Ampicillin- and Multidrug-Resistant Escherichia coli and Enterococcus spp. in Costa Rican Wastewater and Surface Water

Antibiotic-resistant bacteria (ARB) such as <i>Escherichia coli</i> and <i>Enterococcus</i> released into surface waters have strong potential to impact human health. We assessed the prevalence of antibiotic-resistant bacteria (ARB) and multidrug-resistant (MDR) bacteria in undisinfected wastewater effluent in a tropical estuary that receives the discharge from a major wastewater treatment plant (WWTP) in Costa Rica. <i>Methods:</i> We quantified culturable ampicillin-resistant (ampR) and (MDR) <i>E. coli</i> and <i>Enterococcus</i> in wastewater influent from hospital and residential sources, effluent, and estuarine receiving waters of the secondary-treated effluent of a WWTP. AmpR isolates confirmed to species or genus were tested for resistance against six additional antibiotic classes. <i>Results:</i> The proportion of ampR <i>E. coli</i> (18%) was significantly greater than that of ampR enterococci (4%) but neither differed among sites. AmpR <i>E. coli</i> concentrations were significantly different by site (estuary, 3.9 log₁₀ CFU/100 mL vs. untreated residential wastewater, 6.4 log₁₀ CFU/100 mL), but ampR enterococci concentrations were consistent among sites. MDR <i>E. coli</i> and <i>Enterococcus</i> were most prevalent in hospital wastewater (57% and 45% of ampR isolates, respectively), but were found at all sites. MDR <i>E. coli</i> and <i>Enterococcus</i> isolates resistant to five antibiotics were isolated from the estuary, and gentamicin-resistant <i>Enterococcus</i> were isolated only from effluent. <i>Conclusions:</i> Undisinfected effluent is a source of ARB and MDR opportunistic pathogens in the tropical estuary and has the potential to impact the health of beachgoers. Our findings highlight the importance of rigorous treatment of wastewater effluent, including disinfection, as a contribution to efforts to achieve effective stewardship of antibiotics.