These tests look for bacteria, yeasts and molds in drinking water, wells, surface water, recreational water and building water systems. Each test below names the reference method and how the result is reported: present or absent in the volume tested, a colony count (CFU) or a most probable number (MPN).
| Test | Method |
|---|---|
| Total coliform and E. coli, presence or absence | Standard Methods 9223 B |
| Total coliform counts | Standard Methods 9222 B and 9221 B |
| Fecal coliform counts | Standard Methods 9222 D; 9221 B, C and E1 |
| E. coli counts | No method number listed |
| Enterococci and fecal streptococci | Standard Methods 9230 B and 9230 C |
| Heterotrophic plate count | Standard Methods 9215 B, C or D |
| Anaerobic plate count | No method number listed |
| Yeast and mold count | FDA Bacteriological Analytical Manual, chapter 18 |
| Iron-related and sulfur-reducing bacteria | BART (biological activity reaction test) |
| Pseudomonas aeruginosa | Pseudalert enzyme substrate test |
| Legionella pneumophila | Legiolert culture test |
Coliforms and E. coli
Standard Methods 9223 B, enzyme substrate: the sample is incubated with a medium containing substrates that total coliforms break down to a yellow color (ONPG) and that E. coli breaks down to a product that fluoresces under ultraviolet light (MUG). The result is present or absent in the volume tested, usually 100 mL. This is the usual test for drinking water and private wells.
Standard Methods 9222 B, membrane filtration: a measured volume is filtered, the membrane is incubated on a selective medium, and typical coliform colonies are counted as colony-forming units (CFU) per 100 mL. Standard Methods 9221 B, multiple-tube fermentation: portions of the sample are incubated in a series of tubes, and the pattern of positive tubes gives a most probable number (MPN) per 100 mL, a statistical estimate. For drinking water, coliform-positive results are checked for fecal coliforms or E. coli by Standard Methods 9221 E1 or F.
Fecal coliforms and E. coli. Fecal coliform counts and E. coli counts are separate tests. E. coli is one species within the fecal coliform group, so a fecal coliform count includes E. coli along with other thermotolerant coliforms, while an E. coli count reports E. coli alone. Both are described below.
Fecal (thermotolerant) coliforms grow at an elevated temperature that most other coliforms do not. Standard Methods 9222 D counts them by membrane filtration as CFU per 100 mL, and Standard Methods 9221 B, C and E1 give a most probable number by multiple-tube fermentation.
An E. coli count gives the number of E. coli in a measured volume of water, not only whether E. coli is present. E. coli lives in the intestines of people and warm-blooded animals, so finding it points to fecal contamination. E. coli counts are commonly expressed per 100 mL of water, as colony-forming units (CFU) or as a most probable number (MPN), depending on the counting technique. Numeric E. coli limits appear in recreational water, surface water and discharge permits.
Other indicator organisms
Standard Methods 9230 B (multiple-tube, MPN) and 9230 C (membrane filtration, CFU). Fecal streptococci, and the enterococci within that group, indicate fecal contamination and are often asked for with recreational and marine waters. Reported per 100 mL.
Plate counts
Standard Methods 9215: a portion of the sample is mixed into or spread on a general nutrient agar and incubated, and the colonies are counted. Drinking water uses the pour plate method (9215 B); environmental samples can also use the spread plate (9215 C) or membrane filter (9215 D) method. The result, in CFU per mL, is a general count of bacteria that grow on the medium, not a test for a particular organism.
A count of the anaerobic bacteria in a sample: those that grow without oxygen on the test medium. Like other plate counts, it reflects the organisms that grow under the test conditions, not every bacterium in the water. It is used for wells, process water and systems where oxygen-free conditions develop. Plate counts are expressed as colony-forming units (CFU) per volume tested.
FDA Bacteriological Analytical Manual, chapter 18: the sample is plated on a medium that holds back bacteria, incubated, and the yeast and mold colonies are counted. Reported as CFU per mL.
Nuisance and opportunistic bacteria
BART (biological activity reaction test): the water is added to tubes holding a selective nutrient, and the reactions that develop, and how soon they appear, show whether iron-related or sulfur-reducing bacteria are present and roughly how active they are. These are detection tests for wells and water systems, not exact counts.
Pseudalert, an enzyme substrate test: Pseudomonas aeruginosa breaks down a substrate in the medium and gives a blue fluorescence under ultraviolet light. Results are present or absent in the volume tested, or a most probable number.
Legiolert, a culture test with an enzyme substrate: Legionella pneumophila grows in the medium and breaks down a substrate, giving a brown color or turbidity, and the result is a most probable number per volume tested. The test detects Legionella pneumophila only, not other Legionella species.
Microbiology samples are collected in a sterile bottle and should reach the laboratory soon after collection. Contact us for sampling instructions before you collect the sample.
The method numbers on this page follow the published EPA and Standard Methods designations. These references describe the methods; the TNI LAMS registry lists each method’s code and revision.
Questions about a sample or a method, call 360-967-7010 or email [email protected].
