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JAC Advance Access originally published online on September 1, 2003
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Journal of Antimicrobial Chemotherapy (2003) 52, 623-628
© 2003 The British Society for Antimicrobial Chemotherapy

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

Adam J. Houlihan1 and James B. Russell1,2,*

1 Department of Microbiology, Cornell University, Wing Hall, and 2 Agricultural Research Service, USDA, Ithaca, NY 14853, USA

Received 28 April 2003, returned 9 June 2003, revised 17 June 2003; accepted 2 July 2003

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 µM monensin or lasalocid, but initiated no growth if the concentration was 10 µM. Adapted cultures did not lag with 1 µM monensin or lasalocid, grew well even if the ionophore concentration was 10 µM 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 µM 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.

Keywords: antibiotics, ionophores, resistance, monensin, lasalocid

* Corresponding author. Tel: +1-607-255-4508; Fax: +1-607-255-3904; E-mail: jbr8{at}cornell.edu


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