Teacher Notes

Bacterial Pollution (Coliform)

Super Value Laboratory Kit

Materials Included In Kit

Lactose broth/indicator tubes, 75
Pipets, graduated, sterile, 75

Additional Materials Required

Collecting bottle, sterile (one per site)
Incubator (optional)

Safety Precautions

Field testing should be carried out under the supervision of an instructor. By the very nature of coliform testing, the potential exists for the culturing of pathogenic microorganisms to hazardous levels. Students should never be permitted to open the tubes after they have been inoculated. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines. Please review current Safety Data Sheets for additional safety, handling and disposal information.


Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The culture tubes may be disposed of according to Flinn Suggested Biological Waste Disposal Procedure, Type 1—Potentially Harmful Wastes Due to Microorganism-Type Contamination.

Lab Hints

  • Enough materials are provided in this Super Value Kit for 5 classes of 30 students each working in pairs (75 total student groups). The initial collection period can be done the first class with short observation sessions in each consecutive class until visible results are present.
  • This kit is designed so that 75 separate sites may be sampled (one tube per site), or a simplified form of the multiple tube fermentation procedure can be conducted at 15 separate sites (five tubes per site). In other words each individual class can choose to do 15 individual sites or 3 separate sites (five tubes each).
  • Inoculating several tubes per site results in a much more reliable estimate of bacterial density and reduces the chances of a false negative result. The multiple tube fermentation procedure allows the derivation of a Most Probable Number (MPN)—a statistical probability providing an estimate of the mean density of coliforms present.
  • Sampling is best done on-site to avoid sample contamination and resulting inaccuracy. If on-site testing is not possible, samples should be brought into the lab and tested within one or two hours. Samples can be held up to six hours if packed on ice.
  • To accurately calculate the MPN value, five tubes should be inoculated from each sampling site.
  • Collect samples in small (50100 mL), sterile bottles or flasks. For lake or pond testing, sample away from shore and below the water surface (from a dock, bridge, or small boat if possible). For rivers and streams, the optimum sampling location is midstream and a few inches below the surface. Collection bottles may be shared if different groups are testing the same site.

Correlation to Next Generation Science Standards (NGSS)

Science & Engineering Practices

Using mathematics and computational thinking
Developing and using models
Asking questions and defining problems
Planning and carrying out investigations
Constructing explanations and designing solutions
Analyzing and interpreting data

Disciplinary Core Ideas

MS-LS1.C: Organization for Matter and Energy Flow in Organisms
MS-LS2.A: Interdependent Relationships in Ecosystems
MS-ESS2.C: The Roles of Water in Earth’s Surface Processes
HS-LS1.C: Organization for Matter and Energy Flow in Organisms
HS-LS2.A: Interdependent Relationships in Ecosystems
HS-LS2.B: Cycle of Matter and Energy Transfer in Ecosystems
HS-ESS3.C: Human Impacts on Earth Systems

Crosscutting Concepts

Cause and effect
Structure and function
Systems and system models
Stability and change

Performance Expectations

MS-LS2-1: Analyze and interpret data to provide evidence for the effects of resource availability on organisms and populations of organisms in an ecosystem.
MS-LS2-3: Develop a model to describe the cycling of matter and flow of energy among living and nonliving parts of an ecosystem.
MS-LS2-4: Construct an argument supported by empirical evidence that changes to physical or biological components of an ecosystem affect populations.
MS-ESS3-4: Construct an argument supported by evidence for how increases in human population and percapita consumption of natural resources impact Earth’s systems.
HS-LS2-3: Construct and revise an explanation based on evidence for the cycling of matter and flow of energy in aerobic and anaerobic conditions.
HS-LS2-4: Use mathematical representations to support claims for the cycling of matter and flow of energy among organisms in an ecosystem.
HS-ESS3-4: Evaluate or refine a technological solution that reduces impacts of human activities on natural systems.


American Public Health Association. Standard Methods for the Examination of Water and Wastewater; 16th ed. American Public Health Association: Washington, D.C., 1990.

Campbell, G.; Wildberger, S. The Monitor’s Handbook; LaMotte Co.: Chestertown, MD, 1992.

Jacobsen, C. Water, Water Everywhere: Water Quality Factors Reference Unit; Hach Co.: Loveland, CO, 1991.

Mitchell, M. K.; Stapp, W. B. Field Manual for Water Quality Monitoring; 9th ed. Thomson-Shore, Dexter, MI, 1995.

Student Pages

Bacterial Pollution (Coliform)


Conduct a simple, proven test for the presence of coliform bacteria in water. Depending on which of two suggested procedures are followed, results can be reported as either a qualitative (presence/absence) or semi-quantitative (estimated numerical density) value.


  • Coliform bacteria
  • Indicator
  • Most Probable Number (MPN)


In water quality testing, coliform bacteria are used to indicate the presence of untreated (or under treated) sewage, or fecal contamination. Not necessarily pathogenic (disease causing) themselves, fecal coliforms are nevertheless valuable indicators for two reasons: (1) They are relatively easy to test for, and (2) Their presence typically coincides with numerous potentially pathogenic microbes. If coliforms are found to be present above a designated level (dependent on water use, see the Discussion section), it is assumed that potential pathogens are present, and the water is judged unfit. It would be prohibitive, in time and expense, to conduct individual tests for all potential pathogens.

Coliforms are common in the digestive tracts of cold- and warm-blooded animals, and are passed with feces. Animal wastes, agricultural runoff, and untreated sewage effluent are all likely sources of fecal coliforms in lakes and streams. Untreated sewage effluent may originate from illegal discharge sites, or, frequently, it is the result of heavy storm runoff overwhelming wastewater treatment plants that handle combined storm and sanitary input. Check with your local treatment plant to see if it handles combined input, or if storm runoff and sanitary sewage are handled by separate systems. If combined, ask plant officials if storm runoff ever overwhelms plant capacity, how they handle it, and whether or not they issue warnings or notices when that occurs.

The culture medium in the supplied tubes is lactose broth with chlorphenol red indicator. Most coliform bacteria (particularly those from the guts of warm-blooded animals) are capable of fermenting the lactose present in the medium. Lactose fermentation leads to the production of carbon dioxide (CO2) gas. The CO2 dissolves in the water-based medium and drives the pH (initially around 7) down into the acid range. The chlorphenol red indicator is red at pH 6.8 and above, and turns to yellow below 5.2. Following the incubation period, any tubes that have turned yellow (or greenish yellow) should be considered positive for coliform bacteria.

An MPN (coliform density) can be calculated using the following formula:


A sample calculation for a case with 3 positive tubes out of 5 total tubes, each with 1 mL of sample is shown:


A limitation of this simplification of the MPN procedure is that coliform concentrations higher than 180/100 mL should cause all five tubes to turn. In the event that all five tubes do show positive, results should be recorded as >180/100 mL.

Coliform density standards are established at state and regional levels for several categories of water use (check your local regulations). Typical ranges appear in Table 1.


A typical standard for treated effluent (discharged from a wastewater treatment plant) is on the order of ≤200 coliforms/100 mL. This suggests one potential sampling site: downstream from the discharge point of a nearby treatment plant. The opportunity may exist to visit a treatment facility to discuss their standards and the results of any testing you’ve already conducted.


Collecting bottles or flasks, sterile, one per site
Incubator (optional)
Lactose broth/indicator tube
Pipet, sampling, sterile, disposable

Safety Precautions

Field testing should be carried out under the supervision of an instructor. By the very nature of coliform testing, the potential exists for the culturing of pathogenic microorganisms to hazardous levels. Never open the tubes after they have been inoculated. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.


  1. Uncap the collecting container, immerse it mouth first several inches below the water surface, and turn the bottle mouth into the current. Recap the filled bottle.
  2. Unwrap a sterile pipet and, using sterile technique, transfer a 1 mL sample from the collecting bottle to a lactose broth tube. If several tubes are being inoculated from a single sample, the same pipet may be used. After filling each tube, any water remaining in the pipet should be discarded, and a fresh sample drawn from the bottle.
  3. Label inoculated tubes with the collection site, site characteristics, date and time.
  4. Incubate tubes at 35 °C for 48 hours (72 hours at room temperature).
  5. Check the tubes every 24 hours during the incubation period, and note and record any color changes.
  6. Consult your instructor for appropriate disposal procedures.

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