As a general rule of thumb, heterotrophic plate count (HPC) bacteria testing is beneficial in cooling tower and potable water applications, but are there any disadvantages?
Heterotrophs are broadly defined as microorganisms that require organic carbon for growth which includes bacteria, yeasts, and molds. These bacteria are universally present in all types of water, food, soil, vegetation, and air. Under this broad definition, primary and secondary bacterial pathogens are included, as are coliforms (Escherichia, Klebsiella, Enterobacter, Citrobacter, Serratia etc.). Over the years, a variety of simple culture-based tests that are intended to recover a wide range of microorganisms from water are collectively referred to as “heterotrophic plate count” or “HPC test” procedures.
Cooling Tower Application
Figure 1: Approximate colony forming units per milliliter (CFU/ml)
The advantage of conducting dipslide tests is the ease of use and incubation period (typically 48 hours). Dipslide is a test for the presence of microorganisms and is most frequently used to measure and observe microbial activity in liquid-based systems, often used in testing cooling systems. Dipslides aid in determining the presence of slime-forming bacteria in cooling & industrial water systems and validate the biocidal program efficiency. The test consists of a plastic carrier bearing a sterile culture medium that can be dipped into the liquid sample. Dipslides are then incubated to allow microbial growth to occur, and colonies are estimated by referencing a chart. When running Legionella tests, consider using one of the laboratories that include HPC, if field dipslides are not currently being run. The CTI (Cooling Tower Institute) has historically recommended a target value of <10,000 CFU/ml, although several published articles have disputed the correlation between HPC and Legionella.
However, the 10,000 CFU/ml threshold does aid to indicate aerobic activity and the presence of microbiological growth in the prevention of biofouling. HPC readings above 10,000 CFU/ml can increase the chances of microbiologically induced corrosion (MIC). With higher HPC counts also comes the risk of biofilms that reduce heat transfer and increase energy costs.
Potable Water Application
The EPA (Environmental Protection Agency) has a suggested limit of 500 CFU/ml in potable water. When 500 CFU/ml is exceeded in Health Care settings, the possibility of waterborne pathogens increases and can be the cause of nosocomial (hospital-acquired) infections. These include, but are not limited to, Legionella, Pseudomonas, Burkholderia, Acinetobacter, Stenotrophomonas, non-TB Mycobacterium, and Fungi. HPC testing is used primarily to validate the overall effectiveness of a water management program in accomplishing their objectives, typically to minimize exposure to Legionella or reduce biofilm accumulation. Bacteria testing can also provide a means of monitoring the performance of control measures. HPC values are also used in verification (and by some authorities also for validation) of the efficacy of cleaning in diverse applications including drink vending machines, food processing, and preparation facilities and medical devices.
Unfortunately, there is no universal “HPC measurement.” Although standardized methods have been formalized, HPC test methods involve a wide variety of test conditions that lead to a wide range of quantitative and qualitative results. Temperatures employed range from around 65°F to 105°F, incubation times from a few hours to seven days or a few weeks, and nutrient conditions from low to high. HPC tests do not specify the organisms that are detected either. Only a small proportion of the metabolically active microorganisms present in a water sample may grow and be detected under any given set of HPC test conditions, and the population recovered will differ significantly according to the method used. The actual organisms recovered in HPC testing can also vary widely between locations, between seasons, and between consecutive samples at a single location.
And, although most disease-causing (pathogenic) bacteria are heterotrophic, typical HPC tests will not detect pathogens because special media and test techniques are required for organism growth. “There are opportunistic pathogens that may regrow in water but that are not detected in HPC measurements, including strains of Legionella and nontuberculous mycobacteria. …There is no evidence that HPC levels per se, as measured by established procedures, have a direct relationship to… organisms such as legionellae, Pseudomonas aeruginosa, and non-tuberculous mycobacteria” (WHO 2003). Some studies have suggested that elevated levels of HPC bacteria may actually protect humans by out-competing harmful pathogens. Regarding cooling towers, HPC testing should not be used to validate Legionella control as studies have not proven any correlation.
Interestingly, HPC levels in foods haven’t raised the same concern as in the drinking water industry. It’s not unusual to find very high numbers (>50,000 CFU/gram or ml) of HPC bacteria in a wide variety of foods such as pasteurized milk, cheeses, meats, yogurts, and fresh produce. HPC bacteria consumed with food is generally several orders of magnitude larger than those consumed in water. A million HPC bacteria have been documented on a single gram of leaf lettuce and carrot.
Simply put, HPC testing is a tool. Most thorough cooling tower water treatment programs will recommend weekly dipslide testing as the results may determine nutrient loading and quantify the presence of bacteria. In potable water settings, HPC values are used for verification and validation purposes. Overall, speculation and background knowledge are necessities when evaluating HPC test results in either application.
List of Resources:
 Reynolds, K. A. (2002, July 15). HPC Bacteria in Drinking Water — Public Health Implications? Water Conditioning & Purification International Magazine. https://wcponline.com
 Bartram, J., Cotruvo, J., Exner, M., Fricker, C., & Glasmacher, A. (2004). Heterotrophic plate count measurement in drinking water safety management. International Journal of Food Microbiology, 92(3), 241–247. https://doi.org/10.1016/j.ijfoodmicro.2003.08.005
 Bartram, J. (2013). Heterotrophic Plate Counts and Drinking-water Safety: The Significance of HPCs for Water Quality and Human Health. Water Intelligence Online, 12. https://doi.org/10.2166/9781780405940
 Dignin, B. (2000, March 16). Bacteria Counts and Cooling Tower Water: Debunking the Myths. Water Online. https://www.wateronline.com/doc/bacteria-counts-and-cooling-tower-water-debun-0001
 Mara, D. (2003). Water Quality: Guidelines, Standards and Health. Public Health, 117(6), 459. https://doi.org/10.1016/s0033-3506(03)00178-1
Figure 2: HPC via spread plate method