Enhancement of antibiotic activity by sub-lethal concentrations of enterocin CRL35.

The aim of this study was to evaluate the interaction of several conventional antibiotics with sub-lethal concentrations of enterocin CRL35, a cationic peptide, on Listeria innocua 7. Susceptibility of L. innocua 7 cells to the combination of enterocin CRL35 and non-peptide antibiotics (cefalexin, ampicillin, ciprofloxacin, nalidixic acid, erythromycin, chloramphenicol, vancomycin and tetracycline) was assayed using the broth dilution method and killing curves. Fractional inhibitory concentration (FIC) index was calculated to assess synergy. The transmembrane electrical potential and pH gradient were determined by specific fluorescent probes. We found positive interactions between the cationic peptide and three conventional antibiotics (tetracycline, erythromycin and chloramphenicol) which are excluded from the cells by efflux pumps dependent on the membrane proton gradient. Furthermore, enterocin CRL35 even at sub-lethal concentrations induced the dissipation of both components of the proton motive force (Deltap), i.e. transmembrane electrical potential and pH gradient and hence the alteration of processes dependent on it. We hypothesize that enterocin CRL35 increases the effectiveness of these antibiotics by impairment of the bacterial active efflux systems and the consequent accumulation of these toxic compounds in the cytoplasm.