Wild Yeast
A single-celled contaminant that often rides in on liquid culture or poorly sterilised grain, shown by cloudiness and a sour, yeasty smell.
What it is
Wild yeast is a single-celled contaminant that often rides in on liquid culture or poorly sterilised grain. It competes with the mycelium and can spoil a culture without an obvious mould.
How to spot it
On agar or in liquid it can look like a cloudy haze, a wet sheen, or small cream to grey colonies, and it often gives off a sour, fermented, or yeasty smell. Liquid culture that smells off is a warning sign.
Prevention
Sterilise thoroughly, keep sugar solutions clean, and test liquid culture on agar before using it, since yeast is easier to see on a plate. Good sterile technique when making and using liquid culture is the main defence.
What to do
Discard any culture you suspect carries yeast rather than risk it spreading to grain. Start again from a clean plate or a trusted culture.