Candidate Phyla Radiation, an Underappreciated Division of the Human Microbiome, and Its Impact on Health and Disease

Candidate phyla radiation (CPR) is an emerging division of the bacterial domain within the human microbiota. Still poorly known, these microorganisms were first described in the environment in 1981 as ultramicrobacteria with a cell volume under 0.1 mu m(3) and were first associated with the human oral microbiota in 2007. The evolution of technology has been paramount for the study of CPR within the human microbiota. In fact, since these ultramicrobacteria have yet to be axenically cultured despite ongoing efforts, progress in imaging technology has allowed their observation and morphological description. Although their genomic abilities and taxonomy are still being studied, great strides have been made regarding their taxonomic classification, as well as their lifestyle. In addition, advancements in next-generation sequencing and the continued development of bioinformatics tools have allowed their detection as commensals in different human habitats, including the oral cavity and gastrointestinal and genital tracts, thus highlighting CPR as a nonnegligible part of the human microbiota with an impact on physiological settings. Conversely, several pathologies present dysbiosis affecting CPR levels, including inflammatory, mucosal, and infectious diseases. In this exhaustive review of the literature, we provide a historical perspective on the study of CPR, an overview of the methods available to study these organisms and a description of their taxonomy and lifestyle. In addition, their distribution in the human microbiome is presented in both homeostatic and dysbiotic settings. Future efforts should focus on developing cocultures and, if possible, axenic cultures to obtain isolates and therefore genomes that would provide a better understanding of these ultramicrobacteria, the importance of which in the human microbiome is undeniable. Candidate phyla radiation (CPR) is an emerging division of the bacterial domain within the human microbiota. Still poorly known, these microorganisms were first described in the environment in 1981 as ultramicrobacteria with a cell volume under 0.1 mu m(3) and were first associated with the human oral microbiota in 2007.

Automated summary

Candidate phyla radiation (CPR) is a division of bacteria within the human microbiota that is still poorly understood. These microorganisms, first described in 1981, have a small cell volume and were associated with the human oral microbiota in 2007. Novel imaging technology has allowed for their observation and classification, while advancements in next-generation sequencing and bioinformatics tools have facilitated their detection in different human habitats. Dysbiosis affecting CPR levels is linked to various diseases. Future research should focus on developing culture techniques to gain a better understanding of these microorganisms' importance in the human microbiota.