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Taxonomic status of Pneumocystis

Gerald L. McLaughlin, Ph.D gmclaugh at IUPUI.EDU
Mon Feb 16 13:23:49 EST 1998

Most microbiologists and mycologists, and even the majority of
protozoologists, now place Pneumocystis as an atypical ascomycete.
Historically, perhaps because Pneumocystis trophozoites look by light
microscopy, much like trophs of Plasmodium, some specialists placed
Pneumocystis with the Sporozoa, roughly lumped with what later broke off as
the Apicomplexa, even though they lack an apical complex.  Nonetheless, the
early workers on Pneumocystis were protozoologists rather than mycologists,
also because mycologists hate to work with organisms they can't culture,
while parasitologists do so frequently.  Compounding this confusion were
several differences in drug susceptibity from normal fungi, including folic
acid pathway inhibitors and C14 sterol inhibitors.  Once rDNA sequencing
(and sequence analysis of many other proteins) placed Pneumocystis with the
fungi, other characteristics (glucan wall, silver stain positive for
"cysts") that were fungal in nature were recognized as such.  There remains
a finite possibility that among the sometimes-estimated 500,000 species of
fungi and the similarly numerous "lower fungal-like protists" sometimes
called the archaeomycetes, for many variants, since there is little sequence
data from free-living relatives.  So far, Pneumocystis clearly looks like a
reasonably advanced fungus by sequence analyses.  A lung-dwelling microbe
without recognized sexual or mycelial stages, makes Pneumocystis one of the
odder fungi; a fungus in all likelihood and now classified that way by NIH
and recent taxonomic refrence books and reviews (Bergey's...), but still
worked on in part, by nostalgic protozoologists.
Terminology probably should be changed; e.g. trophs and cysts are not terms
appropriate for fungi; but we actually don't know what the correct fungal
stages or terms are!  If the cyst is an ascus and is the troph is a gamete,
this would be the first warm-developing such stages for a fungus. If the
mycelial form exists in the environment, what does it look like?  One of the
taxonomically closer-related fungi, is an ascomycete pathogen of peach
trees.  We also think that Pneumocystis is spread by an airborne route, as
well as remaining cryptic in lungs until immunosuppression. 


At 04:18 PM 2/16/98 +0000, James Mahaffy wrote:
>It had been my impression that Pneumocystis had definitely been moved
>to the fungi.  But the latest edition of Roberts and Janovy 1996 list it
>under the Family Cryptosporididae with its "taxonomic position still
>undetermined" and some discussion on why it is sometimes classified as a
>It is not critical, but I will be talking about the chap Wed to my
>Zoology Class and it would be helpful to know if it has been removed
>from the protistids. 
>James F. Mahaffy                   e-mail: mahaffy at dordt.edu
>Biology Department                 phone: 712 722-6279
>Dordt College                      FAX 712 722-1198
>Sioux Center, Iowa 51250
Gerald McLaughlin, Ph.D.

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