Acquired efflux mechanisms
Another acquired fluoroquinolones resistance mechanisms is represented by efflux mechanisms. The qepA gene, initially characterized on a conjugative plasmid from a clinical isolate of E. coli (Perichon et al., 2007), encodes a MFS efflux pump. It has been recently recovered from the metagenome of river sediments impacted by improperly managed urban waste waters (Cummings et al., 2011). Environmental reports of this gene are rare; however, Deng et al. (2011) have highlighted the possible spread of this gene by animal and human related bacterial strains in water compartments. Similarly, the detection of the OqxAB efflux pump, conferring resistance to fluoroquinolones, olaquindox, and chloramphenicol, remains rare in environmental samples. The oqxAB, found on a conjugative plasmid in E. coli strains, represents the only example of transferable RND efflux pumps, so far (Hansen et al., 2004). Recently, Zhao et al. (2010) have reported an E. coli strain, isolated from a water pond in a farm environment, harboring the oqxAB gene (Table 1).
Resistance by acquired efflux mechanisms to other drug classes than fluoroquinolones has been extensively reported in the literature (Poole, 2004; Piddock, 2006; Nikaido and Pages, 2011). Studies conducted in water habitats such as aquaculture, impacted by anthropogenic activities, and notably by the application of antibiotics, demonstrated the risk of selection of acquired efflux pumps. Fernandez-Alarcon et al. (2010) reported the presence of different Gram-negative species from aquacultures in Chile expressing the floR gene, which codes for a chloramphenicol and florfenicol exporter, drugs intensively used in veterinary medicine. Alarmingly, those strains resistant to florfenicol also demonstrated a multi-drug resistance, suggesting a process of co-selection. Gordon et al. (2008) characterized the floR gene, in Aeromonas bestiarum strains from freshwater streams in France, located on a 25-kb-plasmid harboring also the tetracycline efflux gene tet(Y), strB-strA, conferring resistance to streptomycin, and sul2 conferring resistance to sulfonamides (Table 1). Interestingly, this plasmid contained sequences with high nucleotide homologies to other genetic elements recognized in different aquatic bacterial species such as V. cholerae and Photobacterium damselae. This demonstrated the contribution of horizontal gene transfer in the spread of these resistances in aquatic habitats. Furthermore, genes encoding tetracycline efflux mechanisms have been found to circulate between farm environments and ground water (Aminov et al., 2002). Propagation of tetracycline resistance genes, by efflux or by ribosomal protection, has been linked to the extensive usage of this drug class in animal feeding, and although the usage of this antibiotic has been restricted, tetracycline resistance genes seem to persist in the food chain and in the environment.