Pharmaceutical Microbiology: Growth Promotion Test

Growth Promotion Test (GPT): Challenges & Solutions

Ashutosh Mohan

1/17/20251 min read

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Why the Growth Promotion Test is considered to be crucial?

A Growth Promotion Test (GPT) is performed on each new lot of Microbial Culture Media received from the suppliers or manufacturers. Regardless of the nature of media whether it is Dehydrated or ready to use, any new lot/batch of media should undergo a GPT.

All the critical microbiological assays in the Pharmaceutical industry heavily rely on Microbial Culture Media to perform their routine but crucial tests like Sterility, Non-sterility, EM, Media Fill, AET/PET, Antibiotic assays, etc. Here, GPT plays a huge role in identifying: Inferior quality media, lot-to-lot variations, etc.

Key Points for GPT

  • All the microbial culture media (Dehydrated or ready to use) must undergo GPT

  • Always acquire the certified reference cultures from the recognized culture collection (mentioned by regulatory bodies like USP/EP/JP, ISO, etc.) to avoid observation

  • Passage numbers should not exceed 05 passages (due to the possible risk of mutation)

  • Use inoculum containing 100 CFU or less for GPT, and indicative test. Whereas, use at least 100 CFU for an inhibitory test challenge

  • GPT can be Qualitative (Presence or absence) or Quantitative (Enumeration)

  • For Qualitative Tests, growth should be compared to a previously approved lot. (On Solid or Agar plates verify phenotypic, morphological or Biochemical characteristics etc., whereas for liquid or broth, check for turbidity)

  • For Quantitative Tests, growth should be within Factor of 2. Ex: If the mean CFU value for a particular organism is 30 CFU. Then applying Factor of 2 the value will be calculated as 30/2= 15 CFU & 30X2= 60 CFU. Thus, the GPT value for the new lot media should be within the mentioned lower and upper range obtained from Factor of 2.

  • Include Environmental isolates in GPT to verify whether the utilized media is capable of growing these cultures

Factors influencing GPT performance

  • Analyst qualification, experience, and training

  • Media (media components, pH, preparation, Brand to brand, lot-to-lot variation)

  • Instruments (Qualification, & Calibration)

  • Temperature & duration of incubation

  • Management (Budget, human resources, etc.)

  • Storage temperature of Reference culture

Author

Ashutosh Mohan, Certified Six Sigma Black Belt, & cGMP

Founder & CEO, Consult MicroBio

Email: ashutosh@consultmicrobio.com