SOP: Enterobacteriaceae Enumeration
The Enterobacteriaceae family includes the coliforms plus Salmonella, Shigella, and friends, so its count is a broader hygiene indicator than coliforms alone. Dried infant formula and spices are tested for it routinely because the family flags process failures.
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 Enterobacteriaceae by selective plating with overlay for hygiene monitoring.
Scope
Applies to dried products, spices, infant formula, and ready-to-eat foods.
Equipment and Materials
- Violet red bile glucose (VRBG) agar
- Incubator at 37 C
- Stomacher, diluent blanks, pipettes
- Confirmation: oxidase negative and glucose fermentation
- Colony counter
Procedure
- Prepare the 1:10 homogenate and decimal dilutions.
- Pipette 1 mL into duplicate petri dishes and pour VRBG agar cooled to 44 to 46 C.
- After solidification, overlay with a thin layer of VRBG agar to suppress spreading and create semi-anaerobic conditions.
- Incubate at 37 C for 24 plus or minus 2 hours.
- Count purple-red colonies at least 0.5 mm in diameter, with or without precipitation halos.
- Confirm at least 5 typical colonies: oxidase negative and glucose fermentation positive.
- Calculate the confirmed count from the proportion confirmed.
Recording Results
Report CFU per g to two significant figures. For infant formula, any detection may trigger action per the specification; know your product limits.
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
- Pale or tiny colonies: under-incubation or agar too hot at pouring. Verify time and tempering.
- Spreading despite overlay: overlay too thin. Pour a full second layer of 5 mL.
- High oxidase-positive background: confirm a larger proportion of colonies.
- 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
- The family includes pathogens. Treat plates as Biosafety Level 2.
- Autoclave before disposal.