CELLULAR AND MOLECULAR-BIOLOGY OF PNEUMOCYSTIS-CARINII

This chapter discusses Pneumocystis carinii (P. carinii) and current concepts of the organism in three major areas: cell biology, molecular genetics, and interaction with the host. P. carinii exists primarily as an extracellular pathogen in mammalian alveoli, where clusters comprised of various morphological forms of the organism accumulate within the lumen. The two predominant forms of the organism are the small unicellular trophic form or trophozoite stage (1–4μm) and the larger, multinucleated cystic stage (5–8 μm). The cyst stage is the most easily recognized of the developmental stages of P.carinii. The cyst contains up to eight individual daughter forms, each complete with double-membrane envelopes, nuclei, mitochondria, free ribosomes, and endoplasmic reticula. Trophic forms may occur as individual entities, but are more commonly found in the clusters that populate the alveolar lumen. Two themes are common to almost all the life cycles of P.carinii: (1) replication of the trophic form by an asexual process such as binary fission and (2) a developmental pathway leading to cyst formation. The P. carinii genome is rich in adenine and thymine. The P. carinii genome contains a substantial amount of repetitive DNA, i.e., sequences that are present in more than one copy per genome. © 1991, Elsevier Science & Technology. All rights reserved.