By Yuchen Lin
Water is an important source of life, and high-quality drinking water is critical for the health of human beings. Poor quality sources of water or poorly treated transport systems for water may result in high potential of microorganisms accumulating on the surfaces of water pipes. Such collections of bacteria patches on the inner surfaces of pipes form biofilms, which are responsible for the microbial contamination of water. The development of biofilms leads to the propagation of mixed microbial populations inside water distribution networks and acts as the major source of bacteria present in the drinking water. Biofilm formation results in health risks associated with the emergence of low-quality drinking water, which presents an issue in both low-income countries and high-income countries with advanced water-treatment facilities.
Biofilms found in water pipes constitute complex multispecies bacterial communities. Within the heterotrophic bacteria in the biofilms formed in drinking water systems, pathogenic bacteria, especially the opportunistic pathogens, including Enteropathogenic Escherichia coli, Pseudomonas aeruginosa, Stenotrophomonas maltophila, and Aeromonas and Legionella species, are the most frequently identified.1 Apart from filamentous and appendaged bacteria, other microorganisms, including fungi, viruses, and parasites, can also be found in these biofilms. Viruses and parasites do not grow in biofilm, but they attach as contamination.2
Biofilms vastly improve the survival and growth conditions for the microorganisms in environmental niches, and the surrounding extracellular polymeric substances protect pathogenic bacteria from disinfection, providing them with tolerance to antimicrobial agents.3 Hence, biofilm formation is now a huge threat to public health. It is responsible for the loss of disinfectant potential, increased bacterial levels, reduced oxygen levels, and changes in the taste and odour in the drinking water distribution systems. Bacteria in the biofilms also contribute to the formation of disinfection by-products toxic to humans.4 The growth of biofilm depends on a variety of environmental conditions in the water distribution system, such as the presence of biogenic compounds, temperature, hydrodynamics, hydraulic conditions, biocides, nutrient levels, stagnation periods, water velocity, disinfectants, and the material of the pipe.1,5
In the aqueous environment, microorganisms can adhere to solid surfaces to form biofilms, which involves several stages. The life cycle starts from the initial bacterial attachment that is reversible. Free-floating bacteria move in the liquid environment using their flagellum, and they approach and attach to the inner surfaces of the water pipes. Flagella is the first bacterial part that touches the surface where bacteria decide to commit or not, depending on whether the environment is inappropriate for survival. Under optimal conditions, the second step of irreversible attachment occurs, and bacteria start the colonisation. They move on the inner surface and perform clonal multiplication. Bacterial fimbriae, which cover the whole bacterial surface as long appendages, are required for such attachment and the interactions between bacteria forming the biofilm. One appendage, the type IV pili, also plays an essential role in the attachment. This extracellular appendage is not localised all around the bacterial cell but on the side where it promotes bacterial motility on the solid surface, stimulating the microcolony development, which is the third step of biofilm formation. After the firm attachment, the biofilm life cycle enters early maturation, during which microcolonies begin to form and engulf more bacterial cells. Type IV pili retract and extend to pull bacteria forward, shaping the overall structure of the biofilm and allowing bacteria to climb on each other.
In the maturation step, different parts of the biofilm with different bacterial densities connected by aqueous channels are formed. Bacteria embedded within such extracellular polysaccharide matrix-encased microbial communities are protected from being killed by antibiotics, detergents, chemical biocides, disinfectants, or environmental stress such as UV irradiation, pH fluctuation, osmotic shock or shearing force.1 Their type IV pili spread antibiotic resistance genes within the community rapidly via horizontal gene transfer. In the final disperse stage, some portions of the biofilm may break open, releasing some component bacteria to go to colonise other places of the water pipe, initiating the whole life cycle again elsewhere. Such bacterial dispersal increases the risk of spreading bacteria into the consumed water.
Water and water systems harbour a diverse range of microorganisms. When the nutrients for bacterial growth, such as carbon and phosphorus, are transferred to water and accessible for bacteria, biofilm formation is sped up. Switching water distribution network materials from cast-iron and galvanized steel to plastics and polyester resins has minimized secondary water pollution like the red or black water problems due to iron or sulphate-reducing bacteria. There has been an increase in the usage of plastic pipes in drinking water distribution systems and household installations in recent years. However, despite the corrosion resistance, the plastic surfaces still have biofilm formation, with Legionella pneumophila being the most common species. Previous studies identified a range of chemical compounds released by plastic pipes that support the first step of biofilm development or provide carbon sources for it.6 Different materials used in constructing water pipes alter the conditions for microorganisms to adhere and duplicate as well as the diversity and richness of the microbial communities within the biofilm, but the intensity of biofilm formation can be the same regardless of the type of pipe material.
Until now, none of the materials is shown to be capable of eliminating biofilm formation in the water distribution networks. Fortunately, researchers have found that the formation process is heavily influenced by water temperature and the flow conditions inside water pipes. Therefore, physical control approaches, such as the high velocities of water flushing through the pipes, can remove the mature biofilms, highlighting the need for frequent cleaning and maintenance for water pipe interiors. Complete removal of biofilms is practically impossible to achieve or maintain, but booster disinfection can help keep the level of disinfectants that reduce nutrient availability throughout water distribution systems, decreasing the expansion of biofilms.7 Organic substances and biogenic elements, including phosphorous and nitrogen, must be eliminated as the low levels form the biologically stable water that can effectively decrease biofilm growth.5
In conclusion, reducing biofilms in water distribution systems is a significant yet challenging task that is particularly critical in hospital settings because patients can be more vulnerable to the microorganisms present in drinking water. Further research is needed to ensure an extremely low level of nutrients for biofilm growth or expansion on interior surfaces of water pipes. To provide a safer drinking water system, it is necessary to constantly monitor the occurrence of opportunistic bacteria in water and whether the conditions in pipes favour their survival and biofilm formation.
- Rozej, A. et al., (2014) Structure and microbial diversity of biofilms on different pipe materials of a model drinking water distribution systems. World J Microbiol Biotechnol. 31, 37–47. Doi: 10.1007/s11274-014-1761-6
- LeChevallier, M.W. (1999) “10 Biofilms in Drinking Water Distribution Systems: Significance and Control.” National Research Council. Identifying Future Drinking Water Contaminants. Washington, DC: The National Academies Press. Doi: 10.17226/9595.
- Moore, G., et al., (2015) Biofilm formation in an experimental water distribution system: the contamination of non-touch sensor taps and the implication for healthcare. Bioadhesion and Biofilm Research. 31(9-10): 677-687. Doi: 10.1080/08927014.2015.1089986
- Abokifa, A.A. et al., (2019) Investigating the Role of Biofilms in Trihalomethane Formation in Water Distirbution Systems with a Multicomponent Model. Water Res. 104: 208–219. Doi: 10.1016/j.watres.2016.08.006
- Papciak, D. et al., (2019) The Impact of the Quality of Tap Water and the Properties of Installation Materials on the Formation of Biofilms. Water. 11(9), 1903; Doi: 10.3390/w11091903
- Mulamattathil, S.G. (2014) Biofilm formation in surface and drinking water distribution systems in Mafikeng, South Africa. S. Afr. j. sci. vol.110 n.11-12. Doi: 10.1590/sajs.2014/20130306
- Rose, J. B. (2016) Biofilms and Drinking Water Quality. Water quality and health. https://waterandhealth.org/safe-drinking-water/drinking-water/biofilms-drinking-water-quality/ [Accessed: 28th Feb 2022]