Determination of fecal coliforms in surface water is a fundamental microbiological test used to evaluate fecal contamination, sanitation risk, and compliance with surface water quality standards such as GB 3838-2002. This technical article explains the indicator significance of fecal coliforms, applicable sample types, recognized testing methods—including multiple-tube fermentation, membrane filtration, and enzyme substrate techniques—and their practical applications in environmental monitoring and regulatory compliance.
Technical Overview
Fecal coliforms, also known as thermotolerant coliforms, are a subgroup of the total coliform group defined by their ability to ferment lactose with gas and acid production at an elevated temperature of 44.5 °C. They are Gram-negative, non-spore-forming, facultatively anaerobic bacilli that originate predominantly from the intestinal tracts of warm-blooded animals, with typical genera including Escherichia, Klebsiella, Enterobacter, and Citrobacter.
Because fecal coliforms are shed in large numbers in animal and human feces and are relatively easy to culture, they serve as the classic indicator organism for assessing recent fecal contamination in surface water. A positive detection signals the possible presence of enteric pathogens such as Salmonella, Shigella, and enteric viruses, which are far more difficult and costly to detect directly.
In China, fecal coliforms are a mandatory microbiological parameter in the Surface Water Environmental Quality Standard GB 3838-2002, with class-based limits expressed as MPN/L (most probable number per liter):
- Class I (source water/national nature reserve): ≤ 200 MPN/L
- Class II (drinking water source, first-class protection zone): ≤ 2,000 MPN/L
- Class III (drinking water source, second-class protection zone; fishery waters): ≤ 10,000 MPN/L
- Class IV (industrial water use, non-direct recreational contact): ≤ 20,000 MPN/L
- Class V (agricultural and landscape water use): ≤ 40,000 MPN/L
Depending on the analytical method used, results are reported either as MPN/L (statistical estimate from multi-tube or well-based enumeration) or as CFU/100 mL (direct colony count from membrane filtration).
Testing Samples
River and Stream Water
Flowing water bodies are sampled at representative cross-sections, typically at established monitoring stations. Sampling near sewage outfalls, tributary confluences, or livestock operations may be required for pollution-source investigations. Because fecal coliform levels can vary across the channel width and depth, depth-integrated or width-integrated sampling is recommended for representative results.
Lake and Reservoir Water
Static or slow-moving water bodies often exhibit thermal stratification and spatial heterogeneity in microbial distribution. Samples are collected at defined depths near water intake structures, in recreational zones, or at routine grid monitoring points. For lakes serving as drinking water sources, fecal coliform data directly support source-water risk classification under GB 3838-2002.
Estuarine and Coastal Surface Water
Brackish and marine surface waters, including shellfish harvesting areas and bathing beaches, are routinely tested for fecal coliforms because salinity, tides, and urban runoff strongly influence bacterial survival and transport. Membrane filtration methods with salinity-tolerant media are commonly applied to these matrices.
Recreational and Bathing Water
Swimming areas, landscape water bodies, and artificial water features are monitored to protect public health during recreational exposure. Elevated fecal coliform counts trigger beach advisories, source investigations, and remediation of sanitary sewer overflows or stormwater inputs.
Sample collection and preservation requirements are critical to data validity:
- Collect directly into sterile, pre-autoclaved glass or plastic bottles; avoid rinsing and prevent contamination of the bottle mouth and cap.
- Add sodium thiosulfate (approximately 0.1 mg per 100 mL capacity equivalent) when sampling chlorinated or disinfected waters to neutralize residual disinfectant.
- Fill the bottle to about 80 % capacity to allow mixing; leave a small headspace.
- Transport samples in the dark in an insulated container, chilled to approximately 2–8 °C.
- Analyze as soon as possible, ideally within 6 hours of collection; the maximum holding time is 24 hours when refrigerated, in accordance with standard methods such as HJ 347.1-2018 and HJ 347.2-2018.
Testing Items
The core testing item is the enumeration of the fecal coliform group in surface water. In practice, fecal coliform testing is frequently bundled with related microbiological parameters to provide a complete sanitary profile:
- Fecal coliform group count (thermotolerant coliforms), reported as MPN/L or CFU/100 mL
- Total coliform group count
- Escherichia coli (E. coli) count
- Heterotrophic plate count (HPC), as a general bacterial load indicator
- Fecal streptococci / intestinal enterococci, as a complementary fecal contamination indicator
Testing Methods
Three principal categories of methods are recognized for the determination of fecal coliforms in surface water. Method selection depends on the expected bacterial density, sample turbidity, salinity, required turnaround time, and the applicable regulatory standard.
Multiple-Tube Fermentation (MPN) Method
The multiple-tube fermentation technique is the classical confirmatory approach, referenced in HJ 347.2-2018, Standard Methods 9221 E, and ISO 9308-3. The procedure involves two stages:
- Presumptive test: replicate tubes of lactose or lauryl tryptose broth are inoculated with decimal dilutions of the sample (for example, five tubes each of 10 mL, 1 mL, and 0.1 mL) and incubated at 36 ± 1 °C for 24–48 hours; gas and acid production indicates presumptive coliforms.
- Confirmed test: gas-positive cultures are transferred to EC broth and incubated at 44.5 ± 0.5 °C for 24 ± 2 hours; continued gas and acid production at the elevated temperature confirms fecal coliforms.
The number of confirmed positive tubes in each dilution series is used to derive the most probable number (MPN) from statistical MPN tables. The method is applicable to turbid or highly contaminated samples, but it is labor-intensive and requires several days for a confirmed result.
Membrane Filtration (MF) Method
In the membrane filtration method, a measured sample volume (typically 100 mL, or a suitable dilution) is passed through a 0.45 µm sterile membrane filter that retains bacteria. The membrane is then transferred onto m-FC agar or M-TEC medium and incubated at 44.5 ± 0.5 °C for 24 ± 2 hours. Fecal coliform bacteria produce characteristic blue colonies on m-FC medium due to the selective action of bile salts and aniline dye. Colony counts are expressed as CFU/100 mL. This approach is consistent with US EPA Methods 1600 and 1603 and Standard Methods 9222 D. It is fast and quantitative but unsuitable for highly turbid samples, where filter clogging or non-target colony overgrowth may interfere.
Enzyme Substrate (Defined Substrate) Method
The enzyme substrate method, specified in HJ 347.1-2018 and Standard Methods 9223, uses a defined medium containing chromogenic or fluorogenic substrates. Reagent is added to a 100 mL sample, which is incubated at 44.5 ± 0.5 °C for approximately 18–24 hours. Fecal coliform bacteria metabolize the substrate and generate a quantifiable color or fluorescence signal, with enumeration performed using 51-well or 96-well quantitation trays, presence/absence formats, or MPN tubes. This method offers rapid turnaround, minimal confirmatory work, low detection capability, and high analytical precision, making it well suited to routine surface water monitoring programs.
Method Comparison and Quality Control
Regardless of the method chosen, a robust quality assurance program is essential:
- Sterility controls: incubate one uninoculated medium lot per batch to verify no background contamination.
- Positive and negative culture controls: verify method performance using reference strains of E. coli (positive) and non-thermotolerant organisms (negative).
- Temperature control: incubators must maintain 44.5 ± 0.5 °C, verified with calibrated thermometers, because even modest temperature deviation changes selectivity.
- Replicate analysis and dilution suitability: high-count samples require serial dilution so that counts fall within the statistically valid range of the method.
- Biohazard management: all cultures are handled under biosafety level 2 practices and autoclaved before disposal.
Applications
Fecal coliform determination in surface water supports a wide range of regulatory, operational, and scientific objectives:
- Surface water quality classification and compliance assessment against GB 3838-2002 for rivers, lakes, and reservoirs.
- Routine environmental monitoring by government agencies, research institutes, and accredited third-party laboratories.
- Discharge compliance verification for municipal and industrial wastewater treatment plants, where effluent limits (for example, GB 18918-2002 Class 1A ≤ 1,000 MPN/L and Class 1B ≤ 10,000 MPN/L) directly affect receiving water quality.
- Pollution source tracing and sanitary investigations near sewage outfalls, combined sewer overflows, livestock operations, and urban stormwater discharge points.
- Drinking water source protection and early warning of microbial contamination events at raw water intakes.
- Recreational water risk management, including beach and bathing area health advisories.
- Environmental impact assessments, water restoration project evaluation, and academic research on microbial water quality dynamics.
FAQ
What is the difference between total coliforms and fecal coliforms?
Total coliforms ferment lactose at 36 ± 1 °C and occur widely in soil, water, and vegetation as well as in feces. Fecal coliforms are the thermotolerant subgroup that continues to ferment lactose at 44.5 °C, indicating a higher probability of warm-blooded animal origin and thus a more specific signal of fecal contamination.
Why is incubation at 44.5 °C used?
The elevated temperature of 44.5 ± 0.5 °C selectively inhibits environmental coliforms that cannot tolerate heat while allowing intestinal coliforms originating from warm-blooded animals to grow and ferment. This temperature discrimination is the defining criterion that separates fecal coliform testing from total coliform testing, which is why precise incubator control is mandatory.
Why is sodium thiosulfate added to sampling bottles?
Sodium thiosulfate immediately neutralizes any residual chlorine or other oxidizing disinfectant in the sample. Without it, disinfectant continues to kill bacteria during transport and storage, causing falsely low results that underestimate the true microbial load.
What do MPN and CFU mean, and how do they differ?
MPN (most probable number) is a statistical estimate of bacterial density derived from the pattern of positive and negative tubes or wells, and is the reporting format used by GB 3838-2002. CFU (colony forming unit) is a direct count of visible colonies on a membrane or plate. Both are valid quantitative expressions, but they are method-dependent and should not be interconverted arithmetically.
How quickly must samples be analyzed?
Microbial populations in water samples change rapidly after collection. Samples should be analyzed as soon as possible, ideally within 6 hours, and in no case later than 24 hours when held at 2–8 °C in the dark. Field blanks and transport blanks are recommended for sampling programs requiring regulatory-grade data quality.
