Total bacterial counts, heterotrophic plate counts, total heterotrophs etc etc . Well firstly…what do the words mean? Secondly what do they mean as test results in different water supplies? Hopefully we can shed some light on the subject!
Latin and greek dominates the language of terms microbiologists use. Sometime it really does seem like a different language – but there is an order (mostly). For example ‘hetero’ means mixed, ‘oligo’ means a few, and ‘auto’ means self. ‘Troph’ or ‘trophy’ means food or feeding. So what is a heterotroph?
Using the quick lesson in Latin and Greek above heterotrophs are ‘mixed feeders’. What this means is they are capable of using a range of food sources. So a heterotroph will happily eat different sugars, fats and other organic matter. The vaste majority of microorganism that we know are heterotrophs. Principally, this is be cause they are not fussy eaters which makes them easier to grow and identify in a laboratory.
Other bacteria are more fussy. An example is oligotrophs – which following the logic means ‘eat a few things’. Legionella fits into this category. Legionella use amino acids (the building blocks of proteins) as their main source of energy. Particularly an amino acid called cysteine seems to be important. Sugar or fat doesn’t interest them. This explains why they are less easy to grow and took the US Centers for Disease Control six months of continual work to grow and identify them back in 1976.
So you can see that Legionella and heterotrophs are fundamentally different purely because of what they eat. This explains why Legionella won’t grow on media (agar plates etc) designed for heterotrophs. This also shows that heterotrophs will grow where Legionella will not. So finding heterotrophs in a water sample may not be an indicator of the presence of Legionella.
Heterotrophic Bacterial Counts and the Built Environment
Heterotrophs are everywhere, literally everywhere. On your skin, in your gut, on your food and in your water. In the built environment they inhabit cooling towers, spa pools, and potable water systems – as well as a few other places. If there is a mixed diet available they will turn up for the feast. Of course the size of the feast affects how many will turn up. So the available nutrients is important. Unlike Legionella heterotrophs are not as reliant on biofilm to survive. Many will multiply whilst floating free in the water. If we break the different water systems into different parts we can understand how heterotrophic plate counts may or may not be helpful.
As part of their normal operation cooling tower suck air laden with nutrients into their water system. A small to medium size cooling tower can suck in as much as 700 grams an hour of nutrients from dust in the air while operating. This is washed into the re-circulating warm water. Of course this is heterotroph heaven and they flourish. They form biofilms, multiply in the water and basically clog up the system unless controls are in place. Of course if biofilms are present then protozoa and Legionella are going to join in! The Legionella find their amino acids as excretions from other bugs, and from parasitizing protozoans.
So cooling towers are disinfected to control the heterotrophs and to control Legionella. A normal part of cooling tower use is making huge volumes of aerosol. So the cleaner the aerosol the better. Cooling towers had disinfection long before Legionella came on the scene. This was to keep the bacteria down and stop the clogging. Legionella coming from cooling towers suddenly made the disinfection process much more important.
Traditionally heterotrophs are a measure of how well the disinfection is working. The hope is that if heterotrophs are under control then Legionella is too. Sadly this is not always the case. Studies show there is no link between numbers of Legionella and numbers of heterotrophs in a cooling tower. Probably this is because of two different diets and the fact that Legionella will not multiply floating free in the water. Take into account they don’t grow on the same media and it means heterotrophic bacterial counts don’t say much about Legionella. However they are a good measure of the disinfection.
Spa pools are confusing. While not in use they are clean and filtered and disinfected. Just sit in one and everything changes within seconds! But they are warm and do collect nutrients – principally from users! In this low(-ish) nutrient environment biofilms can flourish if not controlled. The best indicator of these establishing biofilms is heterotrophic bacterial counts. In this case the count is measuring the status of the biofilm. Since the disinfection kicks in when you get out – there is little multiplication for bacteria floating in the water. So it also relates to the presence of Legionella. Published evidence show a positive link between high heterotroph counts and Legionella in spa pools. This is a direct contrast with cooling towers. So water about drinking water?
Potable (drinking) Water
Potable water systems are low nutrient environments. This means the biofilm bacteria will be the dominant population. So will heterotrophs indicate the presence of Legionella? The simple answer is ‘no’. The World Health Organization states that:
There is no evidence, either from epidemiological studies or from correlation with occurrence of waterborne pathogens, that HPC values alone directly relate to health risk. They are therefore unsuitable for public health target setting or as sole justification for issuing “boil water” advisories. Source: WHO 2003
The new enHEalth guidelines (2015) suggest:
Measuring the concentration of total bacteria (usually expressed as total bacterial count – TBC, heterotrophic colony count – HCC, or heterotrophic plate count – HPC) in water samples at the point of entry of water to the facility and at several distal outlets throughout the facility may be useful to indicate problem areas of microbial regrowth occurring within the facility water distribution system. However, TBC is not an indicator of health risk and the results should be interpreted accordingly.
The Queensland Health Department put it this way:
It is not recommended to include heterotrophic colony count (HCC), or heterotrophic plate count (HPC) in a verification monitoring program as heterotrophic bacteria are not hazards in themselves. However, monitoring the changes in HCC between water entering a facility and reaching more distal points can be used to indicate parts of the water distribution system where stagnation may be occurring, leading to microbial growth. Source: Queensland Health Department
In support of the above opinions studies have shown no links between heterotrophs and Legionella in building water systems. Other studies have shown possible links at Legionella concentrations well below current standard test methods. In the end they are of little value in controlling Legionella. If you have other problems with your water quality bacterial counts might be useful but they don’t need to be part of your Water Safety and Risk Management Plan for a potable water supply.
Bargellini, A. et al (2011) Parameters predictive of Legionella contamination in hot water systems: Association with trace elements and heterotrophic plate counts. Water Research 4 5: 2315-2321
Bentham R. (1993) Environmental factors affecting the colonization of cooling towers by Legionella spp. in South Australia. International Biodeterioration & Biodegradation 31(1):55-65
enHealth Guidelines for Legionella control in the operation and maintenance of drinking water distribution systems in health and aged care facilities (2015).
Miller, R. Kobel, D. (2002) Prevalence of Legionella in Whirlpool Spas: Correlation with Total Bacterial Numbers. In: Legionella. ASM Press, Eds. Marre et al pp.275-7
Wiik, R. Krovel, A. (2014) Necessity and Effect of Combating Legionella pneumophila in Municipal Shower Systems. PLOS ONE | DOI:10.1371/journal.pone.0114331
World Health Organization (2003) Heterotrophic plate counts and drinking-water safety: The significance of HPCs for water quality and the human health