Pneumocystis (Pneumocystis Pneumonia)

What is Pneumocystis?

Pneumocystis is a protozoal disease caused by Pneumocystis carinii, which is one of the most common causes of pneumonia in people with weakened immunity.

Causes of Pneumocystis

The causative agent of pneumocystis, Pneumocystis carina (Delanoe M., Delanoe P., 1912), is a single-cell microorganism. According to morphological features and sensitivity to antiprotozoal drugs, they were long referred to the protozoa type. Gradually, the pneumocysts began to identify features characteristic of fungi. Attempts to attribute them to the fungi or the simplest ones met with vagueness of systematic criteria. According to some indicators, they are close to Protozoa: according to morphology, the presence of meiosis, the presence of cholesterol in the cell membrane (and not ergosterol, like mushrooms, which makes them susceptible to antiparasitic drugs and insensitive to the antifungal agent Amphotericin B), the impossibility of their cultivation on nutritious environments for fungi. With mushrooms, they are brought together by the fact that pneumocysts contain structures similar to fungi ascospores, as well as phytosterols, which animal cells lack.

Metabolism in P. carinii (synthesis of aromatic amino acids, thymidine, etc.) occurs as in mushrooms; besides, the pneumocysts have no sporogony characteristic of protozoa. In 1988, Edman and Stringer, using the methods of molecular genetics, showed that the nucleotide sequence in t-RNA pneumocyst has a high degree of homology with the nucleotide sequence t-RNA of yeast-related fungi (Saccharomycetes serevisiae). For comparison, similar sequences were also studied in various types of protozoa: Toxoplasma, Plasmodia, Acantameb, etc. According to genetic signs, pneumocystas were closest to fungi. But these are mushrooms that have a number of features that bring them closer to the simplest ones. Thus, P. carinii m-DNA contains the genes for dehydrogenase and cytochrome oxidase, which are 60% similar to the genes of fungi and only 20% to the protozoan genes. Similar properties were found in other species of fungi.

According to the latest data, the pneumocysts are referred to as fungi, possibly as actinomycetes or higher fungi, asko- or basidomycetes. At the same time, some researchers consider P. carinii a phylogenetically ancient pathogen of an indefinite systematic position, which in the process of evolution lost its characteristic taxonomic features (T. W. Beyer, 1989). Modern molecular biological and molecular genetic analysis can not definitively determine their species. To address this issue, an ultrastructural study of the pathogen, the study of its phylogenesis and the improvement of pneumocysts cultivation techniques are necessary.

P. carinii is an extracellular parasite tropic to lung tissue. In the development cycle, the pneumocystics secrete a vegetative form (trophozoite), a precyst and a cyst with intracystic bodies.

Trophozoites – cells 1-8 microns, coated with a thin shell. Their form is diverse and mobile (oval, ameba-like, etc.). It depends on the shape of the structures to which they fit. From the surface of the trophozoite, micro-growths and long processes in the form of filopodia depart. At their expense, the pneumocytes are interconnected and attached to the host cells. The core is limited to two membranes. Mitochondria, free ribosomes, small and large vacuoles, polysaccharides, lipid drops are detected in the cytoplasm. Sometimes there are dual cells, which may indicate division or copulation. According to electron microscopic studies, trophozoites tightly adhere to first-order alveolocytes. Filopodiums can go deep into the cell of the lung of the host without breaking its membrane. At the same time, the membrane of the alveolocyte can enter the body of the trophozoite without damaging it. Physiological contact of the pathogen with the host is carried out through a system of micropinocytic vesicles.

The predzista are oval cells with a diameter of 2–5 µm, lacking pellicular processes. Nuclear fission occurs in them. P. carinii cysts are round or oval, 3.5–10 µm in size, sometimes up to 30 µm (Fig. 20). They are covered with a dense three-layer shell and an outer membrane. The cytoplasm contains from 1 to 8 intracystic bodies that go into the lumen of the alveoli after rupture of the cysts. Empty cysts have an irregular shape, often in the form of a crescent, sometimes they contain intracystial bodies. The size of the last 1-2 microns. The pellicle intracystic cells re-forms after leaving the destroyed cyst. They become extracellular trophozoites and begin a new round of the life cycle.

Among AIDS patients, pneumocystosis is one of the most frequent opportunistic infections (more than 80%) and, if left untreated, almost always leads to death. In other patients with a weakened immune system, pneumocystis is excreted in 40% of cases. According to animal experiments, the incubation period lasts from 4 to 8 weeks.