Lactoferrin interaction with salmonellae potentiates antibiotic susceptibility in vitro.

Interaction of lactoferrin (Lf) with the cell envelope (CE) and outer membrane (OM) of Salmonella typhimurium-type strain ATCC13311 was tested by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western-blot analyses. The peroxidase-labeled bovine Lf (BLf) and human Lf both recognized a heat-modifiable protein with an estimated molecular mass of 38 kD in the OM. Simultaneous immunoblotting with an antiporin monoclonal antibody specific for a conserved porin domain in members of enterobacteriaceae confirmed that the Lf-binding protein is a porin. Such Lf-binding porin proteins (37-39 kD range) were readily detected in nine other common Salmonella species: S. dublin, S. panama, S. rostock, S. abony, S. hartford, S, kentucky, S. pullorum, S. thompson, and S. virchow. The latter six species also demonstrated one to three weak Lf-reactive bands of low molecular weight in their CE. The antibiotic susceptibility of Salmonella in the presence of Lf was examined. A mixture containing sub-minimum inhibitory concentration (MIC) levels of Lf (MIC/4) and cefuroxime (MIC/2) inhibited the bacterial growth. Lf strongly potentiated the action of erythromycin (eightfold), whereas it increased the activity only by two-fold for ampicillin, ciprofloxacin, chloramphenicol, and rifampicin; similarly, these antibiotics also reduced the MIC of BLf by twofold in S. typhimurium. Such antimicrobial potentiation was not observed with BLf mixtures containing cefalexin, gentamycin, or polymyxin B against strain ATCC13311. BLf and cefuroxime also demonstrated potentiation of varying degrees (two to 16-fold) with nine other Salmonella species. These data established the binding of Lf to porins in salmonellae and a potentiation effect of Lf with certain antibiotics.