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Index of Typical Cell Culture Contaminants

Trouble-shooting Guide: If you have a contamination problem, select what you believe to be the organism from the short descriptions below and click on the image to go to a more detailed page.

A photo of bacteria in cell culture under a microscope.


Bacterial contamination is usually manifest by a sudden change in pH, cloudiness in the medium, sometimes with a slight whiteish film on the cell surface of plates, dishes, or on the bottom of bottles of medium that typically dissipates when the vessel is moved. Under low power (100x) spaces between cells will appear granular or you may see very small black dots. In some cases under higher magnification (400x) cocci or “rods” may be distinguished and there may be motibility of the bacteria.

Bacteria can usually be distinguished from media components such as serum protein by the regular particulate morphology of the organism versus an irregular shape in the case of serum protein cryoprecipitates.

A photograph of fungus in cell culture under a microscope.

Fungus (Mold)

Fungi (molds) show up as thin filamentous mycelia and sometimes as denser clumps of spores. Fungi are usually very slow growing but in advanced stages of contamination often overtake a culture as a fuzzy growth (either whiteish or black) that can be seen without a microscope.

A photograph of mycoplasma in cell culture under a microscope.


Mycoplasmas cannot be detected by the naked eye or even by typical light microscopy. Therefore, mycoplasma contamination must be detected by alternative testing methods. The image provided here is of a mycoplasma “colony” after being propagated on special agar. The colony has a typical “fried egg” appearance.

A photograph of yeast in cell culture under a microscope.


Yeasts appear as round or ovoid particles that are smaller than mammalian cells and are mostly seen as “budding” from other yeast particles in chains of two, three, or more. In advanced stages, they can appear as multi-branched chains of particles.