The Persistent Challenge of Pneumocystis Growth Outside the Mammalian Lung: Past and Future Approaches

Abstract

The pathogenic fungi in the genus, Pneumocystis, have eluded attempts to continuously grow them in an ex vivo cultivation system. New data from transcriptomic and genomic sequencing studies have identified a myriad of absent metabolic pathways, helping to define their host obligate nature. These nutrients, factors, and co-factors are acquired from their mammalian host and provide clues to further supplementation of existing media formulations. Likewise, a new appreciation of the pivotal role for the sexual cycle in the survival and dissemination of the infection suggests that Pneumocystis species are obligated to undergo mating and sexual reproduction in their life cycle with a questionable role for an asexual cycle. The lack of ascus formation in any previous cultivation attempts may explain the failure to identify a sustainable system. Many characteristics of these ascomycetes suggest a biotrophic existence within the lungs of the mammalian hosts. In the present review, previous attempts at growing these fungi ex vivo are summarized. The significance of their life cycle is considered, and a list of potential supplements based on the genomic and transcriptomic studies is presented. State of the art technologies such as metabolomics, organoids, lung-on-a chip, and air lift cultures are discussed as potential growth systems.

Pneumocystis spp. in Pigs: A Longitudinal Quantitative Study and Co-Infection Assessment in Austrian Farms

“…In light of the findings that Pneumocystis spp. has a significantly reduced genome size and lacks many crucial biosynthetic pathways required to survive outside of the host [4], it is currently accepted that this pathogen is an obligate biotroph [45][46][47][48][49], depending on the host lungs for nutrients. Yet, this study demonstrated that P. suis is able to cope with several co-infecting agents at the same time, even though this might not be valid for an extended period of time, thus explaining its decreased prevalence at four months.…”

Emerging and re‐emerging fungal threats in Africa

“…This impedes the development of better treatment options, understanding drug interactions particularly with ART, and in vitro assays that would clarify host/pathogen interactions. Recently discussed by Cushion et al, finding new approaches to identify a viable culture system for Pneumocystis would revolutionize the approach to combating PCP 71 . Now that the genome has been published important information is available on how Pneumocystis uses host factors for survival 62 .…”

Analysis of Pneumocystis Transcription Factor Evolution and Implications for Biology and Lifestyle

“…Despite its major impact upon human health, we remain remarkably ignorant about the biology and epidemiology of Pneumocystis . The inability to culture Pneumocystis in vitro , despite 3 decades of research, makes this pathogen uniquely difficult to study ( 9 ). The lack of in vitro culture methods also delayed the generation of full genome sequences, further slowing research on the Pneumocystis genus.…”

“…These have highlighted, for example, the importance of metabolic adaptation, micronutrient assimilation, and stress responses for the establishment of infections by Candida albicans , Cryptococcus neoformans , and Aspergillus fumigatus ( 21 – 24 ). It is not yet possible to culture Pneumocystis in vitro ( 9 ). Therefore, using bioinformatic approaches, we have explored more deeply the evolution of a range of families of transcriptional regulators in Pneumocystis to gain further insights into the reduced biology of these pathogens.…”