The virulence of representative strains of the five species of was compared in C57BL/6 and BALB/c mice in terms of LD50 values and ofbacterial growth kinetics and histological changes in mouse livers. and showed relatively low LD50 values and much bacterial growth for 2-3 days before viable counts declined. Histological changes in infection resembled those caused by , with early development of neutrophil-rich micro-abscesses and hepatocyte necrosis followed by macrophage infiltration and formation of granulomas. By contrast, and were almost entirely avirulent as shown by high LD50 values, early elimination of viable bacteria and no evidence of growth. Histological changes consisted only of slight, transient infiltration of the liver with neutrophils. Both groups of bacteria were seen infrequently in Kupffer cells early in infection, but only the highly virulent species appeared to replicate. LD50 values for and were (10-20)-fold greater, and for the less virulent bacteria at least two-fold greater, in C57BL/6 than in BALB/c mice. This difference in host susceptibility was not reflected in detectable histological differences between the two mouse strains.


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