Pneumocystis

Abstract 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 species‐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.