11.4 Microbiological Testing & Quality Assurance

9.3 Microbiological Testing & Quality Assurance

Microbiological compliance testing protects public health by verifying that drinking water and treated wastewater discharges are free from pathogenic enteric microorganisms. Because isolating specific pathogens (such as Salmonella, Shigella, Cryptosporidium, or Norovirus) is complex, hazardous, and costly, water utilities monitor indicator organisms that originate in the intestinal tracts of warm-blooded animals.

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Microbiological Indicator Organisms

Target Bacteria Groups

• Total Coliform Group: Aerobic and facultative anaerobic, Gram-negative, non-spore-forming, rod-shaped bacteria that ferment lactose with acid and gas production within 48 hours at 35.0°C (\pm 0.5^\circ\text{C}). Genera include Escherichia, Citrobacter, Enterobacter, and Klebsiella. Total coliforms serve as an operational indicator of treatment integrity and distribution system cleanliness under the Revised Total Coliform Rule (RTCR).
• Fecal Coliform Group (Thermotolerant Coliforms): A thermotolerant subset of total coliforms capable of fermenting lactose with gas/acid production at an elevated temperature of 44.5°C (\pm 0.2^\circ\text{C}). Fecal coliforms provide a direct link to fecal contamination from warm-blooded animals.
• Escherichia coli (E. coli): A specific species within the fecal coliform group. E. coli is the most definitive indicator of recent fecal pollution and enteric pathogen risk. Under the RTCR, an E. coli positive sample triggers immediate public notification and system evaluations.
• Enterococci: Gram-positive cocci capable of growth in 6.5% NaCl at 45°C. Enterococci serve as the primary fecal indicator organism for marine waters and high-salinity discharge monitoring under EPA Clean Water Act guidelines due to their superior survival rates in saline environments.

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Microbiological Analytical Testing Methods

1. Membrane Filtration (MF) Method

The Membrane Filtration (MF) technique filters a 100 mL sample through a sterile 0.45 (\mu\text{m}) pore size cellulose ester membrane filter disk under vacuum. Bacteria are trapped on the membrane surface, which is placed on a selective nutrient agar or broth pad in a Petri dish.

• Total Coliform Testing (m-Endo Medium): Incubated at 35.0°C (\pm 0.5^\circ\text{C}) for 24 (\pm 2) hours. Total coliform colonies produce a characteristic red to dark red color with a golden-green metallic sheen caused by aldehyde reaction with sodium sulfite/fuchsin in m-Endo. Ideal plate count range is 20 to 80 coliform colonies.
• Fecal Coliform Testing (m-FC Medium): Incubated at 44.5°C (\pm 0.2^\circ\text{C}) for 24 (\pm 2) hours in a sealed plastic bag immersed in a circulating water bath. Fecal coliform colonies develop a characteristic blue color; non-fecal colonies appear gray, buff, or reddish-brown. Ideal plate count range is 20 to 60 colonies.

2. Most Probable Number (MPN) / Multi-Tube Fermentation

The MPN method uses serial dilutions inoculated into glass tubes containing nutrient broth and inverted Durham tubes to trap carbon dioxide gas bubbles.

• Presumptive Phase: Inoculate Lauryl Tryptose Broth (LTB) and incubate at 35.0°C for 24 to 48 hours. Gas formation (bubble in Durham tube) or heavy turbidity indicates a positive presumptive test.
• Confirmed Phase: Aliquots from positive LTB tubes are transferred to Brilliant Green Lactose Bile (BGLB) broth (incubated at 35.0°C for Total Coliforms) or EC Broth (incubated in a 44.5°C water bath for Fecal Coliforms). Gas production within 24 to 48 hours confirms coliform presence. Results are calculated as an MPN index per 100 mL from standard statistical tables based on positive tube combinations (e.g., 5-5-5 or 5-3-1).

3. Defined Substrate Technology (DST) / Colilert

Defined Substrate Technology (DST) (commercially available as Colilert® or Colilert-18®) utilizes target-specific chromogenic and fluorogenic substrate compounds to simultaneously detect Total Coliforms and E. coli in 100 mL samples within 18 to 24 hours at 35.0°C (\pm 0.5^\circ\text{C}).

• Total Coliform Reaction (ONPG): Total coliforms produce the enzyme (\beta)-galactosidase, which hydrolyzes the substrate ONPG (ortho-nitrophenyl-(\beta)-D-galactopyranoside), releasing ortho-nitrophenol and turning the sample yellow.
• E. coli Reaction (MUG): E. coli produces the specific enzyme (\beta)-glucuronidase, which hydrolyzes the substrate MUG (4-methylumbelliferyl-(\beta)-D-glucuronide), producing a fluorogenic compound that fluoresces bright blue under 365 nm ultraviolet (UV) light.
• Formats: Available in Presence/Absence (P/A) single-vial bottles for drinking water compliance, or Quanti-Tray® multi-well trays for MPN enumeration in wastewater effluents.

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Laboratory Quality Assurance & Quality Control (QA/QC)

South Carolina environmental laboratories are regulated under SC DES Regulation 61-81 (State Environmental Laboratory Certification Regulations) to ensure data defensibility and precision.

Chain of Custody (COC) & Sample Integrity

Every compliance sample must be accompanied by a legally binding Chain of Custody (COC) document recording:

1. Unique Sample Identification ID and exact sampling location.
2. Date, time, and collector signature.
3. Sample matrix (potable, surface water, wastewater, sludge).
4. Preservation reagents added (e.g., (\text{HNO}_3), (\text{H}_2\text{SO}_4), (\text{Na}_2\text{S}_2\text{O}_3)).
5. Signatures of all parties transferring and receiving the sample with date/time stamps.

Regulatory Preservation & Maximum Holding Times

Sodium Thiosulfate ((\text{Na}_2\text{S}_2\text{O}_3)) must be added to microbiological sample bottles (100 mg/L) prior to sterilization to neutralize residual chlorine disinfectants.

Essential Laboratory Quality Control Checks

• Method Blanks (Negative Controls): Sterile deionized water processed through identical filtration/incubation steps to verify absence of equipment or reagent contamination.
• Duplicate Samples (Precision): Duplicate sample analysis measures analytical repeatability. Precision is quantified as Relative Percent Difference (RPD): [ \text{RPD (%)} = \frac{|D_1 - D_2|}{(D_1 + D_2) / 2} \times 100% ] where (D_1) and (D_2) are duplicate sample concentrations. Target RPD for routine water parameters is typically < 10% to 20%.
• Laboratory Fortified Matrix / Spikes (Accuracy): Standard reference material added to sample matrix to calculate percent recovery: [ \text{% Recovery} = \frac{\text{Spiked Sample Result} - \text{Unspiked Sample Result}}{\text{Known Spike Concentration}} \times 100% ] Target recovery ranges between 85% and 115%.

SC DES Certification & Equipment Tracking

Under Regulation 61-81:

• Incubator & Water Bath Monitoring: Temperatures must be checked and recorded twice daily at least 4 hours apart using NIST-traceable calibrated glass or digital thermometers. Total coliform incubators must maintain 35.0°C (\pm 0.5^\circ\text{C}); fecal coliform water baths must maintain 44.5°C (\pm 0.2^\circ\text{C}).
• Proficiency Testing (PT): Certified laboratories must analyze blind PT sample sets from an accredited provider twice per year with passing scores for all certified parameters.