Growth promotion testing (GPT) of culture media: what, why and how
13 min read · 15 October 2026
Every result in a micro lab depends on the medium. If a batch of agar cannot grow organisms, a "no growth" result means nothing. Growth promotion testing (GPT) proves that each batch of medium, whether bought ready-made or prepared in-house, does its job before you use it.
GPT is one of the first things a new microbiologist learns and one of the most common sources of audit findings. The rules are simple, but they are easy to apply loosely. This guide explains the rules, the reasons behind them, and how to run GPT in a way that stands up to inspection.
Three properties you may need to test
- Growth promotion: the medium supports growth of the specified organisms. Every medium needs this.
- Inhibitory properties: a selective medium suppresses the organisms it should suppress. For example, mannitol salt agar should not grow E. coli.
- Indicative properties: a differential medium shows the expected reaction, such as pink colonies of E. coli on MacConkey agar or yellow zones around S. aureus on mannitol salt agar.
Non-selective media like soybean-casein digest agar need only growth promotion. Selective and differential media used in tests for specified micro-organisms (USP <62>) need two or all three.
The basic rules
- Inoculate with not more than 100 CFU of each organism for growth promotion and indicative tests.
- For inhibitory tests, inoculate with not less than 100 CFU and check that the organism does not grow.
- Use cultures no more than five passages from the reference culture.
- Incubate growth promotion and indicative tests for no longer than the shortest period stated in the test method.
- Incubate inhibitory tests for not less than the longest period stated.
- For solid media, the count must not differ by more than a factor of 2 from the calculated value of the standardised inoculum, which in practice means recovery between 50% and 200%.
- For liquid media, growth must be clearly visible and comparable to a previously tested and approved batch.
The logic is that a weak medium is most likely to show its weakness at a low inoculum and a short incubation. Use a large inoculum or a long incubation, and almost any medium will pass.
Typical organisms and conditions
Media for total counts (USP <61>)
| Medium | Organisms | Incubation |
|---|---|---|
| Soybean-Casein Digest Agar or Broth (TAMC) | S. aureus ATCC 6538, P. aeruginosa ATCC 9027, B. subtilis ATCC 6633, C. albicans ATCC 10231, A. brasiliensis ATCC 16404 | 30–35 °C, not more than 3 days for bacteria and 5 days for fungi |
| Sabouraud Dextrose Agar (TYMC) | C. albicans, A. brasiliensis | 20–25 °C, not more than 5 days |
Media for specified micro-organisms (USP <62>), examples
| Medium | Property | Organism |
|---|---|---|
| MacConkey Broth | Growth promotion / Inhibitory | E. coli ATCC 8739 / S. aureus |
| MacConkey Agar | Growth promotion + indicative | E. coli |
| Rappaport Vassiliadis Salmonella Enrichment Broth | Growth promotion / Inhibitory | Salmonella Typhimurium ATCC 14028 / S. aureus |
| Xylose Lysine Deoxycholate Agar | Growth promotion + indicative | Salmonella |
| Cetrimide Agar | Growth promotion / Inhibitory | P. aeruginosa / E. coli |
| Mannitol Salt Agar | Growth promotion + indicative / Inhibitory | S. aureus / E. coli |
| Reinforced Medium for Clostridia | Growth promotion | C. sporogenes ATCC 11437 |
| Sabouraud Dextrose Broth and Agar | Growth promotion | C. albicans |
Media for sterility testing (USP <71>)
| Medium | Organisms |
|---|---|
| Fluid Thioglycollate Medium | S. aureus, P. aeruginosa, C. sporogenes |
| Soybean-Casein Digest Medium | B. subtilis, C. albicans, A. brasiliensis |
Always check the current pharmacopoeia chapter for your test; the tables above are a summary. Equivalent strains from other culture collections (NCIM, MTCC, NCTC, NBRC) are acceptable if they are listed as equivalent.
Preparing the inoculum
Most labs now use commercial quantitative culture pellets or loops that deliver a known count, which makes it much easier to stay below 100 CFU. If you prepare your own suspensions:
- Subculture from the working stock, staying within five passages.
- Prepare a suspension in sterile saline or buffer and adjust to a known turbidity.
- Make serial dilutions to reach roughly 10–100 CFU per inoculum volume.
- Plate the inoculum on a non-selective medium in parallel to confirm the actual count.
- Use suspensions within the time validated for their stability, often 2 hours at room temperature or 24 hours refrigerated.
Document the inoculum count for every GPT. Without it you cannot calculate recovery.
Environmental and in-house isolates
For EM media, many sites add one or two of their own frequently recovered isolates to the standard strains, for example a Micrococcus from Grade C corridors or a Bacillus from material airlocks. Regulators like this because it proves the medium grows the organisms you actually find.
Keep in-house isolates under the same controls as reference cultures: identified, preserved, with passage numbers recorded. Review the choice each year based on your organism library.
Ready-to-use media
Buying prepared plates and tubes does not remove the need for GPT. The supplier's certificate tells you how the batch performed at their site. It does not show what happened in transport, often in an Indian summer in a truck. Most sites:
- Perform full GPT on every incoming lot, or
- Qualify the supplier with full GPT on several lots, then perform reduced testing on each lot with periodic full testing.
Either approach is acceptable if it is justified and documented. Always inspect incoming media for cracks, contamination, dehydration and correct labelling, and check storage temperature during transport where possible.
In-house prepared media
For media prepared in the lab, GPT is part of a larger set of checks:
- Weighing and water quality records
- pH after sterilisation, within the specified range
- Sterilisation cycle records (time, temperature, load)
- Sterility check of a sample from each batch
- Visual checks: colour, clarity, gel strength, volume
Only release the batch after all checks pass. If media must be used before GPT is complete, for example in an emergency, the test results must be held and only reported once GPT passes. Document this in a risk assessment.
Documentation
Record:
- Medium name, manufacturer and lot number, or preparation batch number
- Preparation or receipt date and expiry
- Organisms with culture collection number and passage number
- Inoculum count
- Incubation temperature and duration, with incubator ID
- Counts on test plates and on the comparison batch or inoculum control
- Calculated recovery
- Pass or fail decision with signatures and dates
Release the batch in your system only after GPT passes, and make sure analysts can see the release status before using the media.
Common mistakes
- Using an inoculum far above 100 CFU, which hides weak media
- Incubating longer than the method's shortest time
- No inoculum control or comparison batch for solid media
- Releasing media for use before GPT results are available, without a documented risk assessment
- Using cultures beyond five passages, or not recording the passage number
- Skipping inhibitory or indicative tests for selective media
- Treating the supplier's certificate as a substitute for testing
Good GPT costs a few plates and a little time. A medium that silently fails can make months of "no growth" results meaningless.
Educational content. Always follow your site's approved SOPs, the current pharmacopoeia and applicable regulations.