Biological contamination in plant cell cultures originates from two sources, namely, from the tissue used to initiate the culture, and from the laboratory environment. Contaminants transferred in or on the plant material include plant pathogens and environmental microorganisms. Most epiphytes are eliminated by surface sterilants; however, these do not eliminate endophytic organisms. Endophytes may be intercellular pathogens, as well as opportunistic and adapted colonizers originating from the environment; or intracellular viruses, viroids, and bacteria. While thermo- and chemo-therapy are used to eliminate pathogens from stock plants, the main strategy used is meristem tip (┐apical meristem┐) culture. The principle of meristem culture is that most endophytes are absent from apical tissue. In some cases, it may be considered desirable to establish cultures from differentiated plant tissues, for example those expressing the production of secondary metabolites or proteins of interest. Use of the latter explants poses a high risk of introducing endophytes into culture.
Aseptic cultures may become contaminated in the laboratory due to failure of sterile technique or equipment malfunction. Laboratory contamination is caused by plant-associated, environmental, and human-associated bacteria; yeasts; and microarthropods. Most microbial contaminants are expressed (i.e. grow) on plant tissue culture media but some may be latent (i.e. not expressed) or subliminal. Detection, and at least partial identification, is a prerequisite for the control and elimination of laboratory contamination. Some microorganisms are biomarkers for system failures. It may not be possible to control fast-growing microorganisms before they overrun the culture but slow-growing, or latent microorganisms, can be treated with antibiotics.
The preventive approach advocated here is based on screening of the stock plants for pathogens and microbial endophytes, establishment of aseptic cultures via meristem explants, and good laboratory practice (GLP) based on Hazard Analysis Critical Control Point (HACCP). The use of antibiotics to treat chronic infections of cell cultures is also discussed.