Chloraminated drinking water does not generate bacterial resistance to antibiotics in Pseudomonas aeruginosa biofilms

To determine if exposure of Pseudomonas aeruginosa biofilms to chloraminated drinking water can lead to individual bacteria with resistance to antibiotics. Biofilms of P. aeruginosa PA14 were grown in drinking water in a Kadouri drip-fed reactor; the biofilms were treated with either 0·5 mg l⁻¹ or 1·0 mg l⁻¹ of chloramine for 15 or 21 days; control biofilms were grown in water without chloramine. Fewer isolates with antibiotic resistance were obtained from the chloramine-treated biofilms as compared to the control. Minimum inhibitory concentrations (MIC) for selected antibiotic-resistant isolates were determined using ciprofloxacin, tobramycin, gentamicin, rifampicin and chloramphenicol. All of the isolates tested had increased resistance over the wildtype to ciprofloxacin, rifampicin and chloramphenicol, but were not resistant to tobramycin or gentamicin. Under these test conditions, there was no detectable increase in antibiotic resistance in P. aeruginosa exposed as biofilms to disinfectant residues in chloraminated drinking water. Chloramine in drinking water, while unable to kill biofilm bacteria, does not increase the potential of P. aeruginosa to become resistant to antibiotics.