Aeromonas species from non-chlorinated distribution systems and their competitive planktonic growth in drinking water

Impact of flushing procedures on drinking water biostability and invasion susceptibility in distribution systems

Ensuring high-quality drinking water remains challenging, as complaints about odors, discoloration, or contamination persist. In Belgium and beyond, traditional flushing is a common curative strategy that involves discharging large water volumes through hydrants while the network remains in use. In some cases, free chlorine (0.5 mg/L) is added, and consumers are advised not to drink the water. However, flushing can alter water biostability, potentially increasing susceptibility to microbial invasion. This study used a pilot-scale drinking water distribution system with three identical 100 m polyvinyl chloride(PVC) loops (DN 80 mm) to assess the impact of flushing with and without chlorination as practiced in chlorinated networks. Loop 1 was flushed with tap water and sodium hypochlorite (NaOCl), followed by two non-chlorinated flushes, loop 2 was unflushed, and loop 3 underwent three flushes. Biostability was assessed using online flow cytometry, and susceptibility to bacterial invasion (Aeromonas media, Pseudomonas putida, and Serratia fonticola) was evaluated in the days following flushing. The water had a 7-day residence time. Results showed that chlorinated flushing promoted microbial regrowth (3.8 × 105 vs 2.0 × 105 and 1.6 × 105 cells/mL for loops 1, 2, and 3, respectively), primarily of resident Sphingopyxis spp. Biofilm cell densities (~4 × 106 cells/cm2) remained stable across conditions. Bacterial indicators declined over time, with P. pudita and S. fonticola surviving longer (>100 hours) than A. media (13 hours). Decay rates were highest in chlorinated loops, likely due to increased microbial competition. For example, the decay constant of S. fonticola at 20°C was -0.082 h-1, -0.042 h-1, and -0.027 h-1 for loops 1, 2, and 3, respectively.

IMPORTANCE

Traditional flushing is used as a curative strategy to solve unwanted quality issues during distribution, yet its impact on microbial biostability remains poorly understood. This study provides critical insights into how traditional flushing, both with and without chlorination, influences microbial regrowth and susceptibility to invasion. Findings reveal that chlorinated flushing promotes the regrowth of resident drinking water bacteria while accelerating the decay of introduced unwanted bacterial indicators, emphasizing the complex trade-off between microbial control and system stability. Understanding these dynamics is essential for optimizing flushing procedures, minimizing unintended consequences, and improving distribution system resilience.

Exploring the metabolic potential of Aeromonas to utilise the carbohydrate polymer chitin

Members of the Aeromonas genus are commonly found in natural aquatic ecosystems. However, they are also frequently present in non-chlorinated drinking water distribution systems. High densities of these bacteria indicate favorable conditions for microbial regrowth, which is considered undesirable. Studies have indicated that the presence of Aeromonas is associated with loose deposits and the presence of invertebrates, specifically Asellus aquaticus. Therefore, a potential source of energy in these nutrient poor environments is chitin, the structural shell component in these invertebrates. In this study, we demonstrate the ability of two Aeromonas strains, commonly encountered in drinking water distribution systems, to effectively degrade and utilize chitin as a sole carbon and nitrogen source. We conducted a quantitative proteomics study on the cell biomass and secretome from pure strain cultures when switching the nutrient source from glucose to chitin, uncovering a diverse array of hydrolytic enzymes and metabolic pathways specifically dedicated to the utilization of chitin. Additionally, a genomic analysis of different Aeromonas species suggests the general ability of this genus to degrade and utilize a variety of carbohydrate biopolymers. This study indicates the relation between the utilization of chitin by Aeromonas and their association with invertebrates such as A. aquaticus in loose deposits in drinking water distribution systems.

Impact of operational conditions on drinking water biofilm dynamics and coliform invasion potential

Biofilms within drinking water distribution systems serve as a habitat for drinking water microorganisms. However, biofilms can negatively impact drinking water quality by causing water discoloration and deterioration and can be a reservoir for unwanted microorganisms. In this study, we investigated whether indicator organisms for drinking water quality, such as coliforms, can settle in mature drinking water biofilms. Therefore, a biofilm monitor consisting of glass rings was used to grow and sample drinking water biofilms. Two mature drinking water biofilms were characterized by flow cytometry, ATP measurements, confocal laser scanning microscopy, and 16S rRNA sequencing. Biofilms developed under treated chlorinated surface water supply exhibited lower cell densities in comparison with biofilms resulting from treated groundwater. Overall, the phenotypic as well as the genotypic characteristics were significantly different between both biofilms. In addition, the response of the biofilm microbiome and possible biofilm detachment after minor water quality changes were investigated. Limited changes in pH and free chlorine addition, to simulate operational changes that are relevant for practice, were evaluated. It was shown that both biofilms remained resilient. Finally, mature biofilms were prone to invasion of the coliform, Serratia fonticola. After spiking low concentrations (i.e., ±100 cells/100 mL) of the coliform to the corresponding bulk water samples, the coliforms were able to attach and get established within the mature biofilms. These outcomes emphasize the need for continued research on biofilm detachment and its implications for water contamination in distribution networks.

IMPORTANCE

The revelation that even low concentrations of coliforms can infiltrate into mature drinking water biofilms highlights a potential public health concern. Nowadays, the measurement of coliform bacteria is used as an indicator for fecal contamination and to control the effectiveness of disinfection processes and the cleanliness and integrity of distribution systems. In Flanders (Belgium), 533 out of 18,840 measurements exceeded the established norm for the coliform indicator parameter in 2021; however, the source of microbial contamination is mostly unknown. Here, we showed that mature biofilms, are susceptible to invasion of Serratia fonticola. These findings emphasize the importance of understanding and managing biofilms in drinking water distribution systems, not only for their potential to influence water quality, but also for their role in harboring and potentially disseminating pathogens. Further research into biofilm detachment, long-term responses to operational changes, and pathogen persistence within biofilms is crucial to inform strategies for safeguarding drinking water quality.

Opportunistic Pathogens in Drinking Water Distribution Systems-A Review

In contrast to "frank" pathogens, like Salmonella entrocolitica, Shigella dysenteriae, and Vibrio cholerae, that always have a probability of disease, "opportunistic" pathogens are organisms that cause an infectious disease in a host with a weakened immune system and rarely in a healthy host. Historically, drinking water treatment has focused on control of frank pathogens, particularly those from human or animal sources (like Giardia lamblia, Cryptosporidium parvum, or Hepatitis A virus), but in recent years outbreaks from drinking water have increasingly been due to opportunistic pathogens. Characteristics of opportunistic pathogens that make them problematic for water treatment include: (1) they are normally present in aquatic environments, (2) they grow in biofilms that protect the bacteria from disinfectants, and (3) under appropriate conditions in drinking water systems (e.g., warm water, stagnation, low disinfectant levels, etc.), these bacteria can amplify to levels that can pose a public health risk. The three most common opportunistic pathogens in drinking water systems are Legionella pneumophila, Mycobacterium avium, and Pseudomonas aeruginosa. This report focuses on these organisms to provide information on their public health risk, occurrence in drinking water systems, susceptibility to various disinfectants, and other operational practices (like flushing and cleaning of pipes and storage tanks). In addition, information is provided on a group of nine other opportunistic pathogens that are less commonly found in drinking water systems, including Aeromonas hydrophila, Klebsiella pneumoniae, Serratia marcescens, Burkholderia pseudomallei, Acinetobacter baumannii, Stenotrophomonas maltophilia, Arcobacter butzleri, and several free-living amoebae including Naegleria fowleri and species of Acanthamoeba. The public health risk for these microbes in drinking water is still unclear, but in most cases, efforts to manage Legionella, mycobacteria, and Pseudomonas risks will also be effective for these other opportunistic pathogens. The approach to managing opportunistic pathogens in drinking water supplies focuses on controlling the growth of these organisms. Many of these microbes are normal inhabitants in biofilms in water, so the attention is less on eliminating these organisms from entering the system and more on managing their occurrence and concentrations in the pipe network. With anticipated warming trends associated with climate change, the factors that drive the growth of opportunistic pathogens in drinking water systems will likely increase. It is important, therefore, to evaluate treatment barriers and management activities for control of opportunistic pathogen risks. Controls for primary treatment, particularly for turbidity management and disinfection, should be reviewed to ensure adequacy for opportunistic pathogen control. However, the major focus for the utility's opportunistic pathogen risk reduction plan is the management of biological activity and biofilms in the distribution system. Factors that influence the growth of microbes (primarily in biofilms) in the distribution system include, temperature, disinfectant type and concentration, nutrient levels (measured as AOC or BDOC), stagnation, flushing of pipes and cleaning of storage tank sediments, and corrosion control. Pressure management and distribution system integrity are also important to the microbial quality of water but are related more to the intrusion of contaminants into the distribution system rather than directly related to microbial growth. Summarizing the identified risk from drinking water, the availability and quality of disinfection data for treatment, and guidelines or standards for control showed that adequate information is best available for management of L. pneumophila. For L. pneumophila, the risk for this organism has been clearly established from drinking water, cases have increased worldwide, and it is one of the most identified causes of drinking water outbreaks. Water management best practices (e.g., maintenance of a disinfectant residual throughout the distribution system, flushing and cleaning of sediments in pipelines and storage tanks, among others) have been shown to be effective for control of L. pneumophila in water supplies. In addition, there are well documented management guidelines available for the control of the organism in drinking water distribution systems. By comparison, management of risks for Mycobacteria from water are less clear than for L. pneumophila. Treatment of M. avium is difficult due to its resistance to disinfection, the tendency to form clumps, and attachment to surfaces in biofilms. Additionally, there are no guidelines for management of M. avium in drinking water, and one risk assessment study suggested a low risk of infection. The role of tap water in the transmission of the other opportunistic pathogens is less clear and, in many cases, actions to manage L. pneumophila (e.g., maintenance of a disinfectant residual, flushing, cleaning of storage tanks, etc.) will also be beneficial in helping to manage these organisms as well.

Taxonomic diversity and biomass of the invertebrate fauna of nine drinking water treatment plants and their non-chlorinated distribution systems

Invertebrates such as Asellus aquaticus, halacarid mites, copepods and cladocerans are known to regularly occur in drinking water distribution systems (DWDS). An eight-year study investigated the biomass and taxonomic composition of invertebrates in the finished water of nine Dutch drinking water treatment plants (using surface water, ground water or dune-infiltrated water) and their non-chlorinated distribution systems. The main aims of the study were to examine the source waters' influence on invertebrate biomass and composition in the distribution networks and to describe invertebrate ecology in relation to the habitat of filters and the DWDS. Invertebrate biomass of the finished drinking waters of the surface water treatment plants was significantly higher than in the finished waters of the other treatment plants. This difference was due to the higher nutrient levels of the source water. The main part of the biomass in the finished water of the treatment plants consisted of rotifers, harpacticoid copepods, copepod larvae, cladocerans and oligochaetes, which are small-sized, euryoecious and tolerate broad environmental conditions. Most of them reproduce asexually. Most species found in the DWDS are known to be detritivores, but all are benthic and euryoecious organisms, many of which have a cosmopolitan distribution. The euryoeciousness of these freshwater species was also shown by their occurrence in brackish waters and ground or hyporheic waters and the ability of many eurythermic species to overwinter in the DWDS habitat. These species are preadapted to the oligotrophic environment of the DWDS and can develop stable populations there. Most species can reproduce asexually and the sexually reproducing invertebrates (Asellus aquaticus, cyclopoids and probably also halacarids) have obviously overcome the potential problem of finding a mating partner. This study also showed a significant correlation of DOC in the drinking water with the invertebrate biomass. A. aquaticus was the dominant biomass component in six out of nine locations and was highly correlated with the Aeromonas counts in the DWDS. Thus, invertebrate monitoring in DWDS is an important additional parameter in understanding biological stability conditions in non-chlorinated DWDS.

Diurnal Variation of Epiphytic Microbiota: an Unignorable Factor Affecting the Anaerobic Fermentation Characteristics of Sorghum-Sudangrass Hybrid Silage

Forage epiphytic microbiota exhibits pronounced changes in composition and function throughout the day. However, the effects of these changes on silage fermentation are rarely explored. Here, we transplanted the epiphytic microbiota of sorghum-sudangrass hybrid (SSG) harvested at 7:00 h (AM), 12:00 h (M), and 17:00 h (PM) to sterilized SSG to evaluate the effects of diurnal variation of epiphytic microbiota on fermentation characteristics. During fermentation, remarkable differences in bacterial community successions were observed between silages inoculated with AM and M microbiota. Compared to AM microbiota, M microbiota inoculation increased the proportions of Pantoea dispersa, Leuconostoc lactis, Enterobacter, and Klebsiella variicola, whereas it decreased the proportions of Weissella cibaria and Lactobacillus plantarum during fermentation. This led to the most rapid pH declines and organic acid production in AM silage and the slowest in M silage. Both M and PM microbiota affected the bacterial cooccurrence patterns, indicated by decreased complexity and stability in the community structures of M and PM silages compared to that of AM silage. The predicted functions indicated that some key carbohydrate metabolism pathways related to lactic acid synthesis were downregulated, while some competing pathways (ascorbate and aldarate metabolism and C5-branched dibasic acid metabolism) were upregulated in M silage compared to AM silage after 3 days of fermentation. Correlation analysis revealed positive correlations between competing pathways and enterobacterial species. The current study highlights the importance of diurnal variation of epiphytic microbiota in affecting the silage bacterial community, potentially providing an effective strategy to improve silage quality by optimizing harvest time. IMPORTANCE Ensiling is a way to preserve wet biomass for animal and bioenergy production worldwide. The fermentation quality of silage is largely dependent on the epiphytic microbiota of the material. Plant epiphytic microbiota exhibit diurnal changes in composition and function. However, the effects of these changes on silage fermentation are rarely explored. The results presented here demonstrated that diurnal variation of epiphytic microbiota could affect the fermentation characteristics and bacterial community during SSG fermentation. Marked bacterial community differences were observed between AM and M silages during the initial 3 days of fermentation. The dominance rate of Lactobacillus plantarum was highest in AM silage, whereas enterobacterial species were more abundant in M silage. The predicted function revealed downregulated lactic acid synthesis pathways and upregulated competing pathways in M silage compared to those in AM silage. This study provides clues for technological-parameter optimization of the fermentation process by the selection of harvest time.

Complete Genome Sequence of an Aeromonas rivuli Strain Isolated from Ready-to-Eat Food

Aeromonads can be associated with diseases in animals and humans. Knowledge regarding Aeromonas rivuli, a species recently discovered in creek water in Germany, is still fragmentary. Here, we announce the complete genome sequence of Aeromonas rivuli strain 20-VB00005, which was recovered from ready-to-eat food.

Climate Change: Water Temperature and Invertebrate Propagation in Drinking-Water Distribution Systems, Effects, and Risk Assessment

This paper provides a summary of the knowledge of drinking-water temperature increases and present daily, seasonal, and yearly temperature data of drinking-water distribution systems (DWDS). The increasing water temperatures lead to challenges in DWDS management, and we must assume a promotion of invertebrates as pipe inhabitants. Macro-, meio-, and microinvertebrates were found in nearly all DWDS. Data in relation to diversity and abundance clearly point out a high probability of mass development, and invertebrate monitoring must be the focus of any DWDS management. The water temperature of DWDS is increasing due to climate change effects, and as a consequence, the growth and reproduction of invertebrates is increasing. The seasonal development of a chironomid (Paratanytarus grimmii) and longtime development of water lice (Asellus aquaticus) are given. Due to increased water temperatures, a third generation of water lice per year has been observed, which is one reason for the observed mass development. This leads to an impact on drinking-water quality and an increased health risk, as invertebrates can serve as a host or vehicle for potential harmful microbes. More research is needed especially on (i) water temperature monitoring in drinking-water distribution systems, (ii) invertebrate development, and (iii) health risks.

Water Lice and Other Macroinvertebrates in Drinking Water Pipes: Diversity, Abundance and Health Risk

Activities to ensure and maintain water quality in drinking water networks, including flushing, are presented after standardized hydrant sampling combined with a stainless-steel low pressure–high flow rate (NDHF) filter and a 100 µm mesh size was used to separate pipe inhabitants. A databank of more than 1000 hydrant samples in European lowland areas was developed and used to analyze the diversity and abundance of macroinvertebrates in drinking water networks. Load classes for water louse (Asellus aquaticus) and oligochaetes are given with three evaluation classes: normal colonization, increased colonization, and mass development. The response of Asellus aquaticus in drinking water networks to environmental conditions are presented as are their growth and reproduction, promotion of a third generation by climate change effects, food limitations, and the composition and stability of their feces. Finally, the health risks posed by dead water lice and water lice feces with bacterial regrowth and the promotion of microbe development on house filters are analyzed.