What are the major types of biological cell culture contamination?

There are five major types of biological cell culture contamination - Bacterial, Mold, Viral, Mycoplasma, and Yeast. 

Bacterial Contamination - Bacteria are among the most common biological cell culture contaminants. This is attributed to their fast growth rates combined by their size and ubiquity. A cloudy turbid appearance and sudden drop in pH of the culture medium are indications that bacterial contamination has set in. 

Mold Contamination - Mold contamination results when highly resistant mold sports encounter suitable growth conditions and get activated. Increasing turbidity and a sharp increase in pH of the culture is an indication of mold contamination.

Viral Contamination - Viruses have very rigid host requirements and only contaminate very specific species. Viral contamination is difficult to detect because of the extremely small size of viruses. Advanced techniques such as ELISA assays, electron microscopy, PCR with appropriate viral primers, or immunostaining with a panel of antibodies are used to detect viral contamination. 

Mycoplasma Contamination - Mycoplasma are simple bacteria that lack a cell wall and measure less than 1μm. Their small size makes it difficult to detect mycoplasma in the early stages of contamination. Mycoplasma contamination can only be detected when the mycoplasma achieves extremely high densities, resulting in the deterioration of the cell culture. Lower cell proliferation rate, agglutination in suspension cultures and agglutination in suspension cultures are an indication of chronic mycoplasma infections. ELISA, fluorescent staining, immunostaining, PCR, or autoradiography can be used to detect mycoplasma contamination. 

Yeast Contamination - The culture undergoes very little change in the early stages of yeast contamination. As the yeast contamination advances, the culture becomes more turbid and its pH increases.