Microbial growth in drinking water distribution systems (DWDS) depends on a great number of factors, and its control represents a great challenge for management of these engineering systems. The present case study assessed the influence that a pair of factors—water chlorination and flow velocity—had on the biofilms formed in a model DWDS in 626 days. The culturable bacteria number and biomass of the biofilms developed under the flow velocities of 0.3 m/s, 0.5 m/s, 0.7 m/s and 1 m/s were determined during three consecutively applied regimes of water chlorination to 0.05 mg/l (in 380 days), 0.42 mg/l (in 46 days) and 0.14 mg/l free chlorine (in 200 days). The results demonstrated that biofilm formation was a prolonged process directly depended on flow velocity at drinking water chlorination to 0.05 mg/l. The increase in the water chlorination to 0.42 mg/l chlorine resulted in both the reduction in culturable bacteria number and biomass removal, but the bacteria killing and the biofilm removal were distinct processes. The biocide action of chlorine was faster and more effective than its biomass removal effect. The chlorine decreasing from 0.42 to 0.14 mg/l resulted in increasing the biofilm HPC densities, although the biomass removal process was still continuing. The study carried out contributes for better understanding the biofilm behavior in DWDS and demonstrates that biofilm formation could be managed within a DWDS through operational decisions on parameters that can be changed and controlled as flow velocity and chlorination to safeguard drinking water quality.

Waste water treatment plants (WWTP) are considered as a hotspot for the acquisition and dissemination of antimicrobial resistance (AMR). The present study aimed to assess the AMR rate of the waste water microbiome in a selected WWTP and the treatment efficiency. Culture-dependent and PCR methods were used in the AMR study of raw and treated waste water (TWW) microbiomes. The population proportion of heterotrophic plate count (HPC) bacteria resistant to five antibiotic classes was assessed, as well as the AMR phenotype of a total of 238 Enterobacteriaceae and 259 Enterococcus spp. strains. Waste water treatment increased tetracycline- and ciprofloxacin-resistant bacteria by 67% and 61%, as well as the incidence of Enterobacteriaceae resistant to ciprofloxacin, co-trimoxazole, and cephalosporins. Multiple resistance increased, and 8.8% of TWW isolates exhibited an ESBL-producing phenotype, most often encoded by blaTEM and blaCTX-M genes. The most common resistance among Enterococcus spp. was to erythromycin and tetracycline, and despite the increased AMR rate among TWW isolates, only the increase in tetracycline resistance and the decrease in high-level gentamicin resistance were significant. All parameters analysed demonstrated limited removal of resistant HPC or faecal indicator bacteria in the studied WWTP and a positive selective effect towards some of them, most often to ciprofloxacin.

Drinking water quality and safety are of great concern for public health. The aim of this study was to assess the prevalence of antimicrobial resistance (AMR) among the heterotrophic bacteria in drinking water provided by a Bulgarian drinking water supply system (DWSS). Culture-dependent methods and conventional PCR assays were used to study drinking water sampled from six locations on its way from the water source to the consumers' taps. The populations of bacteria resistant to nine antibiotics (ABs) from different classes were quantified and the occurrence of seven antibiotic resistance genes (ARG) was determined. The species composition of the bacterial community and the AMR phenotype of isolated bacteria were determined. The AMR level underwent changes within the DWSS network and the population's proportion of bacteria resistant towards the tested ABs differed depending on the sampling site. The increased level of resistance towards some ABs in drinking water emphasize the role of DWSS as a reservoir of antibiotic-resistant bacteria (ARB) and ARGs that could pose a potential human health risk. Being focused on a Bulgarian DWSS, our study will contribute to establishing health hazards associated with ARB and ARGs in drinking water.

Antimicrobial resistance (AMR) is one of the major threats to human health and is becoming an environmental challenge for water resources too. Our study’s aim was: to assess the AMR of heterotrophic bacteria in drinking water-associated biofilms against six clinically important antibiotics; to compare the prevalence of antibiotic resistant bacteria (ARB) in drinking water and in the associated biofilms; to estimate biofilm formation ability of selected isolates. Culture-dependent methods were used in the population-based study of the biofilms and in assessment of the single-species biofilm formation ability and the AMR phenotype of the isolated strains. The population proportion of the bacteria resistant to each tested antibiotic significantly differed in the biofilms formed in drinking water from different sampling points. In all biofilms, the abundance of tetracycline- and ampicillin-resistant bacteria was low, and of streptomycin-resistant bacteria was high. An increased proportion of the bacteria resistant to ciprofloxacin, chloramphenicol and streptomycin was detected in the biofilms compared to those found in the drinking water. The prevalence of ARB in the biofilms implies an impact on the drinking water quality and an assessment of the attached and the planktonic bacteria is needed to clarify the prevalence of AMR in the drinking water distribution system.

The biofilm formation potential of a drinking water supply system is related to the chemical, microbiological and hydrodynamic characteristics of water, and to the pipe materials in contact with water flow. The goals of this study were: to determine the biofilm dynamics in a model of four drinking water installations, to simulate daily household water consumption; to compare the biofilms developed on different polymer pipe materials and their influence on bacteriological water quality. The results demonstrated that bacterial density of biofilms depended on pipe material type and was influenced by water temperature. The biofilms on polyvinylchloride chlorinated and polyethylene materials had higher bacterial density than biofilms on polypropylene (PP) brands. The effect of the materials, and respectively the biofilms, on drinking water quality was stronger in the overnight stagnation periods, especially during the initial weeks of model operation, than in periods of water consumption. Heterotrophic plate count (HPC) in stagnant or in flowing waters and water temperature followed a similar curve pattern, demonstrating significant seasonal variations. In summer, the HPC values of stagnant waters were raised up to seven times higher than in winter and those of the outlet waters (during the consumption periods) were raised up to four times.