SOP: Coliform Count by Petrifilm

Petrifilm plates shrank the coliform test from a bench full of glassware to a stack of cards. The built-in indicator dyes turn coliform colonies red with gas bubbles, so counting takes minutes. Plants use them for fast hygiene checks on lines and products.

Microbiological testing protects consumers from pathogens and tracks hygiene across the plant. Because living organisms are the analyte, aseptic technique and precise incubation conditions are not optional extras; they are the measurement itself. A single lapse in sterility or temperature control invalidates the result.

Purpose

To enumerate coliforms rapidly using 3M Petrifilm Coliform Count plates for in-process hygiene monitoring.

Scope

Applies to dairy, beverages, produce wash water, and environmental samples. For regulatory testing, verify the method is accepted in your jurisdiction.

Equipment and Materials

• Petrifilm Coliform Count plates (check expiry and storage)
• Sterile diluent blanks and 1 mL pipettes or syringes
• Petrifilm spreader
• Incubator at 35 plus or minus 1 C
• Stomacher and sterile bags

Procedure

1. Prepare the 1:10 homogenate and decimal dilutions.
2. Place the Petrifilm on a flat surface and lift the top film.
3. Pipette 1 mL of the appropriate dilution onto the center of the bottom film.
4. Lower the top film and press the spreader gently to distribute the sample over the growth area. Do not slide the spreader.
5. Leave undisturbed 1 minute for the gel to solidify.
6. Incubate plates clear side up at 35 C for 24 plus or minus 2 hours, in stacks of no more than 20.
7. Count red colonies with associated gas bubbles as coliforms. The grid aids counting; estimate when colonies are numerous.
8. Count plates with 15 to 150 colonies for best accuracy.

Recording Results

CFU per g or per mL equals average count times dilution factor. Report to two significant figures. Record plate lot numbers.

Quality Control

Verify each new lot of culture media with reference strains before use: the target organism must show typical growth and morphology, and a non-target strain must be inhibited or show atypical growth as expected. Include a positive control (reference strain) and a negative control (sterile diluent) with every test batch. Incubate an uninoculated plate from each poured batch as a sterility check. Monitor incubator temperatures twice daily with a calibrated thermometer; do not use incubators found outside tolerance until corrected and re-verified. Track contamination rates in blanks; a rising blank contamination rate signals a systemic problem requiring investigation. Participate in proficiency testing at least annually and investigate any unsatisfactory scores with documented corrective action.

Troubleshooting

• Gel not solidifying: plates expired or stored badly. Check storage at or below 8 C and expiry dates.
• Sample spreading beyond the growth area: too much pressure on the spreader or plates not flat. Use gentle even pressure.
• Gas bubbles confused with colonies: only count red colonies associated with bubbles; random bubbles are artifacts.
• High background: contaminated diluent. Prepare fresh blanks.
• Incubator temperature alarm overnight: assess all plates incubated during the excursion; document the impact decision and repeat affected tests.
• Reference strain atypical: the culture may have mutated or been mislabeled. Obtain a fresh certified strain and re-verify media.

Interferences and Limitations

Injured or stressed cells may fail to grow on selective media, causing false negatives; the resuscitation steps in this procedure address this but cannot recover all injured populations. Viable-but-nonculturable organisms are not detected by culture methods. High background flora can mask target colonies on plating media, which is why confirmation of suspect colonies is mandatory rather than optional. Antimicrobial residues in the sample (sanitizers, preservatives) suppress growth and must be neutralized during homogenization. Results apply to the analytical unit tested; heterogeneous contamination requires larger or composite sampling plans.

Safety Notes

• Treat used plates as microbiological waste; autoclave or disinfect before disposal per local rules.
• Do not use expired plates for compliance testing.