What is bacterial contamination?

Bacterial contamination looks wet rather than fuzzy: slime, cloudiness or spots, not the fluffy growth or colored spores of mold. Our contamination ID guide has photos.

Where	What you see	Called
Grain spawn	Slimy grey kernels, often at the bottom of the jar; sour smell	Wet spot or sour rot (Bacillus)
Liquid culture	Broth cloudy all the way through, with nothing growing	Bacteria; see our liquid culture guide
Mushroom caps	Pale yellow spots that turn golden yellow or "rich chocolate brown"	Bacterial blotch (Pseudomonas)

The grain row comes from GroCycle (GroCycle) and the blotch row from Penn State, which says the discoloration is superficial, no more than 2–3 mm deep (Penn State Extension). A Cornell Small Farms guide with Fungi Ally says smelling a bag of spawn is a good way to detect bacteria (Cornell Small Farms and Fungi Ally).

Wet spot survives because Bacillus forms dormant endospores that resist heat. GroCycle, a company that sells growing kits and courses, advises soaking grain at room temperature for 12 to 24 hours before sterilizing: the soak makes the endospores germinate, and once germinated they're no longer heat resistant (GroCycle).

Boiling isn't enough. Penn State notes that the spores of botulism bacteria survive 212°F (100°C) and are destroyed at 240°F (116°C), which takes a pressure canner (Penn State Extension). See sterilization vs pasteurization.

Prevent bacterial blotch by keeping water from sitting on caps. Penn State finds spots wherever caps stay wet "for a period of 4 to 6 hours or longer" (Penn State Extension), and GroCycle advises enough air exchange that droplets don't stay on mushrooms (GroCycle). Mist less, add fresh air exchange, and after a soak drain the block for 10 minutes so no water stands in the bag (North Spore help center). GroCycle says early spots usually don't affect taste or texture.

Amber droplets on colonized mycelium are usually exudate, which North Spore calls "natural and harmless" (North Spore help center), not bacteria. Wet slime or a sour smell nearby means bacteria, whatever the color. Overly wet grain is a common cause: our liquid culture guide dries the outside of each kernel before it goes in the jar, because over-hydrated grain invites contamination.

GroCycle says you may be able to scoop out a small patch, but Cornell's farm guide says contaminated spawn "needs to be thrown away" (Cornell Small Farms and Fungi Ally). We toss a sour jar unopened, as our pink oyster grain spawn guide explains.

Hypha's family tree links each jar to the culture it came from, so when several jars go sour you can check whether they share a parent, and the strain view compares contamination rates across strains.