The repercussions of exposure to nalidixic acid for Salmonella Typhimurium: Alterations in phenotype, multi-drug resistance and lactic acid susceptibility

Antimicrobial drug resistance in foodborne pathogens has increased in importance for human and animal medicine as well as the food industry. Fluoroquinolones, like ciprofloxacin, are used to treat foodborne infections such as salmonellosis, and resistance to the first-generation quinolone, nalidixic acid, can predict decreased susceptibility to ciprofloxacin. Nalidixic acid is used in food microbiology research to create marker strains similar to the wild-type strain that can be isolated from background flora in foods. Nalidixic acid resistant mutants of Salmonella Typhimurium were developed from a single wild-type strain and examined for alterations in phenotype, multi-drug resistance and lactic acid susceptibility; 18 mutants were developed for full resistance (¡Ý 32 ¦Ìg/ml nalidixic acid) and 12 mutants for intermediate resistance (16 ¦Ìg/ml or 25 ¦Ìg/ml nalidixic acid). The wild-type and mutant strains were confirmed as Salmonella using standard techniques, validated for resistance level to nalidixic acid and examined for cross-resistance to other antimicrobial drugs. The wild-type strain, a cocktail of multi-drug resistant Salmonella Typhimurium DT104 and six nalidixic acid resistant mutants were examined for susceptibility to 1% lactic acid and growth in tryptic soy broth (TSB). Compared to the wild-type strain, two mutants had altered serotypes, and 12 mutants displayed cross-resistance to other antimicrobial drugs. One strain was resistant to six antimicrobial drugs: nalidixic acid, cefoxitin, chloramphenicol, sulfisoxazole, streptomycin and tetracycline. This resistance pattern was similar to the multi-drug resistance pattern associated with Salmonella Typhimurium DT104. For two mutants, area under the growth curve in TSB was significantly (P<0.01) different from the wild-type strain. In lactic acid, one mutant (resistant to five antimicrobial drugs) had a lower (P<0.05) area under the growth curve compared to the wild-type strain and the other mutants. The DT104 cocktail had a lower (P=0.0696) area under the growth curve than the wild-type strain. Resistance to several antimicrobial drugs appeared to increase susceptibility to lactic acid. The development of resistance to nalidixic acid can result in significant alterations in phenotype, antimicrobial drug resistance and growth characteristics emphasizing the importance of strain validation prior to experimental use and the importance of limiting nalidixic acid resistance in environmental isolates.