This test counts E. coli and total coliform bacteria in colony-forming units per 100 mL using Standard Method 9223B. E. coli is the indicator organism public health agencies use nationwide to judge recreational water safety, because it lives in the intestines of warm-blooded animals and its presence means fecal material has reached the water. Total coliform is the broader group, useful for detecting general contamination and decaying organic matter. This is the test behind beach advisories and closures.