[Growing incidence of nalidixic acid resistance and sensitivity to quinolones in Salmonella typhimurium strains isolated from man or animal].

To determine the prevalence of quinolone resistance in Salmonella typhimurium strains from humans or animals (cattle, poultry, swine), the S. typhimurium strains isolated at a teaching hospital and at the central veterinary laboratory of the same district between January 1, 1995, and December 31, 1996 were studied. Susceptibility to nalidixic acid was determined using the disk diffusion method. Strains with decreased susceptibility to nalidixic acid were subjected to minimal inhibitory concentration (MIC) determination for pefloxacin, ofloxacin, ciprofloxacin, norfloxacin, levofloxacin, and grepafloxacin. Decreased susceptibility to nalidixic acid was demonstrated for 41 of the 309 strains studied and increased from 8.5% in 1995 to 18.6% in 1996. MIC90 values of fluoroquinolones for strains with decreased susceptibility to nalidixic acid were lower than 1 mg/L, which is the cutoff above which a strain is classified as susceptible, but were higher than for strains that were susceptible to nalidixic acid. These low levels of resistance may be the first step in selection of mutant strains with high levels of resistance to fluoroquinolones. This warrants continued monitoring of resistance of Salmonella to fluoroquinolones.