Growth Promotion Test (GPT) of Culture Media in Microbiology: Complete Procedure

Growth Promotion Test is a routine quality control activity performed in pharmaceutical microbiology laboratories to confirm that prepared or purchased media can support the expected growth of microorganisms while maintaining the selective or inhibitory properties of specialized media. Conducting GPT helps laboratories generate accurate microbiological results and comply with Good Manufacturing Practices (GMP) and Pharmacopoeial requirements.

Growth Promotion Test (GPT) of Culture Media

Culture media are the foundation of every microbiological analysis carried out in pharmaceutical laboratories. Whether the test involves microbial limit testing, environmental monitoring, sterility testing, or water analysis, the reliability of the results depends on the quality of the culture media being used.

What is a GPT?

A Growth Promotion Test (GPT) is a microbiological quality control test performed to verify that a culture medium can support the growth of specific microorganisms before it is used for routine laboratory testing.

The purpose of GPT is to ensure that the prepared or commercially supplied media retain their nutritional properties after preparation and sterilization. If a culture medium cannot support microbial growth as expected, any microbiological test performed using that medium may produce inaccurate or misleading results.

Related Articles

LAF (Laminar Air Flow) Performance Qualification Procedure

Incubator Complete Qualification Procedure

Objective for Growth Promotion Test

The objective of the Growth Promotion Test is to verify that microbiological culture media are capable of supporting the growth of specified microorganisms under defined incubation conditions before they are used for routine microbiological analysis.

Performing GPT ensures that the media comply with laboratory quality requirements and produce reliable and reproducible microbiological results.

Scope

This procedure applies to all microbiological culture media used in the pharmaceutical microbiology laboratory for routine testing.

Environmental Monitoring

Microbial Limit Testing

Sterility Testing

Water Testing

Finished Product Testing

Raw Material Testing

In-Process Sample Testing

Why is GPT Important?

The test helps laboratories to:

Verify the performance of prepared culture media.
Confirm the quality of newly received media lots.
Detect media deterioration caused by improper storage.
Ensure compliance with GMP and pharmacopoeial requirements.
Prevent false negative microbiological results.

Responsibility involved:

Designation	Responsibilities
Officer/Executive/Designee Microbiologist	Preparing the inoculum. Preparing and sterilizing the culture media. Performing the Growth Promotion Test. Recording observations. Calculating recovery, where applicable. Documenting the test results. Reporting any failure to the concerned department.

Procedure for Growth Promotion Test:

Preparation of Inoculum for Growth Promotion Test

Select the required challenge microorganism according to the type of culture medium being tested.

Prepare a fresh working culture under aseptic conditions.

Perform serial dilutions of the culture until a dilution containing approximately 10 to 100 Colony Forming Units (CFU) is obtained.

Verify the dilution by plate count or an established laboratory method before using it for GPT.

Use the standardized inoculum immediately to avoid changes in microbial viability.

Preparation of Culture Media

Before performing the Growth Promotion Test, prepare the required microbiological media according to the approved laboratory procedure.

The media should be prepared using the recommended quantity of dehydrated media and purified water. After preparation, sterilize the media using validated sterilization conditions.

Before use, check the media for:

Correct appearance and colour
Proper pH (where applicable)
Absence of contamination
Proper sterilization
Appropriate storage conditions

Growth Promotion Test Procedure

Growth Promotion Testing should always be performed in a controlled microbiology laboratory under aseptic conditions.

The exact procedure varies slightly depending on whether the culture medium is solid or liquid.

GPT for Solid Media

Solid culture media can be evaluated by either of the following methods:

Pour Plate Method
Spread Plate Method

Pour Plate Method

Sterilize the required quantity of plates.

Clean and sanitize the LAF as per SOP.

Transfer the plates Under LAF.

Using sterilized tips of micropipette add 1.0ml quantity of culture from the dilution in which 10 to 100cfu/ml are observed

Take the media flask on which GPT to be perform under the LAF.

Allow to cool the media at 45°C .

Pour about 15-20ml media in the plates in which culture is added.

The Petri plates shall be swirled gently such that no air bubble should be formed and the microbial cells distribution was uniform.

After solidification, the plates shall be incubated as per the desired incubation conditions as per challenge organism.

Prepare one plate as negative control.

Spread Plate Method

Take the required quantity of pre-incubated media plates under the LAF

Aseptically inoculated 0.1ml culture having 10-100 cfu/0.1ml (Use the previous dilution of the culture in which 10-100cfu/ml is observed) of selected microorganisms.

Using sterilized spread evenly the suspension to cover the entire surface of the media.

Incubated the plate at specified temperature as per the microorganism.

Growth Promotion Test for Liquid Media

Liquid culture media are evaluated slightly differently because microbial growth is observed as turbidity rather than colony formation.

Prepare the required quantity of liquid media

Transfer the media tubes under previously cleaned and sanitized LAF.

Take the selected culture and its dilution (10-100cfu/ml) and aseptically inoculate this culture in the media tube using sterilized pipette tips.

For the cultures and the selective media

Incubate the tubes at specified temperature for the challenged microorganism.

Also incubate one media tube as a negative control.

After completion of incubation remove the tubes from the incubator and check for the growth.

Good Laboratory Practices During GPT

To obtain reliable results, the following practices should always be followed:

Perform all inoculations under a sanitized Laminar Air Flow cabinet.
Use freshly prepared microbial cultures.
Use sterile pipette tips and laboratory accessories.
Label all plates and tubes before inoculation.
Avoid repeated opening of culture media containers.
Record incubation temperature and duration accurately.
Handle all cultures using aseptic techniques.
Dispose of contaminated materials according to laboratory biosafety procedures.

Recovery Percentage

For general-purpose agar media, recovery should fall between 50% and 200% compared with the inoculated microbial count.

Recovery (%) can be calculated using the following formula:

Parameter	Formula
Recovery (%)	(Observed CFU/Inoculated CFU) × 100

Inoculated CFU	Observed CFU	Recovery
50	48	96%
60	72	120%
40	20	50%

What Should Be Done if the GPT Fails?

If the GPT fails then prepare a fresh batch of the same media from the same lot and repeat the test in triplicate using the recommended challenge microorganisms. Ensure that the inoculum is prepared correctly, the incubation conditions are appropriate, and all testing is carried out under aseptic conditions.

If the repeat test passes: The media can be accepted and released for routine laboratory use, as the initial failure may have been due to an operational error during testing.
If the repeat test also fails: The media should be considered unsuitable for use. The affected batch should be rejected, and the issue should be documented according to the laboratory’s quality management system.

Media, Test Organisms and Acceptance Criteria

Culture Medium	Growth Promotion Organism	Inhibitory Organism	Incubation Temperature	Acceptance Criteria
Soyabean Casein Digest Agar (SCDA)	E. coli, S. aureus, P. aeruginosa, B. subtilis, C. albicans, A. brasiliensis	Not Applicable	30–35°C (bacteria), 20–25°C (fungi)	50–200% recovery
Soyabean Casein Digest Broth	Same as above	Not Applicable	As specified	Visible growth
Sabouraud Dextrose Agar	Candida albicans, Aspergillus brasiliensis	Not Applicable	20–25°C	50–200% recovery
MacConkey Agar	E. coli	S. aureus	30–35°C	Typical pink colonies with inhibition of S. aureus
Cetrimide Agar	P. aeruginosa	E. coli	30–35°C	Characteristic greenish-yellow colonies
Mannitol Salt Agar	S. aureus	E. coli	30–35°C	Yellow colonies with inhibition of E. coli
XLD Agar	Salmonella abony	Not Applicable	30–35°C	Red colonies with or without black centres
Fluid Thioglycollate Medium	S. aureus, P. aeruginosa, Clostridium sporogenes	Not Applicable	30–35°C	Visible growth
R2A Agar	Standard environmental isolates	Not Applicable	30–35°C	50–200% recovery

Change history:

Effective Date	Details of Revision	Change Control No.	Reason For Revision

Reference Used:

Indian Pharmacopoiea General chapter 2.2.9