COMPARISON OF THE VIRULENCE OF METHICILLIN-RESISTANT AND METHICILLIN-SENSITIVE STAPHYLOCOCCUS-AUREUS

The virulence of methicillin-resistant Staphylococcus aureus (MRSA) wa s compared with that of methicillin-sensitive S. aureus (MSSA), using 13 MRSA and 7 MSSA strains isolated from clinical specimens. The infec tivity and lethality of the two groups were examined as to the inoculu m required to infect 50% of guinea pigs (ID50) and to kill 50% of mice (LD(50)), respectively. The mean ID50 [log(10) colony forming units ( CFU)] for MRSA strains was 7.1+/-0.60 standard deviation, which was 1. 5 higher than that for MSSA strains (P < 0.001). The mean LD(50) (log( 10) CFU) for MRSA strains was 9.0+/-0.42, being 1.1 higher than that f or MSSA strains (P=0.001). Pretreatment of mice with cyclophosphamide decreased the mean LD(50) for MRSA strains more than that for MSSA str ains, resulting in the difference in the mean LD(50) being insignifica nt (P=0.502). These results indicate that MRSA is less virulent than M SSA in normal hosts, but that they are equally virulent in immunocompr omised hosts. The growth of MRSA strains was much slower than that of MSSA strains in the lag phase, although their growth rates were almost the same in the exponential growth phase, suggesting that the differe nce in virulence between them may be at least partly due to such a dif ference in growth.