Virulence in Mice of Epidemic Strains of Salmonella typhimurium Isolated from Children

The virulence of 30 multiresistant strains of Salmonella typhimurium, which produced nine severe outbreaks in children's hospitals, was tested in mice. The arithmetic mean of the 50% lethal dose (LD50) for mice was 1.37 × 107 colony-forming units (cfu) compared with a mean LDso of 2.65 x lOs cfu for 18 normal strains (P < 0.005). Thirteen of the epidemic strains had an LD50 of >107 cfu and on the average were 80 times less virulent than strain CDC 9, which has been kept in the laboratory for >40 years. The unexpected results show that apparently the factors determining virulence for children are not the same as those for mice, and they raise doubts about the validity of using mice to study the salmonella host-parasite relationship in humans. Genetic material carried by the transfer factor may be responsible for the altered virulence of the epidemic strains.