Determination of total antibiotic resistance in waterborne bacteria in rivers and streams in Northern Ireland: Can antibiotic-resistant bacteria be an indicator of ecological change?

Antibiotic resistance (ABR) has now become a major public health issue. Relatively little studies have been published on the incidence of ABR in environmental isolates circulating within the community. A study was performed to determine the diversity of total ABR (intrinsic + acquired resistance) in waterborne bacteria. Surface water from 12 waterways, including 11 rivers/steams and 1 lake, were examined for the presence of ABR phenotypes, using a direct antibiotic susceptibility assay and demonstrated the presence of ABR (in increasing order of resistance), to the following 19 agents: amikacin (17%), gentamicin (17%), ciprofloxacin (33%), colistin (42%), linezolid (42%), tobramycin (42%), vancomycin (42%), ertapenem (67%), erythromycin (75%), meropenem (75%), rifampin (75%), teichoplanin (75%), tetracycline (75%), trimethoprim (75%), fusidic acid (83%), aztreonam (92%), clindamycin (92%), penicillin (92%) and cefoxitin (100%). Multiple resistance to the major classes of antibiotics was noted, which varied from one to six classes, with a mean resistance to 3.7 major antibiotics classes, with diminishing antibacterial effectiveness in the following order: aminoglycosides > fluoroquinolones > glycopeptides > macrolides > tetracyclines > beta-lactams. Overall, these data indicate that waterborne bacteria are an important source of ABR determinants and contribute to the mass balance of ABR in the environment, and may be used as an indicator of ecological change in water ecosystems. The waterborne ABR organisms may potentially act as donors in pathogens, which may acquire these through horizontal gene transfer or other genetic exchange events, thus leading to clinically significant cases in both animal and human health. Therefore, environmental bacteria should not be regarded as being devoid of ABR determinants, simply because they are physically removed from clinical settings. Such bacteria have natural intrinsic resistance, as well as having the ability to acquire determinants from agricultural run-off and human wastewater discharge, which may contain ABR organisms, as well as sublethal concentrations of metabolically active antibiotic. The tracking of such organisms to their source may help determine the source of fecal pollution in aquatic ecosystems.