Heterotrophic Plate Count (HPC) bacteria are naturally occurring organisms commonly found in water systems. While their presence is not unusual, HPC bacteria are not regulated or routinely monitored as contaminants in drinking water due to their high variability and typically elevated concentrations.
HPC bacteria and human health
The presence of HPC bacteria in drinking water is a common occurrence. However, it is important to note that these bacteria do not pose direct hygienic (disease-related) significance. Extensive research conducted by experts in the field consistently fails to establish any correlation between HPC bacteria concentrations in drinking water and threats to human health. These findings provide strong evidence that consuming water with HPC bacteria does not jeopardize our well-being.
Nonetheless, the detection of HPC bacteria in a drinking water system can serve as an indicator of potential underlying issues. These issues may include inadequate treatment, poor maintenance of the distribution network, or elevated contamination of water bearing equipment. It is crucial to ensure the proper monitoring of water quality and take prompt corrective actions when necessary. While HPC bacteria themselves are not harmful to humans, changes in HPC concentrations may indicate changes in water quality. Therefore, it is essential to remain vigilant in evaluating complex microbial systems.
HPC bacteria and Legionella
Heterotrophic plate count bacteria and Legionella are bacteria that are commonly found in water sources. Legionella is a genus of bacteria that can cause Legionnaires’ disease, a severe form of pneumonia. HPC bacteria, on the other hand, are a group of bacteria that are commonly used as indicators of water quality. Though high levels of HPC in water systems can serve as an indicator of elevated microbial activity, it is not an indicator for Legionella itself. Monitoring and controlling the levels of HPC bacteria can help understand changes in water quality that potentially could impact the risk of Legionella contamination and subsequent health risks.
Role of HPC monitoring in water treatment
HPC monitoring plays a vital role in evaluating the effectiveness of municipal water treatment processes. Water providers strive to achieve HPC levels below 500 colony forming units per milliliter (cfu/mL) after treatment to minimize any potential interference with culture-based coliform testing, ensuring accurate and reliable results. With advancements in testing methods, these innovative approaches have successfully mitigated elevated HPC concentrations, thus eliminating any potential impact on coliform test outcomes.
Variability of HPC concentrations
HPC concentrations vary significantly across different water systems. Comparisons between various sources, such as New Jersey drinking water systems, dental water lines, hospital hot water, and Michigan lakes, highlight the diverse ranges in HPC presence.
| Water Source | HPC (cfu/mL) | Reference |
| NJ drinking water system | 320 to 1,000,000,000 | LeChevallier et al., 1987 |
| Michigan lakes | 3,000 to >100,000 | Jones et al., 1991 |
| Tuscon, AZ tap water | >3,000 | Pepper et al., 2004; Chaidez and Gerba, 2004 |
| Apartment building | 300–300,000 | Bagh et al., 2004 |
| Administration building hot water | 24,700–144,000 | Sheffer et al., 2005 |
| Hospital hot water | 60,000 | Sheffer et al., 2005 |
| Dental water lines | >3,000 | Rice et al., 2006 |
| Research building | 16,000 (average) | Siebel et al., 2008 |
| New university building | >10,000 | Nguyen et al., 2008; Nguyen et al., 2012 |
| Hospital hot water | 8,000–27,000 | Zhang et al., 2009 |
| Hospital hot water | 2,900 (average) | Duda et al., 2014 |
| New office building | 1–100,000 cfu/cm2 (biofilm) | Inkinen et al., 2014 |
| Commercial buildings | 3 – 2,100,000 | Pierre et al., 2019 |
Case Study: Managing HPC bacteria challenges
A case study from a California hospital in 2016 demonstrates the practical management of HPC bacteria. While preparing to open a new patient tower, the hospital found bacteria levels exceeding state and federal EPA limits in the building water system, necessitating intervention. By implementing copper-silver ionization to continuously disinfect the water system and sediment filtration to remove incoming sediment, the hospital successfully controlled bacterial levels and reduced corrosion damage to the plumbing system.
While HPC bacteria are common in water systems, they do not pose a significant health threat nor serve as reliable indicators of other waterborne pathogens. Routine HPC monitoring primarily serves to track changes in water quality and treatment efficacy, rather than for health risk assessments.
References
Allen 2004: Heterotrophic plate count bacteria—what is their significance in drinking water?
Pierre 2019: Water Quality as a Predictor of Legionella Positivity of Building Water Systems
