Isolation and genotypic characterization of extended-spectrum beta-lactamase-producing Escherichia coli O157:H7 and Aeromonas hydrophila from selected freshwater sources in Southwest Nigeria

The proliferation of antibiotic-resistant bacteria and antimicrobial resistance is a pressing public health challenge because of their possible transfer to humans via contact with polluted water sources. In this study, three freshwater resources were assessed for important physicochemical characteristics as well as heterotrophic and coliform bacteria and as potential reservoirs for extended-spectrum beta-lactamase (ESBL) strains. The physicochemical characteristics ranged from 7.0 to 8.3; 25 to 30 °C, 0.4 to 93 mg/L, 0.53 to 8.80 mg/L and 53 to 240 mg/L for pH, temperature, dissolved oxygen (DO), biological oxygen demand (BOD(5)) and total dissolved solids, respectively. The physicochemical characteristics mostly align with guidelines except for the DO and BOD(5) in some instances. Seventy-six (76) Aeromonas hydrophila and 65 Escherichia coli O157: H7 isolates were identified by preliminary biochemical analysis and PCR from the three sites. Among these, A. hydrophila displayed higher frequencies of antimicrobial resistance, with all 76 (100%) isolates completely resistant to cefuroxime and cefotaxime and with MARI ≥ 0.61. The test isolates showed more than 80% resistance against five of the ten test antimicrobials, with resistance against cefixime, a cephalosporin antibiotic being the highest at 95% (134/141). The frequency of the detection of the resistance genes in the A. hydrophila isolates generally ranged between 0% (bla(SHV)) and 26.3% (bla(CTX-M)), while the frequency of detection among the E. coli O157:H7 isolates ranged between 4.6% (bla(CTX-M)) and 58.4% (bla(TEM)). Our findings indicate that the distribution of antibiotic-resistant bacteria with diverse ESBL-producing capabilities and virulence genes in freshwater sources potentially threatens public health and the environment.