Purpose: The objective of this study was to determine the effect of sublethal oxidative stress induced by chlorine (sodium hypochlorite) against homologous stress and heterologous stress in two Lm strains, Scott A and V7.
Methods: Lm cells were exposed to increasing subinhibitory concentrations (20 ppm/day) of total chlorine from 250 to 375 ppm in tryptic soy broth (TSB). Changes in minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of Lm cells preexposed to chlorine at 250 (1/2 MIC), 330 (2/3 MIC) and 375 (3/4 MIC) and control (nonexposed to chlorine) were determined by macro-dilution method. Such chlorine-adapted Lm cells were evaluated for changes in Kirby-Bauer disk diffusion assay against 11 antibiotics (amoxicillin and clavulanic acid, ampicillin, ceftriaxone, streptomycin, nalidixic acid, gentamicin, sulphamethoxazole and trimethoprim, ciprofloxacin, rifampin, vancomycin). Also, chlorine-adapted and nonadapted Lm cells were evaluated for changes in MIC values to four antibiotics (ampicillin, gentamicin, streptomycin and tetracycline) as per CLSI guidelines.
Results: The MIC and MBC values of chlorine for Lm Scott A and V7 strains when exposed to sublethal chlorine were higher (600 ppm and 700 ppm) as compared to control (500 pm and 600 ppm). The average zones of inhibition by disk-diffusion assay for chlorine-adapted Lm cells was decreased by 0.5 to 2.2 mm compared to control cells against all 11 antibiotics tested. Also, MIC values doubled for all four antibiotics tested against chlorine-adapted Lm cells compared to control in both strains. However, the changes in zones of inhibition and MIC values to all antibiotics tested for the chlorine-adapted and nonadapted control Lm cells were still under the susceptible range.
Significance: These findings indicate that the continuous exposure of Lm cells to chlorine may induce changes in homologues and heterologous stress adaptation.