Q
I understand that bacteria grow on carbon filters. What does it mean for health risk? What should be done about it?
A
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.
Q
What are heterotrophic bacteria, and should I care?
A
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
~37° 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.
Conclusions
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).
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