MadSci Network: Microbiology

Re: What protist are commonly found in drinking water that are harmless?

Date: Fri Oct 17 14:34:54 2003
Posted By: Dean Cliver, Faculty, Food Safety Unit, Uiversity of California, Davis
Area of science: Microbiology
ID: 1066158901.Mi

Your question is about protists in drinking water. Protists are single-celled organisms, including bacteria, algae, protozoa, fungi, and possibly viruses. Drinking water is water that is allegedly safe for people to drink, with the understanding that standards of potability vary enormously. But, as you point out, people may drink water from wells, springs, or city supplies (and sometimes other sources, depending on what is available). I tried on-line searching and gave up after scanning 150 entries, some of which I opened. Two that were at least amusing, if not directly responsive to your question, were and

After that, for lack of a specific book to recommend, I’ll try a top-of-my-head answer. First, life probably evolved in water, and no natural water is sterile. The protists present depend on where the water has been by the time one takes a sample. For example, surface water (lakes and rivers) is a mixture of rain water (which washes dust and microorganisms out of the air, including mold spores and plant pollen) and run-off, which washes stuff off the surface of the soil and from pavements, including animal feces and what is left of decomposed organisms (e.g., last year’s leaves). Some springs empty into surface waters and some come out on top of the ground. If you have to dig or drill for the water, it’s well water.

So, surface water contains protists that were washed out of the air or off of land surfaces. Well water, ideally, contains protists that have successfully percolated down through a lot of soil, so many of the organisms get trapped or stuck along the way, and only certain ones make it down to the aquifer, to travel with the water flowing underground. Unfortunately, the process of purification differs greatly, depending on how “tight” the soil is, through which the water must pass. A worst-case situation could be a “ karst” (deeply fissured limestone), where water that came out of a septic tank one day is found in a downhill spring the next day, with essentially no purification. For this reason, bacterial indicators called “coliforms” are often used to show that water may have disease agents in it, though the presence of coliforms is not direct evidence of a hazard, and the absence of coliforms is not firm evidence of safety. Well water that has received good purification during percolation will contain some bacteria that cannot cause disease, possibly a few viruses, and nothing larger (i.e., no protozoa, algae, or fungal spores). There’s no way to predict what will be in spring water unless the geology of the area is well known.

City tap water comes either from wells or from surface sources. Since the big outbreak of cryptosporidiosis in Milwaukee in 1993 (403,000 people sick), the Environmental Protection Agency wants all cities — even the small ones — to do what they call “complete treatment” of surface water, and even of ground water whose purity is suspect. This means adding a coagulant, letting the big stuff settle out, filtering through sand or some other filter medium, and disinfection. The coagulation, settling, and filtration are intended to remove Cryptosporidium parvum, which is one of the smallest protozoa in its transmission form (the oocyst), and should also remove all other protozoa, algae, and fungal spores. The disinfection is supposed to kill bacteria and viruses that might infect humans, but it doesn’t sterilize the water. Finished drinking water in distribution (e.g., under the street in front of your house) still has bacteria in it, and some of these attach to the inner surfaces of the water mains and produce glop called “biofilm” that can interfere with the flow of water and make the water look or taste strange. Most city systems flush out their water mains at least once a year by opening fire hydrants to cause a rapid rush of water through the mains. Maybe you have seen this done, or seen the notice (“water may be cloudy”) sent to your home before it happens.

So (back to your question), water that is fit to drink may have quite a lot of bacteria in it if it came from a well or a city supply, but probably none that will cause illness. The ones that hang on longest are able to grow, slowly, at very low levels of nutrients (the dissolved stuff that bacteria live on). The genus names of some of these were Pseudomonas, Flavobacterium, and Achromobacter; some of these names have been changed for various reasons since they were first applied. Most viruses and protozoa can’t grow in water at all (they need living hosts); but some viruses, called “bacteriophages” infect bacteria, including water bacteria. There are always a few algae that sneak through, too: put a bottle of “pure” water where the sun shines on it (algae need light energy) and watch what develops. I don’t think our bodies have been shown to need any of these tiny critters, but they don’t do us any harm, either. Out in the water, they digest things that we would rather not have in our drinking water, so they are our friends in a sense. But they are the kinds of friends that are best met outdoors, rather than bringing them home.

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