5.2 Microorganisms in drinking water

Microorganisms that may be present in drinking water are grouped into the following five categories:

Bacteria: are single cell microorganisms. Bacteria harmful to human health such as Campylobacter and Salmonella are generally unable to replicate in raw and treated drinking water. Non-pathogenic bacteria may produce “endotoxins” that can trigger symptoms in susceptible people if ingested, inhaled or in contact with the skin at sufficient concentrations. The reference pathogen used for bacteria in these Guidelines is Campylobacter.

Viruses: are made up of a core of nucleic acid (RNA or DNA) surrounded by a protein coat, and in some cases a lipoprotein envelope, that helps the viruses to attach to and enter host cells so they can replicate. Although they cannot reproduce without the host cell, they can survive in the environment for extended periods of time. No single virus satisfies all requirements of a reference pathogen. As a result, the reference virus included in the Guidelines is derived from a combination of different virus characteristics (Regli et al. 1991) including occurrence data for adenovirus and dose-response data for norovirus. Further information on the reference virus is provided in Appendix A3.5.

Protozoa: are single cell microorganisms that may cause adverse health effects and can live as parasites in the guts of humans and other mammals. These may form persistent cysts or oocysts that are resistant to environmental stress and are unable to grow in the environment. They may also exist as free-living organisms that may cause serious disease in humans (e.g. Naegleria fowleri). The reference pathogen used for protozoa in these Guidelines is Cryptosporidium.

Helminths: are invertebrates that can be transmitted via water as microscopic eggs. While transmission by drinking water is plausible, other routes of infection are typically more common (WHO 2017). The major helminth (worm) parasites of humans listed by the WHO as being transmitted by water are Dracunculus (WHO 2011a) but they are not endemic in Australia. Due to the relatively large physical size of helminth eggs, the management of protozoan pathogens would also manage helminths. For this reason, protozoa are used as a surrogate reference pathogen for helminths. There is no specific consideration of helminths in drinking water in the Australian context.

Fungi: include organisms such as single-celled yeasts and multi-cellular filamentous fungi. Many fungal species can survive in low nutrient (oligotrophic) environments, through scavenging nutrients from the substrate which they colonise, or the air or water in which they live (heterotrophic). The occurrence and health implications of fungi in drinking water systems have been reviewed in the United Kingdom (De Toni and Reilly 2011). Relatively few studies have investigated the fungi found in treated drinking water and this topic remains poorly understood.

Many of the fungal species that have been isolated from treated drinking water include potentially pathogenic strains. However, generally the highest risk from fungi is through airborne transmission to immunocompromised individuals. While healthy individuals may suffer from superficial or localised fungal infections, there is little evidence that drinking water is a significant source of infection (De Toni and Reilly 2011). Chemicals produced by fungi (growing in water distribution and plumbing systems) may change the taste and odour of drinking water, but do not necessarily pose a health risk. Fungi are not considered a significant health risk in drinking water. There is no reference pathogen for fungi in drinking water.

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Pathogens can be shed from infected hosts via excreted faeces and vomit in very large numbers, often billions per day. Many are rapidly inactivated in water, but some can persist in water and soil for months or even years under favourable environmental conditions.

Based on their transmission pathways and health impacts, microorganisms of concern in drinking water can be divided into three groups:

Enteric pathogens: are microorganisms that cause infection in the gastrointestinal tract of humans and/or animal hosts and are excreted in large numbers in faeces and in vomit. These “faecal-oral” pathogens can persist in the environment and be transported to source waters that might be used for drinking water. If left unmanaged, these pathogens can potentially lead to new infections in any exposed population. The most common and widespread public health risk associated with drinking water is contamination by pathogens from human or animal faeces.

Management of enteric pathogens involves:

• protecting of source waters from faecal contamination

• reducing the burden on treatment systems with water storage and selective abstraction of source water to avoid taking lower quality water

• treating water to remove or inactivate any remaining pathogens

• monitoring and protecting the distribution system to prevent recontamination or regrowth.

Opportunistic pathogens: are microorganisms that occur naturally and may cause disease opportunistically in humans depending on the exposure scenario. The risk to health from these microorganisms is typically low at the concentrations likely to occur in natural environmental waters (with low nutrient concentrations). However, in distribution and plumbing networks where conditions are suitable, opportunistic pathogens (e.g. Aeromonas, Pseudomonas) can grow to very high concentrations that may cause harm, particularly to susceptible population groups. Favourable conditions such as biofilms in water supply systems can also lead to microbial communities that support the proliferation of pathogenic amoebas (e.g. Naegleria fowleri). Naegleria fowleri is a free-living amoeba that can enter the central nervous system via the nasal cavity and cause primary amoebic encephalitis (PAM). While extremely rare, PAM is almost always fatal.

Management of opportunistic pathogens involves:

• treating water to remove or inactivate pathogens

• managing water quality in the distribution and plumbing network

• preventing conditions that support microbial growth or allow recontamination to occur (e.g. minimising stagnation, maintaining effective disinfection residuals and low nutrient levels).

Cyanobacteria: are true bacteria, although they are sometimes termed “blue-green algae” because they resemble true algae. Some cyanobacteria can produce toxins (cyanotoxins) that are harmful to human health. The cell walls of most cyanobacteria contain polysaccharides that may cause skin irritation. Part V contains fact sheets on toxic cyanobacteria and their toxins.

Management of cyanobacteria involves:

• minimising nutrient inputs into source waters

• avoiding water from the surface layer of stratified water bodies

• promoting water movement (e.g. through mixing) in unstratified water bodies

• treating water to remove cyanobacterial cells and taste and odour compounds and inactivate cyanotoxins.

Other pathogens that primarily cause infections of the respiratory system, skin, eyes or other organ systems can be spread via drinking water. In practice the processes for managing enteric pathogen risks will also control these pathogens if residual disinfection is maintained. These pathogens are discussed further in Section 5.5 and in the corresponding fact sheets in Part V.