E. coli in our lakes: What does it really mean?

Editor’s note: This post originally appeared on the personal blog of University of Minnesota associate professor of biosciences Timothy Johnson, Ph.D.
Johnson’s research at the U of M College of Veterinary Medicine includes investigations into antibiotic resistance in bacterial pathogens, microbial communities in the animal gastrointestinal tract, and multidrug resistance of E. coli and Salmonella in both humans and animals.

If you follow the local news, or have children that love swimming, you have probably noticed an increasing number of beaches in Minnesota closed recently due to high E. coli levels. Just in Minneapolis, Lake Hiawatha Beach and Lake Calhoun’s Thomas and 32nd Street beaches were recently closed in response to high E. coli counts in the water. The simple phrase “E. coli” strikes fear into the hearts of anyone who has ever experienced gastrointestinal distress. However, it is important to understand what E. coli actually is and what “high E. coli levels” actually means to our lakes.

What is E. coliE. coli stands for Escherichia coli. This is the formal name for a species of bacteria in honor of the German-Austrian physician Theodor Escherich, who first identified the bacteria associated with digestion in infants. Here are the important take-home messages about E. coli:

1. We all carry about 1,000,000 E. coli cells per gram of feces in our guts. That’s right, over 1 million E. coli per gram of poop! If you are healthy, none of these E. coli are capable of causing gastrointestinal illness. In fact, gastrointestinal disease (i.e., diarrhea) due to E. coli is extremely rare in the U.S. and other industrialized countries. We tend to think of E. coli as bad because of the popular press, but in actuality these are important components of a healthy gut in animals (including us).

2. We are not the only animals that carry E. coli. Nearly every mammal and bird carries E. coli. And, there are many different “flavors” of E. coli. Some can colonize birds, some can colonize humans, some can colonize pigs, some can colonize cattle, and some can colonize all of these animals. So, one E. coli certainly does not equal all.

3. Fecal coliform and E. coli counts do not necessarily mean that pathogens capable of causing disease are in the water. It is very important to understand what “high E. coli levels” means when they are found in lakes. E. coli levels are established, according to the Minnesota Department of Health, through testing of water samples from Minnesota beaches. They take these samples and perform culturing of the samples to determine how many E. coli are present in 100 mL of water (100 mL is slightly more than 3 ounces of water). Over a 30-day period, the number of E. coli cells should not exceed 200 per 100 mL of water, on average. Also, no single sample should ever exceed 1,000 E. coli cells per 100 mL of water. If these criteria are exceeded, then closure of a beach is recommended until the numbers of E. coli go down. Remember, these are generic counts of E. coli cells in the water. The actual source of these E. coli are unknown. In fact, they likely originate from a multitude of possible sources, including human waste, bird droppings, agricultural run-off, or even naturally occurring E. coli present in the soil. In short, an E. coli count of 1,000 cells per 100 mL doe not means that there are 1,000 E. coli cells that can make you sick per 100 mL of water. It is actually quite likely that none of these E. coli will make you sick. The reason that the department of health uses these criteria is based on the likelihood of pathogens (not just E. coli, but other pathogens as well) being present in the water based on the counts of E. coli as an “indicator organism.” This is a very conservative approach to estimate the possibility that pathogens are in the lake water.

4. What about the E. coli that can cause disease? So let’s assume that the lake we are going to swim at does harbor some pathogenic E. coli or other pathogen. We have to consider something called “infectious dose,” or how many cells of the pathogen it actually takes to make you ill. Remember, I said before that at best, a small fraction of the E. coli present in lake water will actually be capable of causing disease in humans. The only way it can make you sick is through oral ingestion (the infamous fecal-oral route). And, for healthy humans, the infectious dose of E. coli (only the ones able to cause disease) needs to be in the range of 100-10,000 cells. And, you can ingest these bacteria even if you don’t drink the lake water. However, typically in order to acquire enough of the pathogenic bacteria you would have to swallow water, in my opinion. If you have young kids, you know all about swallowing water. Yes, it happens.

So why should I be worried? There really shouldn’t be any cause for major alarm when these alerts go out. The department of health is looking out for your best interests, with good reason, to prevent the occurrence of disease acquired through swimming. I am not recommending that you do not heed their warnings! These warnings are established, like I said, through a conservative approach to ensure that you don’t get sick when you swim. In my opinion, most of these alerts are likely benign, and only a small percentage of “high E. coli level” lakes actually contain pathogens capable of causing human disease. However, I am not willing to play the pathogen lottery with my kids or my family, and I wouldn’t recommend that anyone do that. Until we have better and cheaper ways to measure pathogens in lake water, this is the best we have and it is in place for a reason. But there is no reason to panic. Like I said before, these E. coli can arise for a lot of different reasons, they don’t always correlate with microbes that can make you sick, and they will go down over time. So, my advice? Pick a different lake this week, and don’t hesitate to return to your favorite lake when the alert subsides! Oh, and don’t blame the E. coli.

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