Are Members of the Fungal Genus Pneumocystis (a) Commensals; (b) Opportunists; (c) Pathogens; or (d) All of the Above?

Members of the genus Pneumocystis are microscopic yeast-like fungi that reside in the lungs of almost every mammal that has been evaluated for their presence. They grow extracellularly in the alveoli of mammals and are considered to be host obligate, as they cannot grow outside the lung on artificial media. Pneumocystis species (spp.) are typically restricted to the lungs, although extrapulmonary manifestations have been reported [1]. Considered to be zoonotic (able to infect more than one mammalian species), these organisms once were referred to as a single genus and species, ‘‘Pneumocystis carinii’’. It is now clear that distinct species of Pneumocystis infect different mammalian hosts. Each mammalian species has at least one species of Pneumocystis that it harbors which cannot infect another mammalian species (species specificity). Pneumocystis spp. have suffered many identity crises, beginning with their initial identification in 1909, when they were thought to be part of the life cycle of the protozoan parasite Trypanosoma cruzi [2]. In 1914, they were provided an identity of their own and given the genus and species designation ‘‘Pneumocystis carinii’’, which reflected their preference for the lung, ‘‘pneumo’’-; their characteristic morphological form, the cyst, -‘‘cystis’’; and ‘‘carinii’’ to honor the Italian investigator, Antonio Carini, who provided the slides for study. Presumed to be a protozoan parasite, the potential fungal nature of P. carinii was first raised in the 1950s, and the controversy over their protozoan or fungal nature continued into the late 20th century [1,2]. Phylogenetic analyses first based on the nuclear small subunit rRNA sequence alignments then with additional gene sequence comparisons showed that the closest extant relatives to P. carinii were the fission yeast Schizosaccharomyces pombe and the plant pathogen Taphrina deformans [3]. Molecular biological techniques have been essential for providing the breadth of knowledge that is currently available for these intractable fungi. Phylogenetic determinations based on gene sequences not only permitted the fungal identity of the genus to be clarified, but provided the basis for species distinctions within the genus. Although it is anticipated that there are thousands of Pneumocystis species, only five have been formally described: P. carinii and Pneumocystis wakefieldiae, which infect rats; Pneumocystis murina in mice; Pneumocystis oryctolagi in rabbits; and Pneumocystis jirovecii, which infects humans [2]. It should be noted that the nomenclature of Pneumocystis has not been without controversy, particularly with the name P. jirovecii [2]. A number of clinician investigators raised concerns about the change from P. carinii to P. jirovecii, which had the potential to cause confusion among patients and clinicians alike [4]. However, validation of the names through the formal guidelines recommended by the fungal community combined with scientific and biological evidence and recommendations by authorities have contributed to a general acceptance of the current nomenclature, including P. jirovecii [5]. Where Are They Found and How Do They Replicate?