Drinking water distribution systems (DWDSs) may be contaminated to various degrees when different microorganisms attach to the pipe walls. Understanding the characteristics of biofilms on pipe walls can help prevent and control microbial contamination in DWDSs. The biofilm formation, interspecific interactions, and chlorine resistance of 10 dual-species biofilms in polyethylene (PE) and cast iron (CI) pipes were investigated in this paper. The biofilm biomass (heterotrophic bacterial plate count and crystal violet) of dual species in CI pipes is significantly higher than that in PE pipes, but the biofilm activity in CI pipes is significantly lower than that in PE pipes. The interspecific interaction of Sphingomonas-containing group presented synergistic or neutral relationship in PE pipes, whereas the interspecific interaction of the Acidovorax-containing group showed a competitive relationship in CI pipes. Although interspecific relationships may help bacteria resist chlorine, the chlorine resistance was more reliant on dual-species groups and pipe materials. In CI pipes, the Microbacterium containing biofilm groups showed better chlorine resistance, whereas in PE pipes, most biofilm groups with Bacillus exhibited better chlorine resistance. The biofilm groups with more extracellular polymeric substance (EPS) secretion showed stronger chlorine resistance. The biofilm in the PE pipe is mainly protected by EPS, while both EPS and corrosion products shield the biofilms within CI pipe. These results supported that dual-species biofilms are affected by pipe materials and interspecific interactions and provided some ideas for microbial control in two typical pipe materials.

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.