Rapid selection of antimicrobial-resistant bacteria in complex water systems by chlorine and pipe materials

Antimicrobial resistance is a major health issue induced by the overuse of antibiotics and disinfection reagents, e.g. chlorine. Resistant bacteria thus occur in water supply systems, and they transfer genes to other microbial populations, including pathogens. Treatment and inactivation of resistant bacteria are difficult in complex systems because the behaviour of resistant bacteria in such systems is poorly known, as most previous investigations are commonly performed in pure media. Therefore, we tested here the effect of 0.5 mg/mL chlorine and pipe materials made of polyvinyl chloride (PVC), copper and cement, on microbial populations in biofilms, during 5 days. Bacterial survival was monitored by viable counts, and resistant genes were analysed by quantitative polymerase chain reaction (qPCR). Results show that, in 56% of the cases, resistant bacteria became immediately enriched into biofilms due to chlorine exposure. Higher proportion of resistant bacteria were found in biofilms on PVC and copper pipes. Our findings imply that resistant microbial strains are very rapidly selected and that the pipe material has an influence on microbial selection.