Pneumocystis is a genus of opportunistic fungal pathogens, which cause a type of pneumonia by proliferating extracellularly in the lung alveoli in immunodeficient mammalian hosts. In immune suppressed patients, such as in those with acquired immune deficiency syndrome and individuals undergoing immunosuppressive treatment for cancer or organ transplantation, Pneumocystis pneumonia (PcP) continues to represent an important cause of opportunistic infection, respiratory failure and death. The epidemiology, taxonomy and life cycle features as well as issues related to the transmission of Pneumocystis organisms, initiation of infection, organism interactions with the host and the pathogenesis of PcP are areas of research activities. Good culture methods are not available for sustained in vitro cultivation of any species of Pneumocystis, which is currently viewed as a major challenge to performing experiments on these organisms and precludes genetic manipulations. However, several studies have revealed important insights into the biology of this group of organisms.

Key Concepts:

  • Pneumocystis represents a genus of opportunistic fungi that can colonise healthy mammals (transient infections) and cause severe pneumonia in immune compromised mammals.

  • Genetic studies reveal closest phylogeny of Pneumocystis to the ascomycetous fungi and molecular and biochemical studies further indicate that Pneumocystis represents a unique and diverse group of organisms.

  • Each species of Pneumocystis infects a specific mammalian host (host speciesā€specific) and phylogenetic analyses suggest that the association of Pneumocystis species with their mammalian host species is among the longest known example of coevolution.

  • Pneumocystis undergoes asexual and sexual reproduction; most organisms within the mammalian lung are haploid organisms.

  • Pneumocystis organisms attach to type I lung epithelial cells and proliferate extracellularly in the alveolar spaces.

  • PcP is characterised by intense tissue inflammation that leads to respiratory insufficiency, and can be fatal without treatment.

  • Colonisation by P. jirovecii has been implicated as a comorbidity factor in patients with other pulmonary disorders and those undergoing therapies that affect the host's immune system.

Keywords: AIDS; coevolution; colonisation; fungus; genome; lung alveolus; immunodeficient mammals; opportunistic infection; pneumonia

Figure 1.

Phylogenetic tree based on parsimony analysis of 18S (rDNA) nucleotide sequence. The percentages of bootstrapped datasets are shown. From Taylor et al. .

Figure 2.

A scheme proposed for the intrapulmonary Pneumocystis life cycle. From Yoshida .

Figure 3.

Electron micrograph of Pneumocystis carinii infection of rat lung. The organisms adhere to thin type I epithelial cells (I) and to one another but not to the larger type II cells (II). Type II cells secrete lung surfactant, which appears as multilamellar bodies within cytoplasmic vesicles (L) and other forms (S) in the alveolar lumen. Vegetative trophic forms (T), and cyst (C) stages form a thick layer of organisms that present a serious barrier to gas exchange between the gaseous phase of the alveolus (top) and the blood capillary (not shown). The spore case (upper right) is crescent shaped, suggesting that the spores (ICBs) have been discharged leaving an empty spore case. Numerous tubular extensions (E) of trophic form cell surfaces are present. Bar, 1 μm. From Kaneshiro et al. .

Figure 4.

Comparison of Pneumocystis carinii and rat lung cells. (a) Differential interference (Normarski) optics of P. carinii pneumonia rat lung homogenate. (b) The same field taken under fluorescence optics to show nuclei stained with (DAPI). The nuclei of host cells (arrows) are much larger than those of P. carinii (arrowheads). Bar, 10 μm.



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Further Reading

Calderón EJ, Gutiérrez‐Rivero S, Durand‐Joly I and Dei‐Cas E (2010) Pneumocystis infection in humans: diagnosis and treatment. Expert Reviews of Anti‐Infection Therapy 8: 683–701.

Cushion MT (2010) Chapter 37. Pneumocystis pneumonia. In: MerzWG and Hay RJ (eds) Topley and Wilson's Microbiology and Microbial Infections. Medical Microbiology, Part VI. Systemic Mycosis Caused by Opportunistic Fungi, 10th edn, pp. 763–806, Washington, DC: ASM Press.

Dei‐CasE and Cailliez J‐C (eds) (1998) Pneumocystis carinii. FEMS Immunology and Medical Microbiology, vol. 22, pp. 1–189.

Girard P‐M and Bozzette SA (1995) Strategies for Prophylaxis of Pneumocystis carinii Pneumonia. Ballière's Clinical Infectious Diseases, 1st edn, pp. 551–571, London: Ballière Tindall.

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Walzer PD and Cushion MT (eds) (2004) Pneumocystis Pneumonia, 3rd edn. New York: Marcel Dekkar.

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Kaneshiro, Edna S, and Limper, Andrew H(Aug 2011) Pneumocystis. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0002101.pub2]