept at 5° C 

 in an incidence that is proportional to the number of salmonellae 

 (RIA) administered in the primary infection. For mice housed at 

 25° C, a proportionality between primary and secondary infection is 

 not at all apparent. It would seem, therefore, that a combination of 

 cold and salmonellosis predisposes to an invasion of staphylococci. 

 When inocula exceed the LDcq dose for mice at 5° C, few mice sur- 

 vive to be tested for tissue staphylococci and hence an atypical 

 group, the highly resistant animals, survives. Thus, the relationship 

 between primary and secondary infection was unpredictable, as in- 

 dicated by the curves plotted forthelO'^and 10^ inocula. Despite the 

 tendency toward selection of atj^ical survivors, the data obtained 

 with livers cultured for staphylococci continued to show a sub- 

 stantially higher incidence of secondary invasion for mice in the 

 cold than for animals at room temperature. 



The effect of low temperature on salmonella- carrier mice . 

 Results similar to those obtained with strain RIA were noted when 

 the highly virulent strain SR-U was used in experiments with mice 

 which proved to be typhoid carriers. The results of this study, using 

 mice which arrived from the dealer with feces that yielded positive 

 cultures for salmonellae, are shown in Figure 2. Thus, normal 

 mice infected with avirulent salmonellae and exposed to cold show 



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