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Clinical Microbiology Reviews, 10 1996, 489-498, Vol 9, No. 4
JR Stringer
The identity of Pneumocystis carinii has been uncertain for many years.
Until recently, it was widely regarded to be a protozoan because it does
not grow in culture and is not susceptible to antifungal drugs. Although
these and a number of other phenotypic characteristics of P. carinii differ
from those of typical fungi, analysis of DNA sequences has shown that P.
carinii is a member of the fungal lineage of eukaryotes. However, a close
phylogenetic relative of P. carinii has not yet been found. Analysis of
gene sequences has also revealed that P. carinii is not a single entity but
that the genus Pneumocystis contains a complex group of organisms. P.
carinii organisms from one host species do not grow when introduced into
another host species, and P. carinii isolates from different host species
are more genetically divergent from one another than might be expected for
members of the same species. Genetic variation of a lesser degree also
occurs among P. carinii organisms from the same host species, suggesting
that it may be possible to identify strains and to conduct transmission and
epidemiological studies. Results of early studies exploiting genetic
variation among P. carinii isolates from patients have suggested that
recurrent P. carinii pneumonia may not always be caused by reactivation of
latent organisms, as is commonly believed. However, other features of P.
carinii suggest that this microbe has established a long-term relationship
with its host. A striking new development in this regard is the discovery
of a genetic system that is designed to allow variation in the structure of
a major antigen on the surface of P. carinii.
Copyright © 1996 by the American Society for Microbiology. All rights reserved.
Pneumocystis carinii: what is it, exactly?
Department of Molecular Genetics, Biochemistry and Microbiology, College of Medicine, University of Cincinnati, OH 45267-0524, USA. stringjr@ucbeh.san.uc.edu
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