The 5 Scariest Types of Microorganisms Found in Tap Water

We can see when our tap water is brown with rust or cloudy from lots of minerals. We can sometimes taste or smell if our water is “off” and full of chemicals like chlorine. But what about contaminants we can’t see, taste, or smell? What kinds of microscopic creatures can be found in our drinking water? This list will detail the five categories of pathogens and microorganisms that can be found in tap water (and they are cute until they are not).

Microscopic Bugs

Believe it or not, some bugs (or tiny, crustacean-like animals) are too small to be seen by the naked eye, and just small enough to sometimes evade water filtration techniques. Rotifers and copepods are two such creatures. Rotifers are tiny, worm-like creatures that swim through freshwater sources like rivers, lakes, and even puddles. Copepods are tiny crustaceans found in similar environments. Although these critters are usually removed by filtration and other treatment steps, sometimes outbreaks occur. For instance, in 2009, problems with water treatment in Connecticut led to an abundance of rotifers and copepods in the drinking water in this area. Fortunately, copepods and rotifers do not pose health risks. Unfortunately, they can often serve as hosts for some of the pathogens in this list. Additionally, rotifers and copepods are larger than most pathogenic organisms; thus, when rotifers and copepods escape filtration, it indicates a serious problem with the water treatment.


Protozoans are tiny, single-celled critters similar to bacteria. Though not all protozoans are pathogenic, some can cause disease in humans and are cause for concern when found in tap water. Giardia is one of the most common water pathogens, and infection by this organism can cause severe stomach problems. Fortunately, water treatment can generally remove Giardia before the water reaches your tap, so Giardia infections are most often caused by drinking from untreated water in rivers or lakes.

Cryptosporidium is another type of protozoan that can cause gastrointestinal problems. These organisms are found in lakes, rivers, and reservoirs that can serve as sources for public drinking water. Fortunately, filtration steps involved in treating public water sources can eliminate around 99% of cryptosporidium. Unfortunately other purification steps, like chlorine disinfection, are less effective against cryptosporidium.

A third type of protozoan, Naegleria fowleri, has sometimes been referred to as the “brain-eating amoeba.” In keeping with its terrifying name, brain infections caused by this amoeba are usually fatal. Fortunately, infections by Naegleria fowleri are very rare. The only Naegleria fowleri infections from tap water in the United States both occurred in Louisiana, which may have been a result of damage to the water system by Hurricane Katrina. Additionally, Naegleria fowleri is generally harmless if swallowed and only leads to brain infections when water goes up the nose.


As with protozoans, not all bacteria are pathogenic. However, there are a few bacteria that may give cause for concern when it comes to your drinking water. E. coli and Salmonella are both well-known bacteria that can cause health problems in humans. There are many varieties of both E. coli and Salmonella, and while some are virtually harmless, others can cause problems ranging from gastrointestinal issues to kidney failure. Both have also previously been found in tap water. In 2014, Portland, Oregon had to issue a boil water warning due to E. coli detected in the drinking water. Salmonella in the tap water in Colorado led to dozens of patients with health issues.


Fungi, a category containing pathogens like mold, can also be a safety concern for tap water. Fungal spores, like those of Chaetomium sp can sometimes appear in tap water. Fortunately, with Chaetomium sp spores, they pose little health risk and can sometimes even affect the smell or taste of your water, giving you a clue to stop drinking. Rhizopus stolonifer is the type of mold that you often see growing on old bread – its spores can also sometimes be found in tap water. Though this mold can release toxins, it is unlikely to make you sick unless it is present in very, very high amounts.


Viruses are tiny, non-living agents that can cause a variety of diseases in humans. Viruses are the smallest type of pathogen in this list, making them the hardest to remove by filtration techniques. Fortunately, viruses are very susceptible to disinfection techniques used at many water treatment facilities. Viral outbreaks have been seen in water not treated in public treatment facilities, such as private wells. For example, an outbreak of hepatitis A was linked to a shared well in north Georgia in 1990. 

There are a variety of microorganisms that pose a risk to human health. Though most water treatment facilities do an excellent job in eliminating these health risks, outbreaks have occurred in the past. Thus, it’s important to be aware of the potential contaminants that might affect your drinking water.

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