Pathogenicity

Pathogenic yeasts and fungi from clinical sources are held at the National Collection of Pathogenic Fungi. In general the NCYC holds non-pathogenic yeasts but there are certain exceptions to this and the following comments are included as a guide to users. It must be noted that pathogenicity is not an absolutely defined characteristic and so the following should be used as a guide rather than a definitive account of the subject.

Many kinds of microorganism may become opportunistic pathogens if they gain access to the human bloodstream. If the subject has an impaired immune system the consequences of such invasion are very serious and may be fatal.

The most pathogenic yeasts are certain Candida species and Filobasidiella neoformans, the asexual state of which is called Cryptococcus neoformans. The latter species is not held by NCYC.

Hurley et al (1987) list the pathogenic yeasts of candidosis in probable descending order of virulence for man as: C.albicans, C.tropicalis, C.stellatoidea, C.glabrata, C.krusei, C.parapsilosis, C.guilliermondii, C.viswanathii, Clavispora lusitaniae (Candida lusitaniae) and Rhodotorula mucilaginosa (Rh.rubra). (Ref: 'The Yeasts', Vol 1. ed. A H Rose and J S Harrison. Academic Press, 1987: Chapter 4). However only a few Candida species are markedly pathogenic. Some species of Malassezia, Trichosporon and Geotrichum may also be pathogenic.

A number of other yeast species not mentioned here have also been described as pathogens under particular circumstances and it is advised that a literature search is made if a decision is required on any particular strain or species.

Customers should always employ good microbiological practice when working with yeast cultures and use 'Category 2' containment when working with the Candida strains listed above, or any yeast strain suspected of being a pathogen. Full details can be found in: 'Categorisation of biological agents according to hazard and categories of containment', Advisory Committee on Dangerous Pathogens, HSE Publications, ISBN 0 7176 1038 1. The NCYC will be pleased to offer advice on suitable containment levels.

Please note that the NCYC is prohibited from sending cultures which are regarded as human or animal pathogens, or that are classified as 'dual use' to certain countries.