Drinking water biofilm growth after surface disinfection

International audienceDrinking water biofilms represent a reservoir of indigenous microorganisms and potential pathogens. Therefore, preserving the microbiological quality in drinking water systems requires the limitation of this biological deposit accumulation, particularly by disinfection.To document the colonization of drinking water biofilms developed on HDPE (high density polyethylene) after chlorination, we (1) assessed the drawbacks of disinfection procedure currently applied on drinking water networks; and (2) addressed the post-disinfection dynamic of biofilm formation (number of adherent bacteria and diversity of bacterial populations).Non-chlorinated drinking water biofilms classically contained around 107cells/cm2 among which less than 1% were cultivable and more than 90% had non-damaged membranes. Eubacteria accounted for more than 50% of the biofilm bacteria and were dominated by α-, β-, γ-proteobacteria. AFM imaging revealed that attached biomass was dispersed on surfaces and was organized as mainly “big” clusters (300 nm thick and 4 μm2 surface areas) which covered up to 10.8% of the surfaces.Application of chlorine (0.4 to 10 mgCl2/L) resulted in a leaching of deposits and a reduction of nearly 80% of the number of biofilm bacteria. Indeed weakening of deposit/biofilm structure by chlorine oxidation leads to their removing by hydrodynamic shear forces. The remaining attached biomass was altered by the oxidant since 77% of the bacteria showed permeabilized membranes. Biofilm bacterial communities were also modified according to the chlorine exposure dose. In particular, the γ-proteobacteria population (including most of the waterborne pathogens) increased when chlorine increased, and supported that these bacterial sub-populations were less sensitive to chlorine.After chlorination, re-colonization of surfaces was immediate (measurable 3h after the disinfection). We assessed that accumulation of bacterial cells (approximately 8x105 cells/cm²/day) was log-linear. The return to the initial state required 2 to 3 weeks. Within this biomass, the number of cultivable bacteria increased very rapidly (up to 10² CFU/cm²/h), and in some case exceeded the number measured before disinfection. Such patterns could be explained both by continuous deposition of bacteria from the bulk and by the recovery of activity (repair/growth) of a fraction of surviving attached bacteria just altered by chlorination. It is also noteworthy that the kinetics of bacterial accumulation on the chlorine disinfected surfaces was rather similar to those observed on new HDPE surfaces (that have not undergone colonization). This point is in contrast with the generally accepted idea that living-in-place polymers and microorganisms promote biofilm growth.The resilience of biofilm bacteria populations exposed to chlorine will also be discussed as well as its outcomes on drinking water disinfection procedures