ANTIBIOTIC-RESISTANT GRAM-NEGATIVE BACTERIA IN RURAL GROUNDWATER SUPPLIES

More than 250 coliform and noncoliform bacteria, isolated by standard membrane filtration methods from rural, untreated groundwater supplies, were examined for resistance to 16 antibiotics. All of the noncoliforms and 87% of the coliforms were resistant to at least one antibiotic, with resistance most commonly directed toward novobiocin, cephalothin, and ampicillin. The frequency of multiple antibiotic resistance (MAR) within each species was also determined. Approximately 60% of the coliforms were MAR, including 14, 64, and 94% of Escherichia coli, Citrobacter freundii, and Enterobacter cloacae isolates, respectively. In comparison, MAR was demonstrated by more than 95% of the noncoliforms and included isolates of Acinetobacter calcoaceticus, Aeromonas hydrophila, and Serratia marcescens. Ampicillin-resistant environmental isolates were assayed for ability to transfer resistance to ampicillin-sensitive strains of E. coli and Salmonella typhimurium. Environmental E. coli isolates were capable of in vitro transfer to both recipients in a nutrient-rich environment (trypticase soy broth) at frequencies ranging from 1.1 x 10(-5) to 1.0 x 10(-4). Isolates of C. freundii and Klebsiella pneumoniae exhibited resistance transfer to the E. coli recipient under similar conditions at reduced rates. None of the environmental isolates demonstrated transfer of ampicillin resistance when placed in an environment containing filter-sterilized well water. The presence of antibiotic resistant bacteria, and particularly MAR bacteria, in rural groundwater supplies used as drinking water sources may have important public health implications.