Heterotrophic Plate Count
Questions and Answers
What is Heterotrophic Plate Count? Heterotrophic Plate Count (HPC) is a procedure for estimating the total number of live nonphotosynthetic bacteria (natural background flora) in water.
Does water contain HPC? Any place one finds water on earth one finds a natural microbial population. Natural, untreated water will always have a microbial component. As a natural element, water contains some level of heterotrophic bacteria. These background flora are found in many everyday food products including infant formula, Grade A milk and fresh produce. Indoor air and human saliva also contain levels of natural flora.
How are humans exposed to HPC microorganisms? We are exposed to billions of HPC microorganisms throughout our personal environment everyday. For example, tens of thousands of such bacteria are permitted and frequently found in Grade A milk, yogurt, pickles, wine, apple cider, fruits, and vegetables while such foods are still healthful to eat.
Are HPC microorganisms harmful? No, because most bacteria are not harmful. Secondly, HPC are naturally occurring. In general, HPC organisms serve to prevent the growth of opportunistic pathogens because they have the strength to help overcome harmful bacteria and pathogens that may pose a health threat. There has never been a documented illness associated with them. Neither the U.S. Environmental Protection Agency (EPA) nor the Food and Drug Administration (FDA) believe there is a health risk associated with background flora in drinking water, including bottled water, and neither agency has set a standard for the amount in drinking water.
Why hasn’t the federal government established a safe level for the amount of HPC microorganisms that can be in either food or water? Since HPC microorganisms are both naturally occurring and not harmful neither EPA nor FDA felt it was necessary to establish a maximum contaminant level (MCL) for HPC. There has never been an association of any particular HPC concentration with a health risk. According to Stuart E. Richardson, chief of California’s Department of Health Services, heterotrophic bacteria are widely regarded as harmless to human health. HPC is not known to have a correlation to the presence of pathogens in food. (WATER TECHNOLOGY, November 1998)
What is the federal government doing about HPC? The U.S. EPA conducted an extensive review of the health effects of HPC prior to promulgating new drinking water regulations in 1991. Furthermore, the U.S. FDA conducted a similar review in generating its new bottled water regulations. Based on science and public health significance, both the EPA and FDA have determined that it is not relevant to impose an HPC limitation requirement in either of their regulations.
Do both bottled water and tap water have HPC in them? Both tap water and bottled water have some HPC content regardless of treatment method or residual disinfection concentrations. No treatment process used in mass production of drinking water yields a sterile product. Even tap water with high residual chlorine levels has HPC present.
Is the HPC in bottled water harmful to human health? The HPC in bottled water which do multiply in the bottle do not possess appreciable virulence factors associated with human disease. It should be emphasized that the mere presence of an environmental microbe does not mean that it poses an infectious threat even though it may appear in a list of potential pathogens in a textbook. The correct conditions must exist or an infection will not occur. A very particular series of circumstances is necessary in order for a microbe to cause disease.
What does the FDA – the agency that regulates bottled water – say about HPC in bottled water? In 1993, the FDA examined the presence of HPC in bottled water and concluded:
“FDA still believes that, when bottled waters are free of microorganisms that are of public health significance (i.e., indicated by the absence of coliforms) and are bottled under sanitary conditions in compliance with the CGMP regulations (21 CFR, part 129), the presence of heterotrophic bacteria that are part of the natural flora in those bottled waters normally will not pose a health risk because these organisms do not colonize the digestive tract of humans.” (FR FDA 10/06/93 PROPOSED 58 FR 52042 – QUALITY STANDARDS FOR FOODS WITH NO IDENTITY STANDARDS; BOTTLED WATER – AGENCY: Food and Drug Administration, HHS. – ACTION: Proposed rule.
Does the amount of HPC in bottled water increase after the water is bottled? Heterotrophic plate count bacteria do increase in numbers in a sealed vessel after bottling. No drinking water is absolutely sterile.Once water enters the bottle, the small number of HPC present do what they do in nature – they utilize available nutrients and multiply. The dead HPC deteriorate returning available carbon and nitrogen, in a different form, to the water. Thus, individual species increase and decrease in numbers over a long period of time. Accordingly, the microbiological “landscape” of bottled water varies widely both in number of HPC and species present over time. Thus, sampling a particular bottle will only produce a snapshot of the HPC content at that one point time. Eventually, all the nutrients are utilized and the HPC counts become very low, or even undetectable.
Does the amount of HPC in the water entering the bottle impact the number of HPC that multiply once the bottle is sealed? The concentration of HPC reached in the bottle, and the cycles of multiplication and decrease, are independent of the initial concentrations of HPC entering the bottle. Even though the specific species may differ, a similar situation occurs in tap water. If tap water is allowed to stand, the chlorine dissipates and the HPC counts increase. However, it is important to understand that if the water entering the bottle is free of pathogens, none can develop in the sealed environment provided by the bottle.
Will drinking water with HPCs detrimentally effect someone with a suppressed immune system? There is no evidence that AIDS patients, or individuals with any other immuno-compromised condition, are any more susceptible to naturally occurring HPC than persons not infected with HIV.