Is Your Liquid Culture Contaminated? How to Test Cultures on Agar

By Edward Grand, PhD — mycologist and fungal geneticist

If you're wondering whether your liquid culture is contaminated, looking at the syringe usually won't give you a reliable answer. Healthy mycelium and bacterial contamination can look remarkably similar in liquid. The practical way to check a culture before you commit it to grain is to put a few drops on agar and watch what grows.

The rest of this guide covers why that is, how to run the test, and what each result actually looks like on the plate — healthy mycelium, bacteria, yeast, green mold and the rest.

It has never been easier to obtain genetics. Liquid culture, spore prints and swabs, Petri plates, all a few clicks away. But depending on where those genetics came from, you may want to verify the integrity and cleanliness of the culture before you commit it to a grow.

This is why we need agar.

You cannot tell by looking at the syringe

Let's deal with this first, because it's the question everyone actually arrives with.

You are holding a syringe of liquid culture. It looks a bit cloudy. There are stringy bits drifting around in it. Is that mycelium, or is that a problem?

You can't tell. Neither can anyone else.

Healthy liquid culture is supposed to look like that. The cloudiness is suspended mycelium — that's the product working as designed. Wispy strands, a bit of sediment, a slightly off-white tint: all of that is perfectly normal in a good culture.

And all of that is also perfectly normal in a culture that's been overrun by bacteria.

This is the trap. The appearance that means "this is ready to use" and the appearance that means "this will destroy your grow" are the same appearance. People stare at syringes trying to read tea leaves, convince themselves it looks fine, and inoculate anyway. Then three weeks later a bag of grain goes sour and they blame the bag.

There are a few cases obvious enough to call — a culture that's gone frankly yellow or orange, that smells sour through the cap, that's separated into layers. Throw those out. But the dangerous ones aren't the obvious ones. The dangerous culture is the one that looks completely fine.

So if you can't read it in the syringe, where can you read it?

Why agar is the cultivator's diagnostic tool

Agar is a gelatinizing substance that turns liquid media into a solid, two-dimensional surface. That flat surface makes observing mycelium — and any contaminants riding along with it — far easier than it would ever be inside a syringe or a bag of grain.

Everything that was invisible suspended in liquid becomes a discrete, countable, identifiable colony on a plate. Bacteria that were just "cloudiness" become shiny wet dots. A mold spore that was nothing at all becomes a green colony you can see from across the room.

Agar has myriad uses in mycology:

• Germinating spores from prints and swabs
• Expanding mycelium to use as inoculum
• Sharing genetics between growers
• Checking a culture for impurities

The simplest forms of agar media — usually just called "agar" — contain a carbohydrate source such as malt extract, honey or potato flakes, a small amount of nitrogenous material such as yeast or peptone, and the agar powder itself.

For our purposes here, that last use is the one that saves grows.

How to test liquid culture on agar

The test itself is almost insultingly simple.

Squirt a small amount of your liquid culture onto an agar plate — a few drops is plenty. You're looking for what grows, not trying to colonize the plate. Using less also makes it easier to spot isolated contaminant colonies against the media instead of having everything overgrow into one mass.

Then wait. Incubate at normal temperature and give it a few days.

That's the whole procedure. One plate, a few drops, a few days of patience. And at the end of it you have an actual answer instead of a guess.

The arithmetic here is not close. A plate costs you about a dollar. A contaminated grow costs you a bag of grain, a bag of substrate, six weeks of calendar time, and the shelf space it occupied while it failed. Testing is the cheapest insurance in the hobby, and it's the single habit that most separates growers who succeed consistently from growers who don't.

Test before you commit. Not after the bag goes bad.

What healthy mycelium looks like

Before you can spot a problem, you need a baseline.

Healthy mushroom mycelium on agar is typically opaque white, grows as an even radiating colony from the point of inoculation, and looks dry and fibrous — fuzzy or stringy, never wet.

Treat that as a reference point rather than a specification. Growth habit varies with species, genetics, media and conditions, and a healthy culture can be lobed, uneven or slow without anything being wrong with it. What matters more than any single feature is knowing what your culture normally looks like on your media — so that a departure from its own baseline is what catches your attention.

That's the picture to hold in your head. Everything below is a deviation from it.

What contamination looks like on agar

Bacteria

The most common contaminant, and the easiest to spot once it's on a plate. After a few days bacteria show up as small, slimy, sometimes shiny dots where the liquid culture was applied.

The giveaway is that they look wet, and they grow as separate round colonies rather than spreading as a connected mat. Mycelium radiates outward as one organism. Bacteria pile up as hundreds of individual dots.

They may be cream, white, tan or yellowish. Color matters less than texture and growth habit. Once you've seen the difference, you won't mistake it again.

This is also exactly the contamination you could never have seen in the syringe. In liquid, a bacterial bloom is just… cloudiness. On a plate, it's unmistakable.

Yeast

Yeast behaves similarly and is often lumped in with bacteria, but it looks distinct enough to learn separately.

Yeast colonies are larger than bacterial colonies — often one to three millimetres — cream-colored, opaque, domed and glossy, with a sharply defined circular edge. They have a slightly waxy or pearly look, like small drops of candle wax on the agar.

What they share with bacteria is the thing that matters: no filaments. A yeast colony has a hard, smooth boundary. Mycelium never does — it always has a fuzzy, feathery edge where hyphae are pushing outward. If the margin is sharp, it isn't mycelium.

Trichoderma (green mold)

If there's one contaminant worth learning by sight, it's Trichoderma. It is aggressive, it is common, and it will outrun mushroom mycelium given any opening.

The hard part is that young Trichoderma is white, and at that stage it looks very much like what you're trying to grow. Growers lose plates and whole grows to this every day — they see white, assume mycelium, and move it forward.

What gives it away is the color change. As the colony matures, the center turns green — usually a distinct forest or olive green — and spreads outward from the middle while the advancing edge stays white. That white ring around a green center is the classic signature. It typically shows up within a few days of the colony appearing.

If you see it, discard the plate. Don't open it near your other work — Trichoderma spores travel readily, and a disturbed colony puts them into the air of the room you work in.

Other molds

Other mycelial contaminants work the same way, and they're trickier early on. During the first few days of growth, they can be extremely difficult to tell apart from true mushroom mycelium.

Patience is the diagnostic. Contaminants such as Penicillium and Aspergillus start to change color as their asexual spores (conidia) mature on the colony — usually somewhere in the 3 to 10 day range. Penicillium tends toward blue-green. Aspergillus ranges from yellow-green through gray-green to nearly black, depending on species. That color shift is your answer.

If a plate looks ambiguous at day three, don't throw it out and don't use it. Wait.

At a glance

The single most useful distinction in that table is the last column. Mycelium grows as one organism spreading outward. Bacteria and yeast pile up as many separate colonies. If what you're looking at has a hard, smooth boundary, it isn't mycelium.

Cryptic contaminants

Some things you simply won't catch. Slow-growing bacteria and viruses can be nearly impossible to detect without specialized media and techniques. This is a real limit of home testing, and it's worth knowing about rather than assuming a clean-looking plate is a guarantee.

When you're not sure

If you're having trouble identifying something, there are many forums and social media groups that will help. Post a clear photo and someone will usually have an answer within minutes. The community is good about this.

Confirming you have mushroom mycelium

Colony morphology

Two growth patterns are worth learning to recognize on agar.

Rhizomorphic growth is stringy and root-like, with defined strands reaching outward across the plate. Tomentose growth is fluffy and cottony, spreading as a diffuse mat.

Many cultivated species shift toward rhizomorphic growth on low-nutrient media — around 1.5% malt extract, for example. This makes sense: on nutrient-deficient media, the fungus forms rhizomorphic conduits as it searches for new food sources. It's the organism foraging.

Neither pattern is inherently good or bad, and it varies by species and by media. But knowing which pattern your culture normally shows on a given medium gives you a baseline — and a culture that suddenly deviates from its own baseline is worth a second look.

Clamp connections under the microscope

Many Basidiomycete (mushroom-forming) fungi show clamp connections when observed under a microscope — small bridges across the septa between cells. A piece of an actively growing colony can be excised and used to make a squash mount. This requires a microscope capable of 400× magnification, which may not be available to the average home grower.

The test only runs one way. If you find clamp connections, you are almost certainly looking at dikaryotic mushroom mycelium. If you don't find them, you have learned nothing — plenty of mushroom-forming fungi never produce clamps at all. Agaricus bisporus, the common button mushroom, is a familiar example. Absence is not evidence of a problem, so don't bin a culture over it.

What to do with a contaminated culture

Honestly? Not much.

It is technically possible to isolate the desired mycelium using techniques such as trenching, sandwiching and antibiotics. These work. The mycelium outruns the contaminant into clean media and you cut from the far side. But they take experience, they take time, and they fail often enough that for most growers the honest answer is to start over with clean material. We'll cover them properly in their own article.

Bag the plate, seal it, and get it out of your workspace.

The real lesson: source clean, start clean

The most important thing for a beginner is to source cultures from reliable sources — or to make your own.

If contaminated liquid culture goes in at the beginning of the process, your chances of success are nearly zero. No amount of good technique downstream rescues a bad starting point.

If pure, vigorous culture goes in at the beginning, the whole process becomes easier and dramatically less frustrating. That one decision, at the very front of the process, does more for your success rate than anything else you'll do.

And you cannot make that decision by squinting at a syringe. Plate it.

Culture is not included with any Spore Sorcery product. We do not sell spores or spore syringes.