Yeasts are unicellular eukaryotic microorganisms in the kingdom of Fungi, ranging in size from a few micrometers (typically) up to 40 micrometers (rarely).
Like bacterial contamination, cultures contaminated with yeasts become turbid, especially if the contamination is in an advanced stage. There is very little change in the pH of the culture contaminated by yeasts until the contamination becomes heavy, at which stage the pH usually increases. Under microscopy, yeast appear as individual ovoid or spherical particles, that may bud off smaller particles.
The simulated image below shows adherent 293 cell culture 24 hours after plating that is infected with yeast.
Figure 2.3: Simulated phase contrast images of 293 cells in adherent culture that is contaminated with yeast. The contaminating yeast cells appear as ovoid particles, budding off smaller particles as they replicate.