aeruginosa (though a microbial ID is recommended to confirm). If growth is observed on a Cetrimide Agar plate, it is almost certainly P. aeruginosa and few other microorganisms can tolerate. This agar contains a germicidal compound, Cetrimide, that P. A USP example of this is Cetrimide Agar, used to screen for Pseudomonas aeruginosa in USP. A selective media might be used to exclude organisms if there is a specific species or group that is of interest. In some cases, a general purpose media is not ideal for the test being conducted. TSA can also be incubated anaerobically, via Bio-Bag or specialized incubator, to recover anaerobic organisms. Outside of testing product, TSA is referenced where environmental monitoring is discussed, since the goal is recovery of the broad range of organisms. Other tests, like USP, may start with TSB as an enrichment step, then move to more specialized media for the test to work properly. In the case of USP sterility testing, where any recovery is a potential Out-of-Specification result (simple pass/ fail criteria), general purpose media (TSB and FTM) may be the only type necessary. General purpose medias are used to conduct USP, USP, USP and USP testing. SDB/A has a lower pH to encourage fungal and limit bacterial growth, making it an excellent general purpose fungal media. Because of this property, FTM can grow both anaerobic (like Clostridium and Propionibacterium) and aerobic bacteria. If an anaerobic media is needed, FTM develops an oxygen gradient when left stationary, generating anaerobic conditions at the bottom and increasing oxygen levels at the top. Tryptic Soy-based media provide a range of common nutrients for bacterial and fungal growth, and is referenced throughout the USP. General growth medias mentioned in USP include TSB and TSA (aerobic), Fluid Thioglycolate Media (FTM, anaerobic), and Sabouraud Dextrose Broth and Agar (SDB/A, fungal). If the goal is to recover or grow the widest variety of organisms, a general growth media that provides a wide range of nutrients is the best choice. Liquid media is more conducive to growth, since nutrients, oxygen, and waste products move around more freely, and temperature is more uniform and constant in a fluid.
![tsb medium tsb medium](https://www.threestarbrand.com/wp-content/uploads/2015/04/Trad-Med-Oil-full-range-1536x1536.png)
Liquid media, like TSB, is generally better at growing organisms, and should be used where possible for recovery of microorganism in drug products.
![tsb medium tsb medium](https://us.vwr.com/stibo/bigweb/std.lang.all/52/36/20935236.jpg)
For conducting passive air sampling, a solid TSA plate is easier to handle and leave out in the hood during compounding. Tryptic Soy Agar (TSA) and Tryptic Soy Broth (TSB), for example, have the same nutrient profile and similar ingredients, but TSA is a solid and TSB is a liquid. Microbial growth media can be prepared in both liquid and solid forms, depending on the application. This might include recovery of the broadest range possible of organisms in a sterility test, screening for a specific organism if it would be particularly dangerous in a certain dosage form, or capturing anything in the compounding environment that might find its way into a finished product. The various types of media serve specific purposes based on the goal of the test. The United States Pharmacopeia (USP) references many of these to demonstrate the quality of both compounded drug products and the control of compounding environments. There are dozens of different types of microbial growth media available to Microbiologists, developed for a wide variety of purposes.
![tsb medium tsb medium](https://miro.medium.com/max/2048/0*bCRbl0AbzRSxYfa5.jpg)
James Zellner, Microbiologist and Technical Sales Representative