Genotyping Pneumocystis jirovecii: Impacting Our Understanding of Interhuman Transmission

Abstract

Pneumocystis jirovecii is an atypical fungus transmitted via the airborne route between humans. This fungus is exclusively associated with humans and almost each individual has encountered it at least once before reaching the age of two. P. jirovecii can be cleared and spontaneously resolutive in immunocompetent, whereas it can be responsible for severe Pneumocystis pneumonia (PCP) in immunocompromised patients. In the next decades, the putative increase of the population of immunocompromised patients is likely to lead to the rise of PCP infections. The detection of low fungal loads has been improved with the advances of molecular technologies and polymerase chain reaction (PCR) assays. This improvement in detection raised several questions. Is PCP consecutive of a persistent colonizing strain multiplication or acquisition of a new one? What is the variety of strains in PCP and non-PCP patients? Are outbreaks due to a specific strain? Genotyping approaches could bring some answers. In this review, we reviewed the main typing methods developed for P. jirovecii characterization, with an emphasis on the typing of low fungal loads, a subject rarely underlined. Secondarily, we present the main results obtained and the importance of including patients with low fungal load in the analyses because of their possible role as reservoirs and their impact on P. jirovecii transmission. Finally, we consider systematic treatment of the patients with low fungal load not only to prevent full-blown PCP, but also to block the transmission chain.