Heterotrophic plate counts

(endorsed 2011)

Guideline

No guideline value has been set for heterotrophic plate counts in drinking water. Immediately after disinfection, numbers would be expected to be low. If used as an indicator of distribution system cleanliness, numbers should be established on a system-specific basis. Marked increases in numbers after disinfection or within distribution systems should be investigated.

General description

Use of heterotrophic plate counts (HPC) has a long history. In the 19th century Robert Koch referred to use of total bacterial counts in assessing performance of sand filtration (Bartram et al. 2003).

Heterotrophs are broadly defined as microorganisms that require organic carbon for growth. They include bacteria and fungi. HPC refer to numbers of organisms grown on non-specific culture media without inhibitory or selective agents. Even though the test is non-selective, only a small proportion of the microorganisms present in water will be recovered. The types of organisms detected by HPC tests vary widely with location and time of year. Tests detect microorganisms that grow over a specified incubation period and at a defined temperature. Incubation periods can range from one day to weeks, and temperatures from 20°C to 40°C.

Microorganisms detected within HPC include:

• vegetative bacteria such as coliforms and other Enterobacteriacae that are sensitive to disinfection processes;

• fungi and bacteria such as Bacillus spp. that form disinfectant-resistant spores; and

• bacteria and fungi that grow in water.

Although HPC can include enteric pathogens and opportunistic pathogens such as Aeromonas spp. and Pseudomonas spp., the vast majority are non-pathogenic.

HPC are one of the simplest tests that can be performed in monitoring of water quality. Other names used for HPC include total plate counts or colony counts.

Source and occurrence

Heterotrophic microorganisms include the naturally occurring microbial flora of water and soil environments (typically non-hazardous) and organisms present in a range of pollution sources. They occur in large numbers in raw water sources. Some drinking-water treatment processes, such as coagulation and sedimentation, reduce the number of HPC organisms in water but growth can occur in other processes such as sand filtration. Numbers of HPC organisms are reduced significantly by disinfection processes; however, they can grow rapidly in drinking water once disinfection residuals have dissipated. Heterotrophs can also grow on surfaces in contact with water as biofilms. The principal determinants of growth or “regrowth” are temperature; availability of nutrients, including assimilable organic carbon; lack of disinfectant residual; and stagnation.

Method of identification and detection

No sophisticated laboratory facilities or highly trained staff are required. HPC are determined using aerobic incubation of non-selective nutrient agar.

Various nutrient media are available to determine HPC, including tryptone glucose yeast agar, R2A agar, and yeast extract agar (APHA et al. 2005, AS/NZS 4276.3.1 2007). It is standard practice to perform two different tests, one at 20-22°C for 3-5 days and a second at 35-37°C for 1-2 days.

Indicator value and application in practice

The use of HPC as an indicator of safety declined with the adoption of testing for faecal indicators such as E. coli. There is no evidence that HPC alone can be used as an index of pathogen presence or directly relate to health risk through normal uses of drinking water by the general population (Bartram et al. 2003).

They can, however, be a useful component of operational monitoring. HPC can be used in conjunction with measuring disinfectant residual or dose for operational monitoring of disinfection processes. In this case, the objective is to keep HPC numbers as low as possible. They can also be used to monitor the integrity, cleanliness and maintenance of distribution systems and the presence of biofilms. For this type of monitoring, absolute numbers are less important than changes in numbers and the objective is to keep HPC numbers within defined limits. Marked increases, measured in orders of magnitude, provide evidence of deteriorating conditions that should be investigated. The causes could include inadequate treatment, loss of disinfection residual, and stagnant water.

---

NOTE: Important general information is contained in PART II, Chapter 5

References

APHA, AWWA, WEF (American Public Health Association, American Water Works Association, Water Environment Federation (2005). Standard methods for the examination of water and wastewater, 21st edition. American Public Health Association, Washington DC.

AS/NZS 4276.3.1 (2007). Water Microbiology Method 3.1: Heterotrophic colony count methods – pour plate method using yeast extract agar. Standards Australia, Standards New Zealand.

Bartram J, Cotruvo J, Exner M, Fricker M, Glasmacher A (2003). Heterotrophic Plate Counts and Drinking-water Safety: the significance of HPCs for water quality and human health. WHO Emerging Issues in Water and Infectious Disease Series. IWA Publishing, London.