Various microorganisms can contaminate your cell culture including bacteria, fungi, yeasts and mollicutes (eg. Mycoplasma). These contaminants are a major concern for scientists as they can negatively affect your cells and experiments, meaning you might have to redo your recent experiments to ensure relevant results are collected. Early detection of contaminants is therefore key. Some contaminants are more easily detected than others. For example, bacteria, fungi and yeasts grow quickly and can sometimes be observed by light microscopy. These contaminations can also cause changes in pH or turbidity of the growth medium, and changes in cell morphology. Other contaminating microorganisms such as Mycoplasma are not visible by microscopy nor by visible changes of the growth medium even at very high concentrations.