Biochemistry of the colour reaction: Coliform bacteria ferment lactose via heterofermentative glycolysis, producing organic acids (primarily lactic, acetic, formic acids) that drop the local pH below the transition point of basic fuchsin. Basic fuchsin (a rosaniline dye) forms a deep red-pink precipitate at acidic pH, creating the characteristic red colony with pink diffusion halo. Erioglaucine (FD&C Blue No. 1) remains blue at neutral-alkaline pH, providing the contrasting blue background that makes red coliform colonies immediately conspicuous.

✅ Advantages

• Unambiguous visual readout — deep red colonies on a blue background is one of the most visually distinct colony-indicator systems in all of environmental microbiology; minimises subjective interpretation errors
• APHA-approved for 90 years — included in Standard Methods for Water and Wastewater and the APHA Compendium of Methods for the Microbiological Examination of Foods; accepted globally for regulatory-compliant coliform enumeration
• Dual selectivity — oxgall + brilliant green is superior to single-inhibitor media; specifically suppresses lactose-fermenting Clostridia that cause false positives in sewage and faecal samples on less selective media
• Direct plating capability — suitable for direct plating of water and food samples without enrichment step; pour plate method most common
• Double-strength formulation — can be prepared at 2× concentration to accommodate 10 mL water sample volumes while maintaining selective inhibitor concentrations
• Multi-matrix validated — water, wastewater, sewage, food, dairy, and materials of sanitary importance all tested and validated by APHA methodology

⚠️ Limitations

• Highly light-sensitive — the most critical limitation; brilliant green degrades rapidly on direct light exposure, causing medium colour change (blue→purple/red) and loss of selectivity; false-positive results possible if plates are exposed to light
• Short prepared-plate shelf life — must be used within 48 h of preparation; cannot be stockpiled; pour plates fresh before each testing run
• Non-differentiating within coliform group — cannot distinguish total coliforms from faecal coliforms or E. coli specifically; complementary use with MUG agar (AS-1308) or m-FC agar (AS-1405) required for faecal coliform/E. coli confirmation
• Some gram-positive spore formers may break through — rare high-inoculum samples can overcome bile + BG inhibition if food matrix weakens inhibitor concentrations; Gram-positive spore formers may produce interfering colonies
• Overheating degrades indicators — excessive autoclaving or re-melting degrades brilliant green and fuchsin indicators; prepare only once and pour immediately

💧 Drinking Water & Source Water Testing

Primary application: direct coliform enumeration from treated drinking water, raw source water (rivers, lakes, bores), and rainwater tanks per APHA Standard Methods. Prepare medium at single strength (35 g/L) for 1 mL samples via pour plate method. For 10 mL sample volumes, prepare at double strength to maintain inhibitor concentrations. Results directly reportable as total coliforms per 100 mL. Complement with m-FC Agar (AS-1405) for faecal coliform count and MUG Agar (AS-1308) for E. coli specific detection.

🏭 Wastewater & Sewage Monitoring

Standard medium for total coliform enumeration in raw sewage, treated effluent, and wastewater treatment plant process monitoring. The enhanced selectivity of the dual bile + brilliant green system is critical in wastewater applications where high densities of Gram-positive spore formers (Clostridium spp.) would produce false positives on less selective media. Works alongside TGE Agar (AS-1360) for total heterotrophic plate count.

🍽️ Food Safety Coliform Testing

APHA Compendium-referenced medium for total coliform enumeration in food products including fresh produce, ready-to-eat foods, processed meats, and environmental surface samples from food processing facilities. Used in conjunction with VRBG Agar (AS-1376) for Enterobacteriaceae counts and Glucose BCP Agar (AS-1240) for glucose fermentation confirmation of presumptive Enterobacteriaceae isolates.

🥛 Dairy Product Quality Control

Coliform enumeration in raw milk, pasteurised milk, cream, yoghurt, cheese, and powdered dairy products. High coliform counts in dairy indicate post-pasteurisation contamination, inadequate temperature control, or faecal contamination of raw milk. Use alongside Plate Count Skim Milk Agar (AS-1329) for total aerobic count and Letheen Broth Modified (AS-1271) for samples with residual sanitiser.

• Recreational Water Monitoring: Swimming pools, spa pools, natural bathing waters, coastal waters — coliform counts for public health compliance
• Irrigation Water: Agricultural water quality testing for food safety compliance (FSANZ, SFA, USDA HACCP)
• Environmental Surface Sampling: Swab/rinse samples from food contact surfaces, equipment, and processing environments post-sanitation

• Hospital & Healthcare Water: Water quality monitoring in clinical settings (cooling towers, dialysis water, endoscopy unit water)
• Pharmaceutical Water QC: Purified water and water-for-injection microbial monitoring alongside Plate Count Skim Milk Agar (AS-1329)
• MPN Confirmation: Confirmation plating from positive presumptive Lauryl Tryptose Broth MPN tubes in coliform enumeration workflows

Standard strength (1×):
Suspend ~35 g/L. Pour 15 mL agar, add 1 mL water sample. Final volume ≈ 16 mL per plate. Inhibitor concentrations maintained.

Double strength (2×) for 10 mL volumes:
Suspend ~70 g/L. Pour 10 mL double-strength agar + 10 mL water sample per plate. Final = 1× concentration throughout — inhibitors maintained despite high dilution ratio.

Preparation:
Suspend ~35 g/L (1×) or ~70 g/L (2×). Heat to boiling with constant stirring until dissolved. DO NOT overheat or re-autoclave — degrades indicators. Autoclave 121°C/15 min. Cool to 45–50°C in the DARK. Pour immediately.

Inoculation method:
Pour plate: add sample to empty plate, pour 15 mL cooled medium, swirl. Alternatively spread plate method. For MPN confirmation: streak from positive Lauryl Tryptose Broth tubes.

Incubation:
35 ± 0.5°C, aerobic, 24 ± 2 h. Extend to 48 ± 3 h for slow lactose fermenters. Keep plates in dark during incubation — do not expose to direct light.

Result reading:
Count all deep red colonies with pink halos as confirmed coliforms. Report as CFU per mL or per 100 mL. Colourless, pale, or blue colonies are not coliforms.