Assessment of the microbiological safety of drinking water in outdoor pipe materials: biofilm formation and chlorine resistance of typical bacteria†

Microbial biofilms that develop in pipe walls may pollute drinking water distribution systems (DWDSs). An understanding of biofilm formation ability and chlorine resistance of bacteria in DWDSs can help to reduce the possibility of microbial pollution. This study investigated the biofilm formation ability and chlorine resistance of five typical bacteria in two representative outdoor pipe materials (polyethylene (PE) and cast iron). In both PE and cast iron pipes, the biofilm biomass of bacteria with excellent adhesion characteristics, namely, Acidovorax, Bacillus, and Acinetobacter, was higher. Crystal violet staining and heterotrophic plate count assays showed that the bacterial biofilm biomass in the cast iron pipe was significantly higher than that in the PE pipe. Bacterial activity (adenosine triphosphate and tetrazolium salt assay) and extracellular polymeric substance (EPS) measurements revealed significantly lower bacterial activity and EPS contents in cast iron pipes when compared with those in PE pipes. In addition, the chlorine resistance of bacteria in different pipe materials also varied. Bacillus exhibited better chlorine resistance in both PE and cast iron pipes, whereas Acidovorax had better chlorine resistance in PE pipes but poor chlorine resistance in cast iron pipes. Corrosion products and EPS protected the bacteria from chlorination, but their protective effects were influenced by the bacterial components, leading to variation in chlorine resistance in bacteria. These results demonstrated that outdoor pipe materials have significant effects on biofilm formation and chlorine resistance, causing different threats to drinking water safety.