Opportunistic Atypical Fungus: Pneumocystis jiroveci



Opportunistic Atypical Fungus


Pneumocystis jiroveci



Objectives



1. Describe the symptoms of Pneumocystis jiroveci infection and the cells affected by this organism.


2. List the appropriate specimen types collected for diagnosis of pneumocystis pneumonia.


3. Discuss the laboratory tests used in the diagnosis of P. jiroveci infection, including the methodology and biochemical principles.


4. List the four stains most commonly used in the diagnosis of P. jiroveci infection.



Genus and Species to be Considered












Current Name Previous Name
Pneumocystis jiroveci Pneumocystis carinii



General Characteristics


In 1999 the name of the organism that causes a pneumonia in immunocompromised humans, commonly called pneumocystis pneumonia (PCP), was changed from Pneumocystis carinii to Pneumocystis jiroveci. (The causative organism for the rodent form of pneumocystis is still called P. carinii.) P. jiroveci is an opportunistic, atypical fungus that infects immunocompromised hosts and mostly manifests as PCP.


P. jiroveci originally was thought to be a trypanosome, but its precise taxonomic categorization remains challenging. Several factors supported the notion that P. jiroveci was a protozoan parasite: its morphology is similar to that of microbes and protozoa, and clinically it responds to antiprotozoal drugs but not to antifungal drugs in patients with pneumocystosis. Inability to maintain and propagate the organism in routine culture has further limited its characterization, although cultivation is possible under special conditions. P. jiroveci exists as three forms in its life cycle: trophozoite, precyst (sporocyte), and cyst (the latter is the diagnostic form).


Although P. jiroveci has been shown to be a fungus, it differs from other fungi in various aspects. Its cell membrane contains cholesterol rather than ergosterol. The flexible-walled trophozoite is susceptible to osmotic disturbances. Also, P. jiroveci contains only one or two copies of the small ribosomal subunit gene, whereas most other fungi contain numerous copies of this gene. DNA sequence analysis of the small ribosomal subunit gene in P. jiroveci has disclosed a greater sequence homology with the fungi than with the protozoa. Two independent analyses that compared the DNA sequences of P. jiroveci with those of other fungi confirmed the placement of P. jiroveci in the fungal kingdom, somewhere between the ascomycetes and the basidiomycetes (the closest yeast is the fission yeast, Schizosaccharomyces pombe). DNA analysis also confirmed the difference between the rodent and human forms of pneumocystis.



Epidemiology


P. jiroveci has a worldwide distribution and most commonly presents as pneumonia in an immunocompromised host. As mentioned, infection appears to be species specific, with P. carinii causing disease in rodents and P. jiroveci causing human disease. The exact transmission of disease is still not known. Some speculate that pneumocystis is transmitted person to person. Immunocompetent mammals may be the reservoir for P. jiroveci, which is transmitted to immunodeficient individuals as a pathogen. Most children by age 2 to 4 have antibodies to pneumocystis. Vargas et al. showed that pneumocystis DNA was present in 24 of 72 infants, as determined from nasopharyngeal specimens, and that seroconversion occurred in 85% of infants by 20 months of age.


Since the onset of the human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS) epidemic in the 1980s, pneumocystis has been defined as the most common opportunistic infection among those with HIV or AIDS in the United States. The introduction of highly active antiretroviral therapy (HAART) for patients with HIV has reduced the incidence of disease. However, PCP remains a significant medical problem, because numerous patients with HIV do not respond to therapy, do not comply with therapy, or do not know they are infected.



Pathogenesis and Spectrum of Disease


After inhalation of P. jiroveci, the pathogen is thought to adhere to type I pneumocytes. The organisms replicate extracellularly while bathed in alveolar lining fluid. With successful replication of the organism, the alveolar spaces fill with foamy material, which can be detected with hematoxylin and eosin staining. These changes result in impaired oxygen-diffusing capacity and hypoxemia. A predominantly interstitial mononuclear inflammatory response is associated with this type of pneumonia. When first described, this pneumonia was known as interstitial plasma cell pneumonia.


Symptoms of PCP include a nonproductive cough, low-grade fever, dyspnea, chest tightness, and night sweats. In patients without HIV infection, the underlying conditions most commonly seen as risk factors for this opportunistic infection are asthma, chronic obstructive pulmonary disease (COPD), cystic fibrosis, systemic lupus erythematosus (SLE), pregnancy, rheumatoid arthritis, infection with Epstein-Barr virus, ulcerative colitis, and high-dose corticosteroid therapy.

Related posts:

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  3. Infections of the Lower Respiratory System
  4. Gastrointestinal Tract Infections

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Aug 25, 2016 | Posted by in MICROBIOLOGY | Comments Off on Opportunistic Atypical Fungus: Pneumocystis jiroveci

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