ABSTRACT
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 x 10(7) colony-forming units (cfu) compared with a mean LD50 of 2.65 x 10(5) cfu for 18 normal strains (P less than 0.005). Thirteen of the epidemic strains had an LD50 of greater than 10(7) cfu and on the average were 80 times less virulent than strain CDC 9, which has been kept in the laboratory for greater than 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.
