By Kelly A. Reynolds, MSPH, Ph.D.

While driving my three kids to school this morning, the oldest (at 10 years of age) asked, “If you have to get a germ, what one would be best to get?” Indecision set in as I pondered various disease-causing organisms, their relative morbidity and mortality rates, population vulnerabilities, economic loss potentials, etc.

As their eyes glazed over, I continued thinking about the difficulties in ranking the importance of microbial pathogens. The process is not as straightforward as one would imagine. While the identification of any waterborne disease agent promotes the immediate need for control, careful consideration of an organism’s characteristics is necessary to develop effective and practical solutions. Aeromonas hydrophila is a great example of the difficulties encountered in prioritizing actions against waterborne pathogens.

The bacterium is surrounded by a significant amount of uncertainty relative to health effects and regulatory needs. POU devices, interestingly, complicate the story.

A repeat offender
As required by the 1996 amendments to the Safe Drinking Water Act, the US EPA must make regulatory decisions on at least five contaminants (microbial and chemical) every five years. In 1998 and again in 2005, Aeromonas hydrophila was one of more than 50 contaminants listed on the first and second issue of the US EPA’s Contaminant Candidate List (CCL 1 and CCL 2).1,2

The CCL is a list of unregulated contaminants known or suspected to occur in water and may require federal regulation. A new, updated CCL is published approximately every five years.

Aeromonas’ repeated presence on the CCL led to the gathering of new and previous information to inform the US EPA about possible directions for understanding and controlling the bacterium in drinking water. Options included developing an enforceable regulation, developing suggested guidelines or to not regulate the organism.

The US EPA publication (2006), Aeromonas: Human Health Criteria Document provides an extensive review of the Aeromonas situation, including physical properties of the organism, transmission in humans and animals (fish, amphibians, reptiles, birds, domestic animals) and possible public health effects. A comprehensive description of what is known about the organism helps to identify key data gaps and inform best practices for appropriate controls.

Aeromonas is associated with human illness, including some severe infections. It is also readily found in just about any aquatic environment, including well water, treated drinking water and purified drinking water.

Why then is this human pathogen not even listed on the CCL 3 list released in February 2008?

Is it a pathogen?
The US EPA website ( lists three primary factors that are considered when determining whether or not to regulate a waterborne contaminant:

  • the extent of occurrence of the contaminant in drinking water
  • projected adverse health effects from the contaminant
  • whether regulation of the contaminant would present a ‘meaningful opportunity’ for reducing risks to health

Aeromonas bacteria are naturally present in all types of waters (ground, surface, marine, drinking and wastewater) around the globe. In drinking water systems, they are partially responsible for the development of biofilms that can alter the efficacy of treatment processes. They are also common contaminants of food (produce, seafood, deli meat, cheese, milk) and often present in feces and sewage even after treatment.

It is not uncommon, therefore, to be exposed to Aeromonas from a variety of sources. In fact, researchers believe that ingestion of Aeromonas is ‘continuous.’3

So, why aren’t more people getting sick? Concentrations in food are generally much higher than in drinking water and thus, control of exposures in water alone might not be meaningful given other environmental transmission routes.

Health effects in humans related to Aeromonas infections have been specifically identified since the late 60s in both healthy and immunocompromised populations. Diarrhea, blood infections, meningitis, eye infections, pneumonia, wound infections and other illnesses are well documented.

Chronic exposure to untreated water and contaminated foods are suspected factors in known cases of illness. Despite consistent levels of the bacteria in treated drinking water, however, no waterborne disease outbreak due to Aeromonas in treated drinking water, from any point source, has ever been reported

This is where the story gets complicated. In an attempt to quantitate the infective dose of the bacterium (i.e., how many are needed to make you sick) researchers conducted a controlled human feeding study. The study, unfortunately, failed to identify significant illness rates in the test population even when 100 million colony forming units (CFU)—orders of magnitude higher than typical water concentrations—were ingested.4

Such data inconsistencies complicated the understanding of Aeromonas. The development of new methods for isolation and characterization of the organism revealed genetic differences in the bacterial populations that coded for variable health effects.

These variations resulted in a previous misunderstanding of the importance of Aeromonas when isolated from a particular environment. New data showed that some species of the genus cause illness in animals and humans while others do not.

In fact, most strains found in the environment (including drinking water sources) are considered harmless for healthy populations. The strain used in the feeding study mentioned above was, in fact, one with minimal virulence factors.

Regrowth in POU devices
Improved characterization of aeromonads was a relief to the drinking water treatment industry as debate raged in the early 2000s regarding regrowth of ubiquitous bacteria in POU treatment devices. Aeromonas was a particular concern in POU purification devices utilizing carbon filtration where studies showed that counts of the bacteria post-treatment were often higher than in the influent.5 In both tap and purified water sources, however, levels were not high enough to cause gastrointestinal illness (10 CFU/mL in tap water and 29.5 CFU/mL in POU effluent) regardless of strain virulence.

Sand filtration and common disinfectants utilized in municipal drinking water treatment are effective at inactivating Aeromonas. Within the biofilm environment, however, populations remain protected and can persist in the drinking water distribution system.

Ubiquitous in soils, distribution systems are easily contaminated during routine maintenance practice. Regrowth is expected relative to nutrient and temperature increases (> 15°C/59°F) and low disinfectant residual (< 0.2 mg/L free chlorine).

Off the hook?
Regulatory standards for Aeromonas are in effect in The Netherlands (drinking water; 200 CFU/100 mL) and Canada (bottled water). No disease outbreaks, however, have been reported in relation to treated drinking water. Neither has a causal relationship with waterborne gastroenteritis and aeromonads been shown. Coupled with insignificant epidemiological data and inconclusive human feeding studies, little evidence exists for implementation of broad-spectrum controls.

Aeromonas is not necessarily off the hook as a pathogen of concern. Persons with diarrhea are more frequently colonized with the bacteria and shed higher numbers in the stool than those with asymptomatic infections. Peak isolation from human stools corresponds to seasonal peak concentrations in the environment during warmer months.

Infections in humans are caused by one of seven species but primarily due to either A. hydrophila, A. sobria or A. caviae. A. hydrophila causes about twice as much illness than each of the latter two. Infection typically occurs in children under five years old, the elderly or the immunocompromised, especially those with cirrhosis, cancer, diabetes or other serious ailment.

An understanding of individual host factors is now needed to determine the causal relationship between Aeromonas species and human disease. Normal flora of the gut is a complicated mixture of bacteria living in a delicate balance that differs among individuals in diversity and relative composition. Disruption or incomplete colonization of the gut flora whether due to age, chronic illness or antibiotic therapy likely plays a role in manifestation of Aeromonas infection.

Although untreated water supplies (specifically well water) have been traced to infection in humans in epidemiological studies, molecular analysis of strains isolated from the feces of ill individuals indicates dissimilar characteristics compared to common environmental isolates. This suggests a different source of infection.

What’s next?
After Aeromonas was listed on the CCL, nearly 300 distribution systems surveyed in the US were monitored for a year. Aeromonads were found in 14 percent of the samples and were more prevalent in smaller (< 10,000 customers served) municipalities and those with a groundwater source.

Typical concentrations in treated drinking water are low (< 10 CFU/100 mL). Concentrations in groundwater are also very low (< one CFU/mL) but are more likely to regrow and increase in the distribution system to > 200 CFU/mL.6 These values are still well below the infectious dose and several orders of magnitude below typical concentrations in food.

Several unidentified factors appear to be necessary to cause illness in humans as correlations with virulence factors or genetic characteristics are not evident. While the role of Aeromonas in human disease remains controversial and not well understood, treated and purified drinking water is no longer a primary area of concern for minimizing exposure of the general population.


  1. USEPA. 1998. Announcement of the drinking water contaminant candidate list; Notice. Federal Register. 63:10274-10287.
  2. USEPA. 2005. Drinking water contaminant candidate list 2; Final Notice. Federal Register. 70:9071.
  3. USEPA. 2006. Aeromonas: Human Health Criteria Document. Health and Ecological Criteria Division. Office of Science and Technology, Office of Water. Washington, D.C. pp. 1-198.
  4. D.R. Morgan, P.C. Johnson, H.L. DuPont, T.K. Satterwhite and L.V. Wood.1985. Lack of correlation between known virulence properties of Aeromonas hydrophila and enteropathogenicity for humans. Infection and Immunity. 50: 62-65.
  5. C. Chaidez and C.P. Gerba. 2004. Comparison of the microbiologic quality of point-of-use (POU) treated water and tap water.
  6. P. Holmes, L.M. Niccolls and D.P. Sartory. 1996. The ecology of mesophilic Aeromonas in aquatic environment. In: B. Austin, M. Altwegg, P.
    Gosling & S.W. Joseph (Eds.) The Genus Aeromonas. John Wiley & Sons, New York, NY, pp. 39-76.

About the author
Dr. Kelly A. Reynolds is an Associate Professor at the University of Arizona College of Public Health. She holds a Master of Science Degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds has been a member of the WC&P Technical Review Committee since 1997. She can be reached via email at


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