Because certain antibiotics can be excreted largely as the parent compound in human waste, questions have been raised concerning the possibilities of antibiotic resistance generation in microbes within and as a consequence of discharges by wastewater treatment plants (WWTPs). The release of low doses of antibiotics into receiving surface waters by WWTPs has therefore been identified as a possible contributor to the problems of resistance. The overarching concern surrounding the constant release of these pharmaceuticals from WWTPs is that this may give rise to the creation of a background level of resistance within environmental bacterial populations, which in turn may be transferred to more virulent human pathogens. The goal of this investigation therefore is to contribute to the body of knowledge surrounding the development of drug resistant bacteria as a consequence of the pervasive subtherapeutic concentrations of antibiotics in the environment. More specifically, the question examined in this study was whether wastewater effluent being discharged into environmental waters contributed to an increase in the numbers of resistant bacteria found downstream. The ubiquitous environmental genus Aeromonas was employed as a marker of the impacts of residual antibiotics on riparian bacterial populations. Opportunistic pathogens themselves, Aeromonas spp. have been shown to be able to transfer resistance to other pathogens such as Vibrio cholera, and they are included on the U.S. Environmental Protection Agency's Unregulated Contaminant Monitoring Rule List 2. Since both antibiotics and bacteria migrate between aqueous and sediment phases, sediment grab samples were taken around the point of effluent discharge from a local WWTP which uses activated sludge treatment and chlorination/dechlorination. Three sample collection points were located approximately 600 meters upstream of the effluent outfall, close to the point of effluent discharge, and 965 meters downstream of the outfall. Water column samples taken contemporaneously with sediment at the up- and downstream sampling points revealed antibiotic residuals in the latter but not the former site. Sediment samples were processed for bacterial isolation. An algorithm of several standrad biochemical tests was employed to identify 50 Aeromonas isolates from each sampling site; these isolates were subjected to resistance testing to four commonly used antibiotics detected in the WWTP effluent, namely ciprofloxacin, tetracycline, trimethoprim, and sulfamethoxazole. Changes in the susceptibility to the synergistic combination trimethoprim/sulfamethoxazole were also assessed. The results of antibiotic resistance tests indicate that the Aeromonas bacteria found at points close to the effluent and downstream of the outfall exhibit an increase in both absolute and intermediate resistance relative to the same genus found upstream at the unimpacted sampling point. Statistical analyses revealed these differences to be significant, and since no other identifiable environmental impact exists at this site, the wastewater treatment plant effluent was identified as the source of this resistance increase. This finding highlights the need for both the implementation of conservative antibiotic prescription practices and disposal that reduce the levels of antibiotics reaching WWTPs and the consideration of alternate treatment processes that remove the last vestiges of these compounds before the effluent enters receiving streams.Master of Science in Environmental Engineerin
