Tryptic soy agar (TSA) is a general-purpose growth medium used in microbiology labs to cultivate a wide variety of bacteria and fungi. It supports both Gram-positive and Gram-negative organisms, making it one of the most commonly used solid media in clinical, pharmaceutical, and research settings. You’ll sometimes see it called tryptone soya agar, CASO agar, or soybean-casein digest agar.
What’s in Tryptic Soy Agar
TSA has a simple formula with just four ingredients dissolved in one liter of water:
- Casein peptone (pancreatic): 15.0 g
- Soy peptone (papaic): 5.0 g
- Sodium chloride: 5.0 g
- Agar: 15.0 g
The casein peptone comes from milk protein broken down by pancreatic enzymes, while the soy peptone comes from soybean meal broken down by papain (a plant-derived enzyme). Together, these two digests supply amino acids, short-chain peptides, and other organic nitrogen that microorganisms need to build proteins and grow. The natural sugars present in the soy peptone serve as the primary energy source. Sodium chloride maintains osmotic balance so bacterial cells don’t burst or shrink, and agar provides the solid gel matrix that lets colonies form on the surface.
Why It Supports So Many Organisms
TSA is considered a “non-selective” medium, meaning it doesn’t favor one type of microbe over another or contain inhibitors that block specific species. The combination of animal-derived and plant-derived peptones creates a nutritionally rich environment broad enough to feed bacteria with very different metabolic needs. Common organisms grown on TSA include Escherichia coli, Staphylococcus aureus, and Moraxella species, along with many environmental bacteria and some fungi.
That versatility is exactly why labs reach for TSA as a starting point. If you need to check whether a sample contains living microorganisms, or you want to grow out whatever is present without biasing toward a particular species, TSA is a reliable first choice.
Where TSA Falls Short
The tradeoff for being broadly nutritious is that TSA doesn’t contain everything. Some fastidious organisms, those with more demanding nutritional requirements, won’t grow on plain TSA. A key example is Haemophilus species, which need two specific growth factors called X factor (hemin) and V factor (NAD). TSA lacks both, so labs either add these factors as test strips on the plate surface or modify the medium entirely.
Adding 7% sterile blood to molten TSA (cooled to about 45°C) creates blood agar, which supports many fastidious bacteria and also lets you observe hemolysis patterns, where bacteria break down red blood cells in characteristic ways that help with identification. TSA can also serve as the base for chocolate agar, where the blood is heated until it turns brown, releasing those X and V factors that Haemophilus needs.
How It’s Prepared
Most labs either buy pre-poured plates or prepare TSA from a dehydrated powder. To make it from powder, you dissolve 40 grams of the dehydrated mixture in one liter of distilled water, then sterilize it in an autoclave at 121°C for 15 minutes. The target pH after sterilization is 7.3 ± 0.2, which is close to neutral and mimics the internal environment of most bacteria. If the pH drifts outside the acceptable range, it can be adjusted with small amounts of sodium hydroxide or hydrochloric acid before pouring.
Once sterilized, the molten agar is cooled to around 45 to 50°C and poured into sterile Petri dishes. At room temperature, it solidifies into a firm, translucent, amber-colored gel ready for inoculation.
Storage and Shelf Life
Commercially prepared TSA plates typically have a shelf life of 30 to 90 days, depending on how they’re packaged. The biggest threat to stored plates is desiccation: as moisture evaporates through the dish, the agar surface dries out and can no longer support growth. Wrapping plates in plastic and storing them at 2 to 8°C in the dark is the standard recommendation to slow moisture loss.
Packaging makes a significant difference. In one study, TSA plates sealed in vacuum-flushed nylon/polyethylene packaging and held at room temperature supported normal bacterial growth for up to 24 months with no visible change in appearance or texture. In contrast, conventionally packaged plates from a different supplier desiccated within four months at room temperature and within seven days at 60°C. If you’re working in a field setting or storing plates outside a refrigerator, the packaging format matters more than the medium itself.
Common Lab Applications
TSA serves several routine purposes in microbiology. It’s used for general culture maintenance, keeping bacterial strains alive for future experiments. It’s used for enumeration, spreading diluted samples across the plate and counting individual colonies to determine how many viable cells were in the original sample. And it’s used for isolation, streaking mixed samples to separate individual species into pure cultures.
In pharmaceutical and manufacturing environments, TSA plates play a specific regulatory role. The formulation meets harmonized standards set by the United States Pharmacopeia (USP), European Pharmacopoeia (EP), and Japanese Pharmacopoeia (JP) for microbial limit testing. Pre-poured TSA settle plates are used to monitor air quality in cleanrooms by exposing them for a set time and counting whatever lands on the surface. They’re also pressed against gloved fingertips to check whether personnel are introducing contamination into controlled environments.
TSA vs. Tryptic Soy Broth
Tryptic soy broth (TSB) contains the same nutritional base as TSA but without the agar, so it remains liquid. The choice between them depends on what you need to accomplish. TSA gives you a solid surface where individual colonies can form, which is essential for isolation, counting, and observing colony shape or color. TSB is better suited for growing large volumes of bacteria quickly, since cells suspended in liquid have more access to nutrients and multiply faster. TSB is also used when you need to enrich a sample, growing a small number of organisms up to detectable levels before transferring them to a solid medium for identification.
Some specialized applications call for modifying TSB with additional ingredients. Adding extra dextrose, sodium chloride, and agar to tryptic soy broth is a recommended approach for cultivating Salmonella Typhi, for instance. The base formula is flexible enough that labs can adjust it for many different purposes while keeping the core nutritional profile intact.