What Causes Pneumocystis Pneumonia?

Pneumocystis pneumonia is a potentially life-endangering disease caused by the fungal pathogen Pneumocystis jiroveci. It is predominantly observed in immunocompromised individuals. This microorganism is the most common causative agent of opportunistic infections in patients who present with acquired immunodeficiency syndrome (AIDS) in developed countries.

The exact name of the species that infects humans has been amended from Pneumocystis carinii to Pneumocystis jirovecii in order to differentiate it from the species that infects and causes disease in rats. The name was given in honor of the Czech parasitologist Otto Jirovec who contributed substantially to the research on this organism.

A Fungus or a Parasite?

Pneumocystis jiroveci is a unicellular eukaryotic microorganism that is widespread and ubiquitous in geographic distribution. Initial researches indicated that this organism is a protozoon ( a single-celled organism with animal-like behavior). Some have suggested that it may be a protist of independent taxonomic position.

Still, such taxonomic positioning of Pneumocystis as a genus of unique protozoan organisms was disputed for several years. Today they are identified as fungi, based on their ribosomal ribonucleic acid (rRNA) and specific homologies of other gene sequences, as well as the composition of cell walls and the structure of their fundamental enzymes. Currently a vast majority of scientists group Pneumocystis jiroveci among ascomycetous yeasts. This is the same group in which Saccharomyces cerevisiae (brewer's yeast) is located.

The strains of this microorganism also show high genetic diversity, predominantly due to gene variations that stem from the presence of specific single-nucleotide polymorphisms. In addition, some polymorphisms are considered exceptionally informative markers that can be employed for comparative analyses between the genotype and specific epidemiological or clinical data.

Trophic Stages and Morphology

Pneumocystis jiroveci shows a biphasic life cycle and has two diverse morphological forms:

  • haploid trophozoites (which are a hallmark of the asexual, proliferative phase of its lifecycle)
  • cysts (which represent its sexual phase)

Cysts are produced during the reproductive stage due to the conjugation of trophozoites, while trophic forms prevail in lungs during the infection.

The primary proliferative life cycle of Pneumocystis jiroveci consists of minute, haploid (containing only one member of each chromosome pair) trophic forms that grow into bigger, diploid (containing two members of each chromosome pair) trophic forms and reproduce asexually by cell division. During ultrastructural studies, investigators have described the shape of the trophic form as pleomorphic or amoeboid. The shape of large trophic forms often appears quite irregular during electron microscopy.

Cysts represent an intrapulmonary life cycle stage of Pneumocystis jiroveci. These are developed from parent cells by meiosis and partitioning of the cytoplasm. The cysts are a thick-walled sexual (or reproductive) stage consisting of eight haploid progeny cells that show focal thickening of the wall. This is visible even by light microscopy in silver-stained specimens.

The nucleus of this fungal pathogen is small (between 0.5 and 1 µm), bounded by a typical nuclear envelope with pores, and contains a noticeable nucleolus. It is not completely clear if mitosis in Pneumocystis jiroveci is closed (fully contained within the nuclear envelope) or semi-open (with a partial breakdown of the nuclear envelope).

The mitochondria of Pneumocystis jiroveci are usually poorly preserved and they appear swollen. The analysis of serial sections of trophic forms has revealed a single mitochondrion that can be branched or that is irregular in shape. There are also Golgi-like vesicles, endoplasmic reticulum and primary lysosomes.

The early stages of Pneumocystis jiroveci proliferation inside the human organism prompt the activation of alveolar macrophages and an increase of proinflammatory cytokines, as well as changes in pulmonary surfactant. Such adjustments occur even when the infection process is characterized by lower fungal burden.


Further Reading

Last Updated: Feb 27, 2019

Dr. Tomislav Meštrović

Written by

Dr. Tomislav Meštrović

Dr. Tomislav Meštrović is a medical doctor (MD) with a Ph.D. in biomedical and health sciences, specialist in the field of clinical microbiology, and an Assistant Professor at Croatia's youngest university - University North. In addition to his interest in clinical, research and lecturing activities, his immense passion for medical writing and scientific communication goes back to his student days. He enjoys contributing back to the community. In his spare time, Tomislav is a movie buff and an avid traveler.


Please use one of the following formats to cite this article in your essay, paper or report:

  • APA

    Meštrović, Tomislav. (2019, February 27). What Causes Pneumocystis Pneumonia?. News-Medical. Retrieved on July 19, 2024 from https://www.news-medical.net/health/What-Causes-Pneumocystis-Pneumonia.aspx.

  • MLA

    Meštrović, Tomislav. "What Causes Pneumocystis Pneumonia?". News-Medical. 19 July 2024. <https://www.news-medical.net/health/What-Causes-Pneumocystis-Pneumonia.aspx>.

  • Chicago

    Meštrović, Tomislav. "What Causes Pneumocystis Pneumonia?". News-Medical. https://www.news-medical.net/health/What-Causes-Pneumocystis-Pneumonia.aspx. (accessed July 19, 2024).

  • Harvard

    Meštrović, Tomislav. 2019. What Causes Pneumocystis Pneumonia?. News-Medical, viewed 19 July 2024, https://www.news-medical.net/health/What-Causes-Pneumocystis-Pneumonia.aspx.


The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
Post a new comment

While we only use edited and approved content for Azthena answers, it may on occasions provide incorrect responses. Please confirm any data provided with the related suppliers or authors. We do not provide medical advice, if you search for medical information you must always consult a medical professional before acting on any information provided.

Your questions, but not your email details will be shared with OpenAI and retained for 30 days in accordance with their privacy principles.

Please do not ask questions that use sensitive or confidential information.

Read the full Terms & Conditions.