Municipal wastewater treatment and antibiotic resistance - a case study

Antibiotics are used for more than 50 years in the treatment of bacterial infectious disease of humans and other animals. Some antibacterial agents have been used also for other purposes, namely growth promotion in husbandry, or infection prevention, for example in aquaculture and poultry. The generalized and massive use of antibiotics has been considered the major reason for the crescent antibiotic resistance observed in some bacteria. As has been widely recognized, bacteria can adapt easily to new and adverse conditions. High rates of genetic mutation allied with the occurrence of horizontal gene transfer are responsible for a rapid and extensive dissemination of antibiotic resistance. Antibiotic resistant bacteria are found worldwide in commensals and pathogens of humans, in food products, in animals and in water. After a brief review on these topics, this chapter reports a study made in a wastewater treatment plant with the objective of assessing the ecology of antibiotic resistant bacteria in this kind of habitat and, thus, the potential of treated wastewater to contribute to the dissemination of resistant organisms. The wastewater treatment plant selected for this study uses an activated sludge process and receives sewage drainage of a municipal area of more then 100 000 inhabitants and a population density above 1000/km2. Enterobacteria and enterococci, isolated from the raw influent and from the treated effluent, were recovered on selective culture media, identified to the genus or species levels and characterized for their antibiotic resistance phenotypes. Wastewater treatment led to reductions of enterococci and of faecal coliforms of about 10 times, however, the relative percentages of resistant bacteria entering the wastewater treatment were maintained or increased after treatment, as was observed for the fluroquinolone ciprofloxacin. These results indicate that a well functioning municipal wastewater treatment plant may discharge into thereceiving medium about 104-105 antibiotic resistant bacteria of enteric origin per litre of treated wastewater. The discharge of these organisms into the natural water streams may have an important impact on the overall increase of antibiotic resistance prevalence in the environment. The study of the factors that contribute to the survival and/or proliferation of these organisms during wastewater treatment and in the receiving environment will allow the implementation of preventive measures to minimize the dissemination of antibiotic resistant bacteria.