Heterotrophic bacteria are the broad family of organisms that live on organic carbon, and they are present in essentially all water that is not sterile. A heterotrophic plate count, usually shortened to HPC, counts the ones that will grow on a laboratory plate. It is an indicator of conditions in the system, not a test for anything that makes people ill.
What the count is and is not
HPC gives a general measure of bacterial population, so it tells you about the environment inside your plumbing: warmth, stagnation, biofilm and available organic carbon. It does not identify pathogens, and a raised count is not evidence that anyone will get sick. The organism that actually matters for safety is coliform, and specifically E. coli, because those indicate contamination from surface water or waste. If you only have budget for one microbiology test on a household supply, that is the one to run.
Where high counts come from in a house
Counts climb wherever water sits still and warm. Common sources are long dead legs of pipe to a rarely used bathroom, a water heater kept at a low temperature, a storage tank on a treatment system, and the media in a carbon filter, which is warm, wet and full of the organic carbon these bacteria feed on. That last one surprises people: a carbon cartridge left in past its interval is a very comfortable place for heterotrophs, which is one more reason to change filters on schedule.
What to do about a raised result
Start with what the sample was actually testing. A tap that had not run for days, or an outside spigot with a hose attached, tells you about the fixture rather than the supply. Flush stagnant lines, service or replace overdue filter cartridges, sanitize storage tanks, and resample properly from a cleaned, flushed cold tap. If coliform is also present on a well, that is the result to act on and shock chlorination is the usual response. Our well water testing guide sets out what to order and when.
Questions people ask about heterotrophic bacteria
Are heterotrophic bacteria harmful to drink?
The group as a whole is not considered a health-based measure, which is why it is used as an indicator of system conditions rather than as a safety pass or fail.
Why did my filtered water show more bacteria than the tap?
Carbon filters remove the chlorine that was suppressing growth and offer plenty of organic carbon, so counts commonly rise after filtration, especially on an overdue cartridge.
Should a household test for HPC at all?
It is most useful when investigating a treatment system or a stagnation problem. For routine safety on a well, coliform and E. coli testing is the priority.