Is Antibiotic Resistance a Selective Advantage to Environment Stresses?

Introduction : The presence of antibiotic resistant bacteria in the food supply chain is a major concern. Different intervention strategies are typically used to eliminate pathogenic organisms from food processing environments. Lactic acid is one of the organic acids that have been used most extensively as antimicrobial. Recurring outbreaks pose a great need to revalidate the effectiveness of these decontamination procedures for multidrug-resistant strains.

Purpose : The purpose of this study was to determine the survival kinetics of antibiotic-resistant non-O157 Shiga-toxin producing Escherichia coli (STEC) strains to environment stresses.

Methods : A total of 6 non-O157 STEC strains (serotypes O26 and O103) with three different antibiotic resistant profiles (susceptible, medium and high resistance) were chosen for intervention studies. Tryptic soy broth (pH 6.5) samples were inoculated (10⁷CFU/ml) and treated with lactic acid (2.5, 3.5, 5%). All samples were serially diluted into buffered peptone water (pH = 7.4) at 0, 1, 2.5, 5, 7.5 and 10 min; and plated on tryptic soy agar (TSA). All plates were incubated at 37°C for 24 h and bacterial counts were determined.

Results : E. coli strains had similar acid survival characteristics. When treated with 5% lactic acid (LA), bacterial counts were reduced by approx. 2 logs CFU/mL within 10 min of the experimental period. Bacterial counts were reduced to approx. 1 log CFU/ml with 3.5% LA for O26 and O103 strains. No reduction in bacterial counts was observed with 2.5% LA treatment for both serotypes.

Significance : The results reported the effectiveness of lactic acid to inactivate antibiotic-resistant non-O157 STEC. These findings suggest that E. coli strains showed high acid tolerance and antibiotic resistance had minimal effects on the adaptation to environmental stresses.