What is Heterotrophic (HPC) / Standard Plate Count?

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Published Date Written by Ute Rothkegel


Heterotrophs are broadly defined as microorganisms that require organic carbon for growth. They include bacteria, yeasts and moulds. Agar plates are used to recover a wide range of microorganisms from water which are collectively referred to as “heterotrophic plate count” or “HPC test” procedures as well as Standard Plate Count test.

Microorganisms recovered through HPC tests generally include those that are part of the natural (typically non-hazardous) microbiota of water, or, in some instances, they may also include organisms derived from diverse pollutant sources.

HPCHistory and use of HPC
By the end of the 19th century, with the development of bacteriology, culture media and the gelatin plate, it became possible to obtain what appeared to be quite accurate counts of germs by counting the number of colonies developing on these plates within a defined set of conditions:Incubation @37° C for 48 hours on suitable agar plates. The simplicity of the method was such that it  was rapidly put to use by the 19th-century sanitarians.  Air, water, soil, food, humans and animals were all studied to determine where and how germs lived, as they were apparently responsible for a wide variety of waterborne and foodborne diseases.

We are now in the early 21st century, and,  there is a striking resemblance between our so-called modern problems and the problems they had to resolve in the past. The questions they raised are the same ones that we are discussing now. In terms of water quality, it is quite fascinating to observe that the orders of magnitude of the numerical values used to define good quality water have remained the same.

Microbial growth in water

Microorganisms will normally grow in water and on surfaces in contact with water . Growth following drinking-water treatment is normally referred to as “regrowth.” Growth is typically reflected in higher HPC values measured in water samples. Elevated HPC levels occur especially in stagnant parts of piped distribution systems, in domestic plumbing, in bottled water and in plumbed-in devices, such as softeners, carbon filters and vending machines.
The principal determinants of regrowth are :

  • temperature
  • availability of nutrients ,and
  • lack of residual disinfectant.

Nutrients may derive from the water body and/or materials in contact with water.

Use of HPC in water management

HPC testing has a long history of use in water microbiology. At the end of the 19th century, HPC tests were employed as indicators of the proper functioning of processes (and of sand filtration in particular) and thereby as indirect indicators of water safety. Use as a safety indicator declined with the adoption of specific faecal indicator bacteria .

HPC measurements nevertheless continue to figure in water regulations or guidelines in many countries. HPC measurements are used:

  • to indicate the effectiveness of water treatment processes, thus as an indirect indication of pathogen removal;
  • as a measure of numbers of regrowth organisms that may or may not have sanitary significance.

This is just one of the tests done in our Environmental Division at UIS to determine whether water is safe or not.

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