Strain-specific differences in the amount of shiga toxin released from enterohemorrhagic Escherichia coli O157 following exposure to subinhibitory concentrations of antimicrobial agents

There is no consensus regarding the benefit versus harm of antibiotic thera py for treatment of disease due to enterohemorrhagic Escherichia coli O157. The effects in vitro of subinhibitory concentrations of 13 antimicrobial a gents on the release of Shiga toxin (Stx) by three different Escherichia co li O157 strains expressing Stx 1 or Stx 2 either alone or in combination we re investigated. The Stx-induced cell death of Vero cells was determined us ing a colorimetric assay based on the measurement of lactate dehydrogenase (LDH) released into the supernatant from the cytosol of damaged cells. Grow th of all O157 strains in broth cultures containing subinhibitory concentra tions of cotrimoxazole, trimethoprim, azithromycin, or gentamicin was accom panied by a marked increase in the release of Stx. Exposure to cefixime, ce ftriaxone, or erythromycin caused a marked increase in the release of Stx b y the O157 strain producing Stx 2 alone, but decreased toxin production was observed with the Stx 1 producer and the strain producing Stx 1 and Stx 2. Exposure to ampicillin caused increased Stx release in the Stx 2-producing strain but had no effect on Stx production in the other two test isolates. Exposure to penicillin G, streptomycin, ciprofloxacin, fosfomycin, or sulf amethoxazole caused an increase in toxin production in two of the three tes t strains in each case, while decreases were observed for the other isolate s. The response of Escherichia coli O157 isolates to subinhibitory concentr ations of antibiotics seems to be highly dependent on the nature of the str ain involved.