10.4 Microbiological Testing: Total Coliform, E. coli (Colilert, Membrane Filtration) & QA/QC
Key Takeaways
- Coliform bacteria serve as sanitary indicators for enteric pathogens; the Revised Total Coliform Rule (RTCR) enforces an absence/presence compliance standard in 100 mL drinking water samples.
- The Defined Substrate Technology (Colilert) uses ONPG to detect total coliform (beta-galactosidase turns yellow) and MUG to detect E. coli (beta-glucuronidase fluoresces bright blue under 365 nm UV light).
- Membrane filtration captures bacteria on 0.45 um cellulose ester grid filters; total coliforms produce distinct golden-green metallic sheen colonies on m-Endo agar at 35.0°C.
- Fecal coliform membrane filtration utilizes m-FC medium incubated in a precision water bath at 44.5°C ± 0.2°C, yielding distinctive dark blue colonies.
- Microbiological QA/QC requires positive controls (E. coli), negative controls (Pseudomonas aeruginosa), sterility blanks, and autoclave sterilization validation at 121°C (15 psi) for 15 minutes.
Microbiological Testing: Total Coliform, E. coli (Colilert, Membrane Filtration) & QA/QC
Waterborne pathogens—including bacterial agents (Salmonella typhi, Shigella dysenteriae, Vibrio cholerae), enteric viruses (Norovirus, Hepatitis A), and protozoan cysts (Giardia lamblia, Cryptosporidium)—are difficult, expensive, and dangerous to isolate directly in water treatment laboratories. Consequently, regulatory programs under the Safe Drinking Water Act (SDWA) and the Revised Total Coliform Rule (RTCR) mandate the use of indicator organisms to evaluate the sanitary integrity of potable supplies and wastewater discharges.
1. Microbial Indicators & The Coliform Hierarchy
An ideal microbial indicator must be universally present in high numbers in the feces of warm-blooded mammals, survive longer in environmental water than enteric bacterial pathogens, exhibit comparable resistance to disinfection, and be rapidly quantifiable through standardized, reproducible laboratory assays.
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| THE COLIFORM BACTERIAL HIERARCHY |
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| TOTAL COLIFORM GROUP (Broad Sanitary Indicator) |
| - Gram-negative, non-spore-forming, aerobic / facultatively anaerobic rods |
| - Ferment lactose with acid and gas production at 35.0°C ± 0.5°C within 24 to 48 hours |
| - Genera: Escherichia, Enterobacter, Klebsiella, Citrobacter (Found in feces, soil, and vegetation) |
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| | FECAL (THERMOTOLERANT) COLIFORMS | |
| | - Subset capable of lactose fermentation at elevated temperature of 44.5°C ± 0.2°C | |
| | - Primarily fecal origin; indicates potential wastewater contamination | |
| | +---------------------------------------------------------------------------------------------+ | |
| | | ESCHERICHIA COLI (E. COLI) | | |
| | | - Specific species within fecal coliform group (~97% produce enzyme beta-glucuronidase) | | |
| | | - Definitive, unambiguous proof of mammalian fecal contamination; triggers acute health boil| | |
| | | water advisories under RTCR | | |
| | +---------------------------------------------------------------------------------------------+ | |
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Revised Total Coliform Rule (RTCR) Compliance Framework
Under CDPHE Regulation 11, public water systems collect distribution compliance samples based on population served (e.g., minimum 1 to 480+ samples/month). All compliance testing is performed on $100\text{ mL}$ sample volumes:
- Maximum Contaminant Level Goal (MCLG): Zero ($0$) for E. coli and total coliform.
- Presence/Absence (P/A) Determination: Any total coliform-positive sample triggers mandatory repeat sampling (within 24 hours: original tap, within 5 service connections upstream, within 5 connections downstream) and analysis for E. coli.
- Treatment Technique Triggers: An E. coli positive repeat sample or coliform hits exceeding 5.0% of monthly samples triggers a mandatory Level 1 or Level 2 Sanitary Assessment across the distribution system.
2. Defined Substrate Technology: Colilert & Quanti-Tray Analysis
Defined Substrate Technology (DST) (EPA-approved Standard Methods 9223B / Colilert method) simultaneously detects and enumerates Total Coliforms and E. coli within 24 hours (or 18 hours for Colilert-18) without requiring secondary confirmatory broths.
Dual Biochemical Enzyme Mechanisms
The reagent medium supplies nutrient substrates linked to specific chromogenic and fluorogenic indicators as the sole carbon sources:
- Total Coliform Detection via ONPG ($\beta\text{-galactosidase}$): All coliform bacteria express the enzyme $\beta\text{-D-galactosidase}$, which hydrolyzes the colorless substrate ONPG ($o\text{-nitrophenyl-}\beta\text{-D-galactopyranoside}$), releasing yellow $o\text{-nitrophenol}$.
- E. coli Detection via MUG ($\beta\text{-glucuronidase}$): Escherichia coli uniquely produces the intracellular enzyme $\beta\text{-D-glucuronidase}$ (present in $> 97%$ of strains). This enzyme hydrolyzes the fluorogenic substrate MUG ($4\text{-methylumbelliferyl-}\beta\text{-D-glucuronide}$), releasing $4\text{-methylumbelliferone}$.
Colilert Analytical Interpretation Summary:
- Clear / Colorless Vessel: NEGATIVE for Total Coliform and E. coli
- Yellow Vessel (Equal to or darker than Ref): POSITIVE for Total Coliform
- Yellow + Bright Blue Fluorescence at 365 nm: POSITIVE for E. coli
Quanti-Tray Most Probable Number (MPN) Enumeration
When quantitative bacterial counts are required (e.g., surface water source characterization or wastewater discharge compliance), the $100\text{ mL}$ Colilert mixture is poured into a 51-well or 97-well Quanti-Tray and heat-sealed in an automated sealer. Trays are incubated at $35.0^\circ\text{C} \pm 0.5^\circ\text{C}$ for 24 hours.
- Count the number of yellow wells (Total Coliform).
- Count the number of yellow wells that fluoresce under a $365\text{ nm}$ ultraviolet lamp (E. coli).
- Cross-reference the positive well counts with the standardized MPN Generating Table to establish the statistical Most Probable Number per 100 mL (MPN/100 mL) with 95% statistical confidence limits.
3. Membrane Filtration (MF) Methodology
Membrane Filtration (Standard Methods 9222) provides direct visual colony enumeration by passing a liquid sample through a sterile micro-porous membrane filter.
Equipment & Filtration Mechanics
- Apparatus: Sterile magnetic filter funnel, vacuum flask, and vacuum pump ($< 25\text{ in. Hg}$ / $\approx 0.7\text{ bar}$ to avoid cell lysis).
- Membrane Filter: Sterile, gridded cellulose ester membrane disc, $47\text{ mm}$ diameter, with a calibrated $0.45\text{ }\mu\text{m}$ nominal pore size. Bacteria are physically retained on the grid surface.
[ 100 mL Sample ]
|
v
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| Sterile Magnetic Funnel |
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| 0.45 um Gridded Membrane | <--- Bacteria Retained on Grid
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|
v (Vacuum Filtration)
[ Filtrate ]
Analytical Protocols for Coliform Media
- Total Coliform on m-Endo Medium (SM 9222B):
- Place membrane on an absorbent pad saturated with m-Endo broth (or m-Endo agar) in a $50\text{ mm}$ Petri dish.
- Invert dish and incubate at $35.0^\circ\text{C} \pm 0.5^\circ\text{C}$ for $24 \pm 2\text{ hours}$.
- Colony Appearance: Total coliform bacteria ferment lactose to produce acetaldehyde, which reacts with sodium sulfite and basic fuchsin to form red colonies with a distinctive golden-green metallic sheen.
- Ideal Counting Range: 20 to 80 coliform colonies per membrane.
- Fecal Coliform on m-FC Medium (SM 9222D):
- Place membrane on m-FC broth containing rosolic acid inhibitor.
- Seal Petri dishes in watertight plastic bags and submerge in a thermostatically controlled circulating water bath at $44.5^\circ\text{C} \pm 0.2^\circ\text{C}$ for $24 \pm 2\text{ hours}$.
- Colony Appearance: Fecal coliforms produce distinct blue colonies (non-fecal organisms appear gray or cream-colored).
- Ideal Counting Range: 20 to 60 fecal coliform colonies per membrane.
Membrane Filtration Calculation Formula
Worked Example 9.4.1: Membrane Filtration Fecal Coliform Calculation
A wastewater laboratory analyst filters a series of final effluent dilutions through $0.45\text{ }\mu\text{m}$ filters and incubates them on m-FC media in a $44.5^\circ\text{C}$ water bath for 24 hours. The $10.0\text{ mL}$ sample volume produces 82 blue colonies (too numerous to count reliably), while the $5.0\text{ mL}$ volume produces 34 distinct blue colonies. Calculate the fecal coliform density per $100\text{ mL}$.
Step 1: Select the statistically valid membrane: The ideal counting range on m-FC is 20 to 60 colonies. The $5.0\text{ mL}$ filter with 34 colonies is within the ideal range.
Step 2: Calculate density:
4. Multiple Tube Fermentation (MTF) / MPN Method
Multiple Tube Fermentation (Standard Methods 9221) is the traditional statistical broth method, indispensable for highly turbid, colored, or clay-laden waters that would clog $0.45\text{ }\mu\text{m}$ membrane filters.
- Presumptive Phase: Inoculate series of Lauryl Tryptose Broth (LTB) fermentation tubes containing inverted Durham vials. Incubate at $35.0^\circ\text{C} \pm 0.5^\circ\text{C}$ for $24–48\text{ hours}$. Gas formation (bubble in Durham vial) and turbidity indicates a presumptive positive.
- Confirmed Phase: Transfer an inoculum loop from positive LTB tubes into Brilliant Green Bile (BGB) 2% Lactose Broth (for Total Coliform at $35.0^\circ\text{C}$) or EC Medium (submerged in a $44.5^\circ\text{C} \pm 0.2^\circ\text{C}$ water bath for Fecal Coliform). Gas within 24 to 48 hours confirms coliform presence.
- Completed Phase: Streak positive confirmed tubes onto m-Endo agar and perform Gram staining (confirms Gram-negative, non-spore-forming bacilli).
5. Microbiological QA/QC & Laboratory Safety
Microbiological testing requires rigorous quality control to verify media sterility, specificity, and equipment performance.
- Positive Culture Controls: Test each new lot of media with known bacterial strains: Escherichia coli (ATCC 25922, produces yellow ONPG and UV fluorescence) and Enterobacter aerogenes (produces yellow ONPG without fluorescence).
- Negative Culture Controls: Test each lot with Pseudomonas aeruginosa (ATCC 27853) or Proteus vulgaris to verify that non-coliform bacteria do not produce false-positive reactions.
- Sterility Blanks: Process sterile dilution water through the filtration apparatus at the start, middle, and end of every filtration series. Sterility blanks must yield zero ($0$) colonies.
- Autoclave Sterilization Validation: Autoclave media, sample bottles, and biohazard waste at $121^\circ\text{C}$ ($250^\circ\text{F}$) at $15\text{ psi}$ for a minimum of $15\text{ minutes}$. Validate monthly with $Geobacillus stearothermophilus$ spore ampoules.
In the Defined Substrate (Colilert) test, what specific biochemical reaction confirms the definitive presence of Escherichia coli?
When performing membrane filtration for Fecal Coliforms on m-FC medium, what incubation parameters and colony appearance are required?
What is the standard operating temperature, pressure, and minimum cycle time required for laboratory autoclave sterilization of bacteriological media and glassware?