Correlation between production of listeriolysin O by variants of Listeria monocytogenes and their virulence for rabbits.

L. monocytogenes serovar 1/2a NCTC 7973 was passaged through rabbits and the severity of infection at each passage was determined by counting viable bacteria from infested organs and recording the time of death. A comparative evaluation of the levels of hemolysin produced in vitro by the original and six variant cultures (V1-V6) was done by determination of hemolytic units (CHU). While virulence of the cultures enhanced at each passage (2.2 x 10(9) CFU/g of the spleen for V6 as compared to 5.0 x 10(6) CFU/g spleen for the parent culture), the CHU decreased considerably, 3 CHU for the V6 as compared to 40 CHU for the parent strain. The results suggest that the level of in vitro production of listeriolysin may not parallel the pathogenicity of L. monocytogenes for rabbits.