The susceptibility of ionophore-resistant Clostridium aminophilum F to other antibiotics.

OBJECTIVE To determine if ionophore-resistant ruminal bacteria are cross-resistant to other classes of antibiotics. Clostridium aminophilum was used as a model organism because this Gram-positive ruminal bacterium can adapt to ionophores (monensin and lasalocid). Non-adapted cultures lagged for at least 12 h with 1 microM monensin or lasalocid, but initiated no growth if the concentration was 10 microM. Adapted cultures did not lag with 1 microM monensin or lasalocid, grew well even if the ionophore concentration was 10 microM and contained cells at least 100,000-fold more resistant than those in non-adapted cultures. METHODS Ionophore-adapted and non-adapted cultures were assayed for their susceptibility to other classes of antibiotics (penicillin G, ampicillin, cephalosporin C, vancomycin, carbenicillin, tetracycline, chloramphenicol, erythromycin, streptomycin, linocomycin, rifampicin, trimethoprim, novobiocin, polymyxin B and bacitracin) using a broth microdilution method. RESULTS Adapted cultures retained their resistance phenotype for at least 28 generations even if ionophore was no longer present. Monensin-adapted cultures were as resistant to lasalocid as those adapted to lasalocid, but lasalocid-adapted cultures lagged with 1 microM monensin. Monensin- and lasalocid-resistant C. aminophilum F cultures were as susceptible to most antibiotics as non-adapted cultures. The only antibiotic that seemed to have a common mechanism of resistance was bacitracin, and the ionophore-adapted cultures had a 32-fold greater MIC. CONCLUSION The use of ionophores in cattle feed and the selection of ionophore-resistant ruminal bacteria does not necessarily lead to other types of antibiotic resistance.