Microbial hazards in plant tissue and cell cultures

A wide range of microorganisms (filamentous fungi, yeasts, bacteria, viruse s and viroids) and micro-arthropods (mites and thrips) have been identified as contaminants in plant tissue cultures. Contaminants may be introduced w ith the explant, during manipulations in the laboratory or by micro-arthrop od vectors. Contaminants may express themselves immediately or can remain l atent for long periods of time. This often makes it difficult to identify t he source of contamination. Disinfection protocols have now been developed for a wide range of plant species including those infected with viruses/vir oids or endophytic bacteria. They may include the selection of pathogen-fre e donor plants or donor plant treatments such as thermotherapy. Also microb iological quality assurance systems (e.g. Hazard Analysis Critical Control Point; HACCP procedures) have been adapted to the needs of commercial plant tissue culture laboratories. These are aimed at preventing the introductio n of pathogens into tissue cultures at establishment and in the laboratory. In established in vitro cultures preventative strategies have proved to be essential, since it is extremely difficult to eliminate environmental bact erial and fungal contaminants using antibiotics and fungicides. In many cas es anti-microbial treatments only inhibit contaminants and low levels of co ntamination persist. In particular, the use of antibiotics against Gram-neg ative bacteria (including plant pathogenic bacteria and Agrobacterium tumef aciens vector systems used in genetic engineering) has been shown frequentl y to be extremely difficult or unsuccessful. Detection of latent contaminat ion may involve the use of general and semi-selective microbial growth medi a or serological and PCR-based molecular techniques for specific pathogens. However, it is often difficult to detect low numbers of latent bacterial c ontaminants (e.g. levels present following antibiotic treatment or when aci dified plant media are used). This poses a particular risk in the productio n of transgenic plants where the elimination or detection of Agrobacterium tumefaciens-based vector systems cannot be guaranteed with the currently av ailable methodologies. Recent research has also shown that there is a risk of the transmission of human pathogens in plant tissue cultures.