Antibiotics in media for isolation of Campylobacter spp. do not enhance resistance

Campylobacter spp. have become the major cause of gastro-enteritis in the Western world. However, isolation and culture of these bacteria from different source samples is difficult as they are fastidious and slow growing, and are thus out-competed by other bacterial species and fungi. This has necessitated the use of laboratory media that contain various antibacterial and anti-fungal selective agents.1,2 Antibiotics have the tendency to select for resistant bacteria. For example, fluoroquinolones can select for mutants with gyrA mutations and/or that overexpress an efflux system.3 Therefore, there has been increasing concern that the use of antibiotics in Campylobacter isolation media may result in biased antibiotic susceptibility data sets.2 The antibiotics cefa-lothin, cefoperazone and rifampicin are incorporated in the cur-rently used selective media. These agents are also substrates of CmeABC, an RND-type efflux system in Campylobacter, which can confer multiple antibiotic resistance.4,5 We hypo-thesized that as these agents are substrates of efflux pumps, these media could select for mutants that overexpress the cmeABC,4 cmeDEF6 and/or other efflux systems in Campylobacter jejuni NCTC 11168. Alternatively, induction of these pump genes could allow Campylobacter spp. to survive the presence of antibiotics in media. C. jejuni NCTC 11168 was cultured on standard Mueller– Hinton (MH) agar and also on three commonly used Campylo-bacter selective media (Table 1). The plates were incubated under microaerophilic conditions for 48 h. Colonies were resuspended in MH broth, MH broth plus Blaser-Wang supplement, MH broth plus CCDA sup-plement and MH broth plus Preston supplement and incubated overnight. The suspensions were used as inocula for MIC deter-mination and gene expression analysis for cmeABC and cmeDEF, as described previously.4,6 The antibiotics tested were ampicillin, cefalothin, chloramphenicol, ciprofloxacin, erythro-mycin, kanamycin, rifampicin, tetracycline and trimethoprim