[Suppressive effect of clarithromycin on the production of verotoxin by E. coli O157]

Effects of low concentration of clarithromycin (CAM) on the production of Verotoxin (VT) by Escherichia coli O157 was investigated in vitro. The production of VT1 was suppressed up to 10 hours when bacteria was incubated with 0.64 micrograms/ml (equivalent to the 1/100th of MIC) or higher concentrations. However, the production of VT1 reached to the control level after 22 hours even with 6.4 micrograms/ml of CAM. On the other hand, production of VT2 by 22 hours was partially suppressed with 0.64 micrograms/ml of CAM and completely with 6.4 micrograms/ml of CAM. When the eight clinical isolates were incubated with 0.64 or 6.4 micrograms/ml of CAM, VT1 and VT2 were suppressed in two and eight strains, respectively. In these strains, similar but less efficient suppression of the toxin production was also observed with erythromycin. In contrast to the macrorides, ampicillin did not inhibit or rather stimulated the production of VT1 and VT2 in some strains.