SOP: Media Preparation for Micro Labs
Every colony on every plate traces back to how the media was made. Under-sterilized agar grows contaminants, over-autoclaved agar loses selectivity, and wrong pH shifts everything. Media prep is quiet, exacting work that underpins the whole lab.
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 prepare, sterilize, and quality-check microbiological culture media consistently.
Scope
Applies to dehydrated media rehydrated in the lab: plate count agar, selective agars, and broths used in routine food testing.
Equipment and Materials
- Dehydrated media from approved suppliers (check expiry)
- Analytical or top-loading balance
- Deionized water
- Heating stirrer or boiling water bath
- Autoclave with calibrated gauges
- pH meter
- Sterile petri dishes
- Reference cultures for growth promotion testing
Procedure
- Weigh the exact amount of dehydrated medium per the manufacturer formula for the volume needed. Record the lot number.
- Dissolve in deionized water with stirring and gentle heat until fully dissolved. Do not boil selective media longer than needed.
- Check and adjust pH to the specified value (usually 7.0 plus or minus 0.2) before sterilization, using the calibrated pH meter.
- Dispense into suitable containers, leaving headspace. Cap loosely for autoclaving.
- Sterilize at 121 C for 15 minutes. Do not exceed the time; overheating destroys sugars and selectivity.
- Cool agar to 44 to 47 C in a tempering water bath before pouring plates.
- Pour plates on a level surface in a clean area, 15 to 20 mL per 90 mm dish. Let solidify, then dry briefly if needed.
- Label with medium name, batch number, preparation date, and expiry (commonly 4 weeks refrigerated for poured plates).
- Quality check each batch: sterility (incubate 2 uninoculated plates), and growth promotion with reference strains (for example E. coli on VRBG, Staph aureus on Baird-Parker).
Recording Results
Log every batch in the media register: medium, lot, weights, pH before and after, autoclave cycle ID, QC results, and preparer initials. Reject batches failing sterility or growth promotion.
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
- Contaminated sterility plates: autoclave failure or post-sterilization contamination. Check cycle records and pouring technique.
- Reference strain fails to grow: overheated medium or wrong pH. Review the cycle and pH records.
- Soft or weeping agar: incorrect weighing or incomplete dissolution. Re-check the balance and mixing.
- Darkened medium: caramelization from overheating. Discard and remake with a shorter cycle.
Safety Notes
- Autoclaves operate under pressure and steam. Trained operators only; follow the site autoclave SOP.
- Hot agar causes burns. Handle flasks with heat-resistant gloves.