Increasing incidence of nalidixic acid resistance and susceptibility to quinolones of Salmonella typhimurium strains isolated in humans or animals

To determine the prevalence of quinolone resistance in Salmonella typhimuri um strains from humans or animals (cattle, poultry, swine), the S. typhimur ium strains isolated at a teaching hospital and at the central veterinary l aboratory of the same district between January 1, 1995, and December 31, 19 96 were studied. Susceptibility to nalidixic acid was determined using the disk diffusion method. Strains with decreased susceptibility to nalidixic a cid were; subjected to minimal inhibitory concentration (MIC) determination for pefloxacin, ofloxacin, ciprofloxacin, norfloxacin, levofloxacin, and g repafloxacin. Decreased susceptibility to nalidixic acid was demonstrated f or 41 of the 309 strains studied and increased from 8.5% in 1995 to 18.6% i n 1996. MIC90 values of fluoroquinolones for strains with decreased suscept ibility 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 stra ins 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 r esistance to fluoroquinolones. This warrants continued monitoring of resist ance of Salmonella to fluoroquinolones.