VIRULENCE OF ANTIBIOTIC-RESISTANT SALMONELLA-TYPHIMURIUM

We show that most Salmonella typhimurium mutants resistant to streptom ycin, rifampicin, and nalidixic acid are avirulent in mice, Of seven r esistant mutants examined, sis were avirulent and one was similar to t he wild type In competition experiments in mice, The avirulent-resista nt mutants rapidly accumulated various types of compensatory mutations that restored virulence without concomitant loss of resistance, Such second-site compensatory mutations were more common then reversion to the sensitive wild type, We infer from these results that a reduction in the use of antibiotics might not result in the disappearance of the resistant bacteria already present in human and environmental reservo irs, Thus, second-site compensatory mutations could increase the fitne ss of resistant bacteria and allow them to persist and compete success fully with sensitive strains even in an antibiotic-free environment.