Antimicrobial Susceptibility Profile of Bacteria Isolated from Raw Sewage at a Wastewater Treatment Plant in Rio de Janeiro

Background and Aim: Given the concentration of microorganisms and emerging micropollutants, wastewater is a favorable environment for the exchange of resistance genes and the selection of antimicrobial-resistant strains, one of the most significant global threats of the 21st century, since inadequate environmental sanitation causes around 88% of deaths worldwide. The present study aimed to identify bacteria isolated from a sewage treatment plant located in Rio de Janeiro and determine their susceptibility profile to antimicrobials.

Methods: A 10 µL aliquot of raw sewage was inoculated onto chromogenic agar and presumptive identification of microorganisms were based on their biochemical profiles. Isolates that could not be characterized using traditional biochemical tests were analyzed using polymerase chain reaction (PCR). Following the Brazilian Committee on Antimicrobial Susceptibility Testing, the resistance profile was determined by measuring the diameter of the inhibition zone and the isolates were classified as susceptible (S), resistant (R), multidrug resistant (MDR), extensively resistant (XDR) or pan-resistant (PDR).

Results: In this study, 38 strains were isolated, of which 24% were identified as Gram positive (n = 9) and 76% as Gram negative (n = 29). Among the isolated strains, Enterobacter sp. (7, 18%) was the most frequent genera and Escherichia coli (6, 15%) and Klebsiella pneumoniae (5, 13%) were the most frequent species. No pan-resistant strains were identified, however, isolates of Klebsiella pneumoniae (60%, n = 3/5), Enterobacter sp. (57%, n = 4/7) and Escherichia coli (50%, n = 3/6) presented the profile MDR and only one Enterobacter isolate was considered XDR (14%). Isolates of Morganella morgani, Proteus mirabilis, and Acinetobacter baumannii were fully sensitive to the antimicrobials tested. Regarding the antimicrobial susceptibility of Gram-positive strains, the Bacillus sp., Enterococcus sp. and Staphylococcus aureus isolates showed different resistance profiles, in which only Bacillus sp. and Microbacterium paraoxydans were considered high risk to health. Conclusion: The occurrence of potentially pathogenic and antimicrobial-resistant microorganisms in sewage treatment plants is a reality that has been explored over the years. Resistance to beta-lactam drugs is increasingly present in clinical practice, and these were the antimicrobials with the highest number of resistant isolates in the present study, including Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa and Enterobacter sp., which are listed as WHO priority. Our results revealed that sewage treatment plants act as reservoirs of bacterial resistance, facilitating the spread of resistance genes between different bacterial species. Contact of these microorganisms with the community may cause public health problems and environmental impacts, increasing the need to implement and improve environmental surveillance of resistant pathogens, establishing stricter control over the use of antimicrobials, to try to mitigate the advance of bacterial resistance.