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: microbiologics.com/item-type/Product/product-format/EZ-Accu-Shot.