Bacteria & E. coli (Coliform)
Also known as: Total coliform, E. coli, RTCR
Coliform is the early-warning indicator that something could let pathogens into the water — the trigger behind most boil-water notices.
E. coli: any positive is a violation. The Revised Total Coliform Rule uses coliform bacteria as an indicator; confirmed E. coli is an acute health-based violation.
Coliform bacteria signal a pathway for fecal contamination. E. coli and related pathogens cause gastrointestinal illness — diarrhea, cramps, nausea — and can be serious for infants, the elderly, and the immunocompromised.
Intrusion of contamination into wells, storage tanks, or distribution lines — from breaks, low pressure, back-siphonage, or inadequate disinfection.
Systems respond with disinfection, repairs, and boil-water notices. At home during an advisory, boil water for one minute before drinking.
Source: EPA Safe Drinking Water Information System (SDWIS) · 2026 Q1
Total coliform bacteria are mostly harmless themselves, but their presence means the barrier protecting the water has a gap that pathogens could exploit. That's why the Revised Total Coliform Rule treats coliform as a warning system and confirmed E. coli as a serious, acute violation.
Coliform-related violations are extremely common among small systems, where a single contaminated sample or a lapse in disinfection triggers a notice. They're the most frequent driver of boil-water advisories.
States with the most bacteria & coliform violations
Community water systems with a related health-based violation since 2016.
| # | State | Systems |
|---|---|---|
| 1 | Texas | 182 |
| 2 | Pennsylvania | 166 |
| 3 | California | 145 |
| 4 | Missouri | 135 |
| 5 | Oregon | 123 |
| 6 | Arizona | 116 |
| 7 | Idaho | 105 |
| 8 | Utah | 84 |
| 9 | Nebraska | 84 |
| 10 | Washington | 81 |
| 11 | Montana | 74 |
| 12 | Connecticut | 74 |
Common questions
What is the EPA limit for bacteria & coliform in drinking water?
E. coli: any positive is a violation. The Revised Total Coliform Rule uses coliform bacteria as an indicator; confirmed E. coli is an acute health-based violation.
Is bacteria & coliform in drinking water dangerous?
Coliform bacteria signal a pathway for fecal contamination. E. coli and related pathogens cause gastrointestinal illness — diarrhea, cramps, nausea — and can be serious for infants, the elderly, and the immunocompromised.
Where does bacteria & coliform in water come from?
Intrusion of contamination into wells, storage tanks, or distribution lines — from breaks, low pressure, back-siphonage, or inadequate disinfection.
How do I remove bacteria & coliform from my water?
Systems respond with disinfection, repairs, and boil-water notices. At home during an advisory, boil water for one minute before drinking.
Which states have the most bacteria & coliform violations?
Across U.S. community water systems, 2,462 have had a related health-based violation since 2016, led by Texas with 182.