These flexible, pleomorphic organisms can be as small as 0.2 to 0.3 µm in diameter. They are considered to be the smallest free- living organisms capable of self replication with a genome in the range of only 600-2200 kb. They have become one of the most prevalent and serious microbial contaminants within cell culture systems used in research and industry today.
Characteristically, they can achieve very high densities in cell cultures (107 to 108 organisms/mL) without any visible changes to medium pH or turbidity. Mycoplasma can be grown on artificial media, but most species require very complex, nutritionally enriched media and carefully defined environmental conditions. Some strains are considered to be non-cultivable and therefore, cannot be detected by direct culture methods. There are currently more than 183 species in 8 genera, many of which are pathogenic (The Manual of Clinical Microbiology, 2003). The vast majority of cell culture contaminants belong to only 6 species primarily of human, bovine or porcine origin. Of these six species, M. orale and M. hyorhinis are the most common historically, accounting for over 50% of all mycoplasma contaminated cultures.