I understand that bacteria grow on carbon filters. What does it mean for health risk? What should be done about it?



Just about everyone is aware that many bacteria grow in water and on wet surfaces, especially when there is no disinfectant residual present. The slippery surface on vegetables stored for a while in the refrigerator is caused by growth of heterotrophic bacteria. Tap water and bottled water are not sterile, although they have very few culturable bacteria present after treatment, and few — if any — frank human pathogens are present.         However, bacteria are ubiquitous in the environment because they are so diverse and there are so many and they reproduce rapidly. Some of them are adaptable to almost any conditions. There is almost always a food supply available, even in very highly treated and distilled water. So, it is no surprise that we find them so often. The issues are whether they might adulterate a food product, including water, and whether they constitute an infection health risk from ingestion or other exposure.



What are heterotrophic bacteria, and should I care?



The types of bacteria that we will be discussing are generally called heterotrophs, meaning that they need an outside source of organic carbon food to reproduce and grow. Heterotrophic bacteria produce all of the fats and proteins that they need from their environment.

Some organisms like plants and algae called autotrophs can fix carbon dioxide and use energy from sunlight (phototrophs) or inorganic compounds (lithoautotrophs) to produce the organic proteins, fats and carbohydrates that they need.

There are numerous types of heterotrophic bacteria. Those that are measurable in water by heterotrophic plate count (HPC) methods, include strains of Pseudomonas aeruginosa, Acenetobacter, Klebsiella pneumoniae and many others, and many are called “opportunistic pathogens.” The good news is that there is no evidence of an association from consumption of any of those with gastrointestinal infection among the general population, but some can cause infections by other routes of exposure and among high risk individuals.

There are other opportunistic organisms like Legionella and non- tuberculosis mycobacteria that regrow in water that are not detected in HPC measurements. Inhalation is a risk factor for those types of organisms. However, HPC counts are not indicative of the presence of those regrowth organisms.


Drinking water health issues

Opportunistic pathogens are microorganisms that can cause disease under some fairly extreme conditions, i.e., very high dosages to a sensitive person. For example, risks of infection from oral ingestion ranged from about 7.3 per billion Aeromonas organisms, to about 9/100 Pseudomonas organisms. This compares to theoretical infectivities of some frank pathogens ranging from about one human virus exposure, to a few Cryptosporidium organisms. Several opportunistic pathogens are causative agents in some hospital-acquired nosocomial infections. Pseudomonas aruginosa can be a cause of some dermal and eye infections.      

Some can be harmful to people on antibiotics, with immunosuppression, or using invasive medical devices. Klebsiella was the cause of a recent outbreak in the National Institutes of Health hospital.


HPC analyses

The analysis methodology can count numerous colony-forming microorganisms including Acenetobacter, Actinomycetes, Micrococcus, Pseudomonas, Aeromonas, Enterobacter, Yersinia and many others. The HPC measurement was originally called Standard Plate Count, and there is some difficulty in comparing results between different samples because of the variability. HPC organisms are quantified by plate culture techniques. The methodology does not identify specific organisms, which requires separate isolation and typing of colonies. Plate culturing is usually conducted at 20-22° C or ° C. A 1 ml sample is frequently used although dilution of the sample is common if very high colony counts are expected. Incubation periods are at least two days and sometimes up to seven days.

Levels in the hundreds of thousands per ml have been detected in extreme examples. The colony counts underestimate the actual numbers of viable microorganisms that are present and capable of growing under some test conditions, partly because of the limitations of any culture medium that is being used, as well as temperature and time.

There are quicker tests utilizing ATP measurements indicating the bioactivity of viable organisms that are analogous to the HPC tests, and that can produce results in minutes. Most waters have much lower counts and would not be distinguishable. ATP testing is particularly useful in food applications where high counts are more expected and quick decisions are important.


Drinking water standards

The interpretation of the health significance of the HPC has changed over time. The lower temperature test is purported to give some indication of the amount of food available for bacterial nutrition, or perhaps growth potential approximating piped water conditions, and the higher temperature being human body temperature possibly reflecting bacteria that would be more likely to be of sanitary significance. However, even in Robert Koch’s original work in Germany in the late 1800s he made it clear that the plate counts in themselves did not necessarily enumerate pathogens, but they might correlate with pathogen presence or treatment effectiveness and the overall quality of the water.

A well operated public water system can usually achieve fewer than 10 HPC colonies per ml at the entry to distribution; however, levels in distribution systems can be significantly higher in dead ends and when the disinfectant residual is lost. Neither Environmental Protection Agency (EPA) nor the Food and Drug Administration specifically regulates HPC organisms in drinking water or tap water as harmful contaminants, and they rely on the traditional indicators of public health significance, coliforms and E. coli. In 2013, the EPA eliminated the total coliforms MCL and now uses them much like HPCs as indicators of treatment performance and system cleanliness.

Some countries have mandatory colony count requirements for drinking water. For example, Sweden, Netherlands and Germany had values of 100 colony counts/ml limits from incubations at 20° C. The U.S. has a benchmark value of 500 colonies/ml under the Surface Water Treatment rule as a substitute for maintaining a disinfectant residual. At one time 500 counts/ml was a prerequisite to the total coliforms measurement, because of their potential interference with enumerating coliforms when lactose media were used, but current media are primarily R2A, which is not lactose based.

Bottled waters are often treated with carbon to remove chlorine residuals and ozonated or UV treated before bottling. The ozone is rapidly decomposed so there will not be a disinfectant residual present and regrowth of HPC organisms will occur in the bottle on the shelf.


Water treatment: Carbon filters

Carbon filters and cartridges are an ideal growth medium for HPC bacteria. The carbon accumulates organic ‘food’ molecules, eliminates the disinfectant residual and provides a large surface area for bacterial colonization. Several countries have in the past considered possibly banning carbon filters due to the microbial growth; however, they all decided against regulating the filters. Most carbon cartridges are impregnated with silver, a bacteriostat, as a measure to reduce microbial growth on the filter, but not to control pathogens in the water. Some reduced counts would be expected, at least initially, but biofilm formation and other deposition would protect microorganisms from contacting the silver, so in the long run counts will increase. That is why in some waters it will be important to replace the cartridge regularly, but in others there has not been significant biofilm formation over many years of use.

There have been studies that indicate that the presence of HPC organisms on a carbon filter might be beneficial. When sterilized filters and HPC containing filters were inoculated with pathogens, the pathogens survived longer on the sterilized filters, indicating that the HPC organisms were able to dominate the environmental niche, perhaps through predation or preferential access to nutrients.



An expert advisory group to the World Health Organization reviewed the issue of health risks from HPC organisms in 2002 and concluded that “there is no evidence, either from epidemiological studies or from correlation with occurrence with waterborne pathogens, that HPC values alone directly relate to health risk. They are therefore unsuitable for public health target setting or as the sole justification for issuing “boil water” advisories. … tests for E.coli or other faecal-specific indicators are essential for determining whether a health risk exists.”

So, the bottom line is that HPC growth in water and on carbon filters is undesirable, but it is controllable, in water at least, by disinfection. Absent other indications are unlikely to be a health risk from ingestion. Excessive growth may cause some negative aesthetic taste and appearance, causing biofilms to slough off.


Resources for additional information:

• Standard Methods for the Examination of Water and Wastewater, APHA, AWWA, WEF.

• Heterotrophic Plate Counts and Drinking Water Safety, 2003, Bartram, Cotruvo, Exner, Fricker, Glassmacher, IWA Publishing.

• HPC, what is their significance in drinking water? Allen, Edberg, Reasoner, Int. J. Food Microbiology, 92 (2004).