title: Enhancement of antibiotic activity by sub-lethal concentrations of enterocin CRL35.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Departamento de Bioquímica de la Nutrición, Instituto Superior de Investigaciones Biológicas (Consejo Nacional de Investigaciones Científicas y Técnicas-Universidad Nacional de Tucumán, San Miguel de Tucumán, Argentina.
authors.forename: Carlos J
authors.initials: CJ
authors.lastname: Minahk
authors.forename: Fernando
authors.initials: F
authors.lastname: Dupuy
authors.forename: Roberto D
authors.initials: RD
authors.lastname: Morero
mesh_terms: D000596:Amino Acids / Q000378:metabolism; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001430:Bacteriocins / Q000494:pharmacology*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D003470:Culture Media; D004357:Drug Synergism; D016983:Enterococcus / Q000187:drug effects; D004996:Ethidium / Q000378:metabolism; D005456:Fluorescent Dyes; D006863:Hydrogen-Ion Concentration; D007930:Leucine / Q000378:metabolism; D008087:Listeria / Q000187:drug effects*; D008564:Membrane Potentials / Q000187:drug effects; D008826:Microbial Sensitivity Tests; D017494:Proton Pumps / Q000502:physiology
chemical_list: D000596:Amino Acids; D000900:Anti-Bacterial Agents; D001430:Bacteriocins; D003470:Culture Media; D005456:Fluorescent Dyes; D017494:Proton Pumps; C121783:enterocin CRL35; D004996:Ethidium; D007930:Leucine