Back to Basics: Best Practices for Growth in Liquid Media

by | Pharmaceutical

Publish Date: November 3, 2022

Microbial culture media is used in many industries to grow, enumerate, and identify microorganisms. Media comes in three different forms: solid, liquid, and semi-solid. The media form used depends on the regulation you are following, the goal of your assay, and the type of product you are testing. Commonly, Growth Promotion Testing (GPT) is performed for liquid media used in Sterility and Microbial Enumeration tests. For those not following the pharmacopoeias, you may find yourself needing to check the quality of a new batch of liquid media you recently received or made. Two commonly used liquid media types are Tryptic Soy Broth (TSB) or Fluid Thioglycollate (FTM). Liquid media can seem tricky to work with because it requires easy-to-forget steps, like venting, that can delay growth and inhibit turbidity. But if you follow a few simple rules, you should be able to use it successfully!

  1. Don’t fill bottles or tubes too full with media. Make sure there is enough head space for proper gas exchange in the bottle or tube.
  2. Maintain sufficient aeration. Loosen the caps or use sterile injection needles plugged with sterile cotton or fitted with a membrane filter.
  3. Check redox indicators before inoculating. If your FTM has the redox indicator resazurin added, make sure no more than 30% of the media has turned pink before use. To reduce the media, use steam or a water bath to heat it until the pink indicator disappears.
  4. Plate in parallel on a non-selective agar plate. Plating in parallel will demonstrate that the media was inoculated with a viable culture. If performing GPT per the pharmacopoeias, it verifies that the liquid media was inoculated with no more than 100 CFU.
  5. Use a traceable and standardized culture. Using standardized cultures can take the guess work out of the culture’s enumeration, as well as its identity and source. You can test your liquid media with confidence knowing the culture used is well characterized.

Growth Promotion Testing of Liquid Media

If you’re wondering where to start for GPT of your liquid media, start with tip #5! Microbiologics EZ-Accu Shot™ products are well characterized, standardized cultures that deliver no more than 100 CFU per 0.1ml inoculum. They provide a fast, convenient, and reliable solution for Growth Promotion Testing, Media Challenge Testing, and more. To learn more about this pre-enumerated control and browse available strains, check our product page:

Written by Kali Sorum

Kali Sorum has been a part of the Technical Support Team at Microbiologics since 2012 and has recently joined the Marketing Team as the Technical Marketing Manager. During her time at Microbiologics, she has provided technical support to Microbiologics’ customers internationally and domestically for our products used as Quality Control reference materials in various industries such as the Pharmaceutical, Food, Cosmetic and Clinical. She addresses how they are used and implemented in Quality Control testing for multiple regulations including but not limited to the USP, BAM, CLSI, FDA, and others. Other duties include investigation of product complaints, planning and executing laboratory investigations, assisting and developing training programs on the use of our products and specific test methods, as well as management of the Technical Support Team. Kali has also been a part of the Internal Audit Team since 2014. Auditing yearly to FDA Part 820, ISO 17025, ISO 13485 and ISO Guide 34 standards. She has a Bachelor of Science degree from Nebraska Wesleyan University in Biology and Pre-Pharmacy as well as a minor in Chemistry. She has been certified as a Registered Microbiologist in Pharmaceutical and Medical Device Microbiology (RM) through the National Registry of Certified Microbiologists (NCRCM) since June of 2016.

You May Also Like


Share This