Heterotrophic plate count (HPC) bacteria is a non-specific classification term for the growth of viable, naturally occurring bacteria in water. These organisms have simple nutrient requirements and are cultured using a non-selective, low-nutrient, solid gel matrix as a food source.

In the past, much controversy has surrounded HPC bacteria. Many questioned whether or not HPC bacteria caused disease, an alarming thought given that HPC bacteria are found everywhere in the environment including the air, food and drinking water. HPC levels in food are typically much higher than found in drinking water. Fruits, vegetables, meats, cheeses, yogurt and even pasteurized milk may host tens of thousands to millions of HPC bacteria that are consumed unnoticed.¹

Growth in POU/POE devices
The drinking water treatment and purification industry has been especially concerned about HPC bacteria and its potential risks. Some of the most widely used home water purification and treatment technologies—designed to remove a variety of taste, odor and health-related contaminants—produce tenfold or higher effluent HPC counts than detectable in the influent water.

HPC bacteria are able to persist and grow in and on point-of-use/point-of-entry (POU/POE) treatment device media, membranes, filters and other surfaces. Granulated activated carbon (GAC), commonly used in POU/POE treatment devices, is known to support growth of HPC bacteria. GAC removes chlorine disinfectant residuals, and other contaminants, from tap water. While this is desirable to improve drinking water taste and odor, the lack of disinfectant residual and presence of a growth surface provide a suitable environment for HPC bacteria to attach and grow, especially following periods of non-use and stagnation. It can also grow in water storage vessels, distribution pipes, pressure tanks and hot water heaters.

No adverse health effects
Reports of high levels of HPC bacteria in treated water fueled debate of their health significance. Many cases were found where large numbers of HPC bacteria were present, but with no correlation to increased disease.^2-6 In fact, a well-respected series of studies in Canada showed POU devices reduced gastrointestinal illness by up to 40 percent, despite increasing HPC counts.^4-6

Some of the confusion with regard to the health effects of HPC bacteria arises from the fact that a variety of HPC bacteria have been associated with infections in severely ill, hospitalized patients, primarily causing wound infections. To date, there are no documented cases of persons outside of the hospital environment becoming ill from HPC bacteria. More specifically, there has never been a documented case of waterborne disease associated with HPC bacteria. In fact, there’s an overwhelming body of evidence suggesting just the opposite. Several studies have shown that HPC bacteria can actually out-compete human pathogens and demonstrate a protective effect to consumers.^{7, 8}

Beneficial role
Based on previous evidence and recent awareness of HPC bacteria and their presence in water, researchers have focused intently on the role of HPCs and POU devices in pathogen inactivation. That is, what impact do HPC bacteria have with regard to pathogen inactivation in POU devices? Preliminary studies at the University of Arizona (UA) recently focused on the survival mechanics between HPC bacteria and human enteric pathogens when present together in POU devices.

In the study, commercially available POU carbon filter devices were placed on home faucets and used for three to six weeks, establishing a background culture of HPC bacteria within the systems. Salmonella typhimurium, E. coli, poliovirus and hepatitis A virus—all known human enteric pathogens—were added to sterile tap water, regular tap water and POU-treated tap water that was high in HPC organisms.

HPC bacteria in the POU-treated water were clearly antagonistic to the pathogenic bacteria used in this study, reducing their counts greater than tenfold in a single day and >10,000-fold in two days. During an oral presentation in December 2002, UA researchers stated that the bacteria in POU devices could be considered “a natural form of disinfectant or barrier to enteric bacterial pathogens.” A similar but less dramatic trend was seen with pathogenic viruses.

Other studies supporting HPC bacteria’s antagonistic effect on pathogens have been conducted.⁷ Three enteric bacterial pathogens—Yersinia enterocolitica, Salmonella typhimurium and enterotoxigenic E. coli—readily grew on sterile GAC; still, in the presence of water containing populations of HPC organisms, pathogen counts gradually decreased. The most dramatic results were seen when bacterial populations from river water were previously established on GAC and a mixture of HPC and pathogenic bacteria added to the media. Pathogens not only decreased at a more rapid rate but were prevented from initial attachment when compared to the sterile GAC filters. These studies suggest an antagonistic effect on pathogenic bacteria due to presence of HPC bacteria on the filters, possibly because pathogenic bacteria don’t grow well in the presence of high HPC bacteria.

Conclusion
The evidence is overwhelming that HPC bacteria don’t cause disease in humans via the waterborne route and don’t correlate with the occurrence of waterborne pathogens. Provided that the water adheres to the World Health Organization (WHO) guidelines for water quality, HPC increases resulting from growth in POU/POE devices—don’t indicate a health risk to the consumer, and may in fact contribute to the overall reduction of harmful human pathogens found in drinking water.

GAC filters alone aren’t designed to eliminate pathogenic microbes; however, an incidental effect of the biofilm within these filters appears to be a hostile environment to introduced pathogens. Additional studies are sure to follow evaluating real-time use conditions of GAC filters and antagonistic effects of HPC organisms on a wider variety of pathogenic organisms.

For more information on the scientific consensus of HPC bacteria and the lack of associative health effects, refer to the proceedings from the WHO/NSF International-sponsored symposium—”Does HPC bacteria re-growth in drinking and packaged waters represent a significant health effects concern?”— which was held in Geneva, Switzerland, April 22-24, 2002.⁹ Contributing to the symposium were more than 180 participants from academia, government, industry, public health organizations, and water supply and trade associations.

References
1. Wadhwa, S.G., et al., “Comparative microbial character of consumed food and drinking water,” Critical Review in Microbiology, 28: 249-279, 2002.
2. Calderon, R.G., “Bacteria colonizing POU, GAC filters and their relationship to human health,” Final report CR-813978-01-0, USEPA, Cincinnati, 1991.
3. Colford, J.M., et al., “Participant blinding and gastrointestinal illness in a randomized, controlled trial of an in-home drinking water intervention,” Emerging Infectious Diseases, 8: 29-36, 2002.
4. Payment, P., et al., “A randomized trial to evaluate the risk of gastrointestinal disease due to the consumption of drinking water meeting currently accepted microbiological standards,” American Journal of Public Health, 81: 703-708, 1991.
5. Payment, P., et al., “Gastrointestinal health effects associated with the consumption of drinking water produced by a POU domestic RO filtration units,” Applied and Environmental Microbiology, 57: 945-948, 1991.
6. Payment, P., et al., “A prospective epidemiological study of gastrointestinal health effects due to the consumption of drinking water,” International Journal of Environmental Health Research, 7: 5-31, 1997.
7. Camper, A.K., et al., “Growth and persistence of pathogens on GAC,” Applied Environmental Microbiology, 50: 1178-1382, 1985.
8. Gerba, C.P., NSF International/Water Quality Center Bi-Annual meeting, Tempe, Ariz., December 2002.
9. WHO/NSF, “HPC and Drinking-water Safety: The Significance of Heterotrophic Plate Counts for Water Quality and Human Health,” 2003: www.nsf.org/conference/hpc/hpc_proceedings.html

About the author
Dr. Kelly A. Reynolds is a research scientist at the University of Arizona with a focus on development of rapid methods for detecting human pathogenic viruses in drinking water. She holds a master of science degree in public health (MSPH) from the University of South Florida and doctorate in microbiology from the University of Arizona. Reynolds also has been a member of the WC&P Technical Review Committee since 1997.