A baseline assessment of Enterococcus in low-contamination surface waters and their antibiotic resistance

Enterococci are fecal indicator bacteria in water quality monitoring but they are also persistent environmental organisms with antibiotic resistance potential. This study provides a baseline assessment of enterococci, species diversity and antimicrobial resistance profiles in low- contamination surface waters using the membrane filtration technique and 16S rRNA sequencing. Overall enterococci concentrations were low (geometric mean: 4 CFU/100 mL), well below the recreational water quality criteria. Enterococci showed no significant correlation with Escherichia coli in coastal waters (r = 0.025) but a strong significant correlation in river waters (r = 0.646). Species diversity was high, with E. faecium (49%), E. gallinarum (33%), E. faecalis (13%), and E. avium (5%) comprising the largest proportion of isolates. Antibiotic resistance was generally low across nine antibiotics tested, with all isolates susceptible to teicoplanin and high-level gentamicin. While resistance was rare, it was more prevalent in isolates collected from higher population city river waters and during warmer months in coastal waters. These results demonstrate the consistent presence of diverse Enterococcus species in low-contamination surface waters and highlight their ability to persist in the environment and carry antibiotic resistance even at low bacterial concentrations.