Prevalence of antibiotic resistance genes in drinking water and biofilms: The correlation with the microbial community and opportunistic pathogens.

The emergence of antibiotic resistance genes (ARGs) and opportunistic pathogens (OPs) in drinking water system posed potential risks to human health. However, the occurrence of ARGs and OPs in drinking water biofilms is still at its infancy. In this study, we investigated the occurrence of ARGs and OPs in both water and biofilm samples from a drinking water system, and the correlation between ARGs and microbial communities was analyzed. The quantitative PCR results showed that the drinking water treatment process effectively decreased the absolute abundances of ARGs. However, the relative concentration of ARGs did not show a significant difference between raw water and treated water samples. Compared with bulk water and tap water samples, biofilms had higher relative abundances of ARGs. 16 S Illumina Miseq sequencing results showed that microbial communities of biofilms were distinguished with water samples. Meanwhile, qPCR results of OPs also showed that biofilms had higher relative abundances of OPs compared with water samples. Furthermore, the Spearman correlation analysis indicated that Dechloromonas, Desulfovibrio, Methylobacterium and Propionivibrio correlated well with the relative abundance of ARGs. The absolute concentrations of OPs and ARGs also showed a significant correlation. Results of this study suggest that biofilms could serve as the reservoirs for the spread of ARGs and the interaction between biofilms and bulk water requires further research.

Coupling changes of disinfectant and bacteria induced by the water stagnation and disinfection strategy.

This paper studied stagnation-induced changes of disinfectant and bacteria using an orthogonal test and kinetic analysis, and then proposed a disinfection strategy. Tap water from a drinking water distribution system and ultrafiltrated water were collected and disinfected with four disinfectants (concentrations were set 0.2-1 mg/L as Cl2. The study had several findings. First, disinfectants expanded lag phases and shortened generation times of the microbiome. Reduction in culturability, substrate responsiveness, respiratory activity, membrane potential and integrity subsequently occurred with increasing disinfection concentration. Second, the disinfectant decay rate decreased with initial disinfection concentration, and the effective disinfection phase (heterotrophic plate count (HPC) was less than 100 cfu/mL) was longer in water samples with lower organic matter. Moreover, the disinfection process was divided into an effective phase and an invalid phase (HPC>100 cfu/mL). Then a disinfection efficiency model was built and the regulation of disinfection by-products (DBPs) production was studied in chlorinated water samples, which provides a general method for other disinfectant studies. The average trihalomethanes (THMs) production during the effective phase (marked as THM/th) and THMs production during the invalid phase (marked as ΔTHM) were proposed to evaluate the DBPs production. The level of THM/th and ΔTHM were lower in ultrafiltrated water than those in tap water. THM/th were negatively correlated with initial chlorine concentration while ΔTHM were positively correlated with initial chlorine concentration. Finally, for the purpose of raising disinfection efficiency and decreasing DBPs, we propose periodic pulse disinfection.

Effect of hydraulic conditions on the prevalence of antibiotic resistance in water supply systems.

The incidence of antibiotic resistance genes (ARGs) in tap water leads to potential risks to human health and draws more and more attention from the public. However, ARGs harbored in drinking water remain largely unexplored. In this study, a simulated water supply system was designed to study the effects of different pipe flow rates on the transmission of antibiotic resistance in water supply systems. We observed that the biofilm in low flow rate pipeline (0.1 m/s, 0.3 m/s) had higher concentration of both antibiotic resistant bacteria (ARB) and ARGs, while high flow rate (0.5 m/s and 0.7 m/s) resulted in low relative abundance of ARB and high relative abundance of ARGs in biofilms. The results showed that the high flow rate led to an abundance in non-culturable bacteria and a scarcity of nutrients in the biofilm, giving rise to its antibiotic resistance. High-throughput sequencing pointed out that the high content of Caulobacteraceae and Paenibacillus were determined in biofilms of high flow rate pipelines. Similarity analysis of microbial community composition of inlet water (IW), biofilms and outlet water (OW) showed that the composition of microbial community in OW was more similar to that in biofilms than in IW. Genera of bacteria in biofilms and OW (Brevundimonas, Brevibacillus and Pseudomonas) which had relationship with sulⅠ, sulⅡ in biofilms (P < 0.05) had higher relative abundance than that in IW. Different flow rate conditions had an impact on the biomass, microbial community, ARB and ARGs composition of biofilms. Thus, the detachment of biofilms can increased the antibiotic resistance of the water.

Impact of disinfectant on bacterial antibiotic resistance transfer between biofilm and tap water in a simulated distribution network.

Bacterial antibiotic resistance (BAR) is profoundly important to human health, but the environmental reservoirs of resistance determinants are poorly understood. BAR of biofilm and tap water were analyzed by using a water distribution simulator where different doses of chlorine and chloramine were used in this study. The results revealed that the disinfectants (≥2 mg/L) suppressed antibiotic resistant bacteria (ARB) in tap water and biofilms, while disinfected water and biofilms had a high relative abundance of ARB. The difference of ARB concentration and ARB percentage between the samples obtained from a disinfected pipeline and a non-disinfected pipeline became smaller over time. Because the water supply system is a unidirectional process, it is unclear how planktonic bacteria in water transfer BAR over time, although biofilm is suspected to play a role in this process. Compared with the biofilm samples without disinfectant, the disinfected biofilm had lower ICC and HPC/ICC percentage, lower AOC and AOC/TOC percentage, indicating that the disinfectant inhibited the bacteria growth in biofilm, and the disinfected biofilm had high proportion of non-culturable bacteria and low biodegradability, which affected BAR in biofilms. High throughput sequencing showed that in biofilms, the relative abundance of genera (uncultured_f_Rhodocyclaceae, Brevundimonas, and Brevibacillus in chlorinated systems, and Brevundimonas, Brevibacillus in chloraminated systems) with multiple antibiotic resistance and high abundance (up to 78.5%), were positively associated with disinfectant concentration and ARB percentage. The major prevalent genera in biofilms were also detected in tap water, suggesting that biofilm growth or biofilm detachment caused by external environmental factors will allow the movement of biofilm clusters with higher ARB concentration and percentage into bulk water, thereby increasing the antibiotic resistance of bacteria in tap water.

Impact of biofilm formation and detachment on the transmission of bacterial antibiotic resistance in drinking water distribution systems.

There is growing awareness of the antibiotic-resistance crisis and its implications for public health among clinicians, researchers, politicians, and the public. We studied bacterial antibiotic resistance transition and the role of biofilms in a drinking water distribution system (DWDS). We tracked several different antibiotic resistant bacteria (ARB) with resistance to tetracycline, sulfamethoxazole, clindamycin, and norfloxacin for one year in a DWDS. The results indicated that the amount of ARB increased in tap water, presumably due to biofilm detachment. The effect of biofilm detachment on the transmission of antibiotic resistance from biofilms to tap water was explored by using a bacterial annular reactor. The percentage of ARB of inlet water, outlet water, and biofilms ranged from 0.26% to 9.85%, 1.08%-16.29%, and 0.52%-29.97%, respectively in a chlorinated system, and from 0.23% to 9.89%, 0.84%-16.84%, and 0.35%-17.77%, respectively, in a chloraminated system. The relative abundances of antibiotic resistance Acinetobacter, Sphingomonas, and Bradyrhizobium were higher in outlet water than in inlet water, as determined by high throughout sequencing. The amount of ARB percentage varied with the concentration of viable but non-culturable (VBNC) cells (r = 0.21, n = 160, P < 0.05) in biofilm, suggesting a higher antibiotic resistance mutation rate in VBNC cells. Our results suggest that biofilm detachment was promoted by disinfectant and affected the overall bacterial antibiotic resistance of microbes in tap water.

Effect of disinfectant residual on the interaction between bacterial growth and assimilable organic carbon in a drinking water distribution system.

Public health is threatened by deteriorated water quality due to bacterial regrowth and uncontrolled growth-related problems in drinking water distribution systems (DWDSs). To investigate the scope of this problem, a two-year field study was conducted in south China. The amount of assimilable organic carbon (AOC), total cell concentrations (TCC), and intact cell concentrations (ICC) of water samples were determined by flow cytometry. The results indicated that ICC was significantly correlated to AOC concentration when the chlorine concentration was less than 0.15 mg/L, and ICC was lower at chlorine concentrations greater than 0.15 mg/L, suggesting that free chlorine level had effect on AOC and ICC. To further analyze the effect of disinfectant on AOC and bacterial growth, we designed an orthogonal experiment with different dosages of two commonly used disinfectants, chlorine and chloramine. The results demonstrated that high concentrations of free chlorine (＞0.15 mg/L) and chloramine (＞0.4 mg/L) were associated with relatively low proportions of intact cells and cultivable bacteria. Compared with chlorine, chloramine tended to cause lower AOC level and intact cells, likely because the chlorinated disinfection byproducts (DBPs) were more easily absorbed by bacteria than the chloraminated DBPs. Based on the statistical analysis of 240 water samples, ICC was limited when AOC concentration was less than 135 µg/L, while temperature and the number of small-size particles showed positive effects on ICC (P＜0.05). We conclude that the use of chloramine and controlling particle numbers should be suitable strategies to limit bacterial regrowth.

Characterization of suspended bacteria from processing units in an advanced drinking water treatment plant of China.

For the drinking water treatment plant (DWTP), the organic pollutant removal was the primary focus, while the suspended bacterial was always neglected. In this study, the suspended bacteria from each processing unit in a DWTP employing an ozone-biological activated carbon process was mainly characterized by using heterotrophic plate counts (HPCs), a flow cytometer, and 454-pyrosequencing methods. The results showed that an adverse changing tendency of HPC and total cell counts was observed in the sand filtration tank (SFT), where the cultivability of suspended bacteria increased to 34%. However, the cultivability level of other units stayed below 3% except for ozone contact tank (OCT, 13.5%) and activated carbon filtration tank (ACFT, 34.39%). It meant that filtration processes promoted the increase in cultivability of suspended bacteria remarkably, which indicated biodegrading capability. In the unit of OCT, microbial diversity indexes declined drastically, and the dominant bacteria were affiliated to Proteobacteria phylum (99.9%) and Betaproteobacteria class (86.3%), which were also the dominant bacteria in the effluent of other units. Besides, the primary genus was Limnohabitans in the effluents of SFT (17.4%) as well as ACFT (25.6%), which was inferred to be the crucial contributors for the biodegradable function in the filtration units. Overall, this paper provided an overview of community composition of each processing units in a DWTP as well as reference for better developing microbial function for drinking water treatment in the future.

Exploring the biological stability situation of a full scale water distribution system in south China by three biological stability evaluation methods.

Bacterial regrowth especially opportunistic pathogens regrowth and contamination in drinking water distribution systems (DWDS) have become an emerging threat to public health in the whole world. To explore bacterial regrowth and biological stability, assimilable organic carbon (AOC), biodegradable dissolved organic carbon (BDOC) and bacterial regrowth potential (BRP) were evaluated in a full scale DWDS and bench tests in South China. A significant correlation between BRP and AOC in both water treatment processes (WTP) and DWDS was obtained. For BRP and BDOC, the correlation was more significant in WTP than in DWDS. Both AOC and BRP were significantly correlated with UV254, total organic carbon (TOC), and heterotrophic plate count (HPC) (p < 0.01), whereas BDOC was only significantly associated with UV254, temperature and chlorine residual (p < 0.01). Through a bench test, when chlorine was higher than 0.5 mg/L, the HPC level was low and AOC concentration almost unchanged. On contrary the HPC level increased quickly and declined slightly, with chlorine lower than 0.15 mg/L, which was in accordance with the large amount of biological stability data obtained from DWDS. Through another bench test, the HPC level was positively correlated to AOC concentration and when AOC was below 135 µg/L, the growth rate of HPC was low, which was verified by the analysis of biological stability data from DWDS.

Ultrasound-promoted extraction of cheap microbial flocculant from waste activated sludge.

Ultrasound was uniquely applied to promote the extraction of cheap microbial flocculant (MBF) from waste activated sludge (WAS) of municipal wastewater treatment plants (WWTPs). Various influencing factors, including ultrasonic conditions (frequency, power density and treatment time) and WAS features (pH, concentration and source), were systematically investigated. The propitious ultrasonic conditions for MBF preparation from WAS were 20 kHz, 2.1 to 2.7 kW/L and 1 to 3 min. Natural sludge pH (about 7) was preferable to the MBF preparation. The major components of the extracted MBF contained polysaccharides, proteins and nucleic acids. The yield of the extracted MBF increased with rising sludge concentration. The wide application potential of the developed method was testified by the successful MBF extraction from the WAS samples of four full-scale municipal WWTPs with different typical processes. The ultrasonic method applied to extract MBF from WAS would not only provide a new way for WAS resource reuse, but also markedly cut down the cost of MBF preparation.