Transitory Change of Bacterial Community Structure in Hot Water Biofilm: Effects of Anti-Legionella Treatments

Hot water networks are known to harbor pathogenic bacteria like Legionella. The present work aims to study bacterial community structure surrounding Legionella in biofilm before (T0) and after (72 h) two heat shocks (HS1 and HS2) followed by chemical treatment (C2). Phylogenetic study shows the dominance of Proteobacteria phylum with three classes (Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria) except for 72h-HS2 where Bacteriodetes dominated the phyla (49%.) Three phylotypes dominate the total of sequences; Aquabacterium (22.8%), Novosphingobium (15%) and Burkholderiales (9.6%) of which none is detected in the biofilm after HS2 and the majority of sequences belong to new unclassified bacteria (60.7%). Aquabacterium and Novosphingobium are quickly detected again at T0 before C2. Sequences possibly close to opportunistic bacteria (11.7%; Legionella rubrilucens, L. anisa, Caulobacter, Leptospira, Burkholderiales, and Mycobacterium) are no more detectable after treatments indicating their disappearance or their minor presence compared to other bacteria. Principal component analysis shows a distinct bacterial community structure after HS2. This structure is back to the initial state after C2. The current findings underline the incomplete and transitory change of bacterial community structure in hot water biofilm by heat shock and chemical treatments. Therefore, Legionella eradication requires complete biofilm elimination.