Bacteria in drinking water
Bacteria are a different kind of problem from the chemical contaminants in this hub. They're a disinfection issue — mostly for private wells — and the right tool is disinfection (UV), not a chemical-reduction filter.
What it is
Water-quality testing for bacteria usually looks for indicator organisms: total coliform bacteria, which signal that surface contamination could be getting in, and E. coli, which specifically indicates fecal contamination and the possible presence of disease-causing organisms. Finding E. coli doesn't mean every pathogen is present — it means the pathway is open and the water should be treated as unsafe until fixed.
Health effects, stated plainly
Unlike the chemical contaminants here, microbial contamination is an acute risk: disease-causing bacteria, viruses, and parasites can cause gastrointestinal illness within days, and are most serious for infants, older adults, and people with weakened immune systems. That immediacy is exactly why the standard is strict and why a positive result calls for prompt action. The CDC has primary guidance for well owners. CDC on private wells →
Where it comes from
Most bacterial contamination is a private-well issue: surface water reaching the well after heavy rain or flooding, a damaged or shallow well cap, nearby septic systems, or aging well construction. Public systems disinfect and monitor continuously, so positives there usually point to a distribution-system problem the utility must address.
How to know if you have it
A coliform/E. coli lab test is the only way to know — and because contamination is often intermittent (tied to rain or seasonal changes), wells should be tested regularly. The CDC recommends private-well owners test for total coliform bacteria at least once a year, and after any flooding or well work.
What the current standard is
As of June 2026, public water systems follow EPA's Revised Total Coliform Rule. The health goal (MCLG) for E. coli is zero, and in practice the standard is effectively zero tolerance: a confirmed combination of total-coliform-positive and E. coli-positive samples is an MCL violation that triggers a Tier 1 public notice, including a boil-water advisory. Private wells aren't covered by this rule, so the standard serves as the health benchmark a well owner should hold their own water to. EPA Revised Total Coliform Rule (verified June 2026) →
What treats it
Because this is a disinfection problem, the treatment is disinfection — most commonly an ultraviolet (UV) system certified to NSF/ANSI 55 Class A, installed where water enters the home so all water is disinfected. UV requires reasonably clear water, so it's typically paired with upstream sediment filtration. Chlorination and (as a stopgap) boiling also disinfect. Critically, reverse osmosis and carbon filters are not disinfection methods and should not be relied on for microbial safety.
Find the right treatment for your water.
Tell the selector your source and test results — for bacteria it routes you to certified disinfection, not a chemical filter that can't help.
Get my recommendationEducational information, not medical or regulatory advice. If a test shows E. coli or coliform bacteria, treat the water as unsafe and follow your local health department's guidance. For your specific water, consult a certified laboratory and your local utility, and confirm any product's current certification for your exact model in the NSF, IAPMO, or WQA database. Standards verified against the U.S. EPA in June 2026 and may change.