Antimicrobial resistance in Escherichia coli isolated in inflow, effluent and sludge from municipal wastewater treatment plants

Abstract

Antimicrobial resistance is a phenomenon of increasing importance. Sewage treatment processes are a vehicle for dissemination of resistant bacteria in both aquatic and terrestrial environments. To assess the number of antimicrobial resistant E. coli present in the wastewater inflow, effluent and sludge from urban sewage treatment plants in Portugal, 42 samples of crude inflow, treated effluent and sludge were collected in 14 municipal sewage treatment plants in Portugal. A total of 940 E. coli strains were recovered and tested, using the diffusion agar method, regarding their sensitivity to 12 different antimicrobial drugs. Resistance to tetracycline, ampicillin, trimethoprime/sulfamethoxazole, streptomycin and ciprofloxacin was found in 42.2, 32.6, 23.3, 26.4 and 11.9% of the isolates, respectively. No correlation was found with regards to medical antimicrobial use in Portugal. The highest resistance rates were found in E. coli strains isolated from the district capitals sampled. Wastewater treatment resulted in E. coli decrease between less than one log to four logs; nevertheless, an average of 1.17 × 10⁶ CFU/100 ml were present in the outflow of the plants. This investigation highlights the necessity for constant monitoring of the final effluent and sludge from these treatment facilities.

Keywords:

• Antimicrobial resistance
• Escherichia coli
• Sludge
• Urban wastewater

Acknowledgements

We are grateful to Mário Gadanho, from Instituto de Ciência Aplicada e Tecnologia, Faculdade de Ciências da Universidade de Lisboa, for data analysis. This work was partially supported by a PhD grant from Fundação para a Ciência e a Tecnologia, Lisbon, Portugal (FCT, BD/9968/2002).