Candidate Phyla Radiation

An overview of the Candidate Phyla Radiation (CPR) in microbiology

Candidate Phyla Radiation (CPR) refers to a large group of bacteria that are part of a major evolutionary radiation. These bacteria are characterized by their small cell size and reduced genomes, and they are often found in a variety of environments, including soil, water, and the human microbiome. CPR bacteria are considered "candidate" phyla because they have not been cultured in the laboratory, and their classification is based primarily on metagenomic data.

Overview

The Candidate Phyla Radiation encompasses a vast diversity of bacterial lineages that are distinct from the well-known bacterial phyla. These lineages were initially identified through 16S ribosomal RNA gene sequencing and later through more comprehensive metagenomic approaches. CPR bacteria are notable for their small genomes, which often lack many of the genes required for independent life, suggesting that they may rely on symbiotic or parasitic relationships with other organisms.

Characteristics

CPR bacteria are typically characterized by:

• Small cell size: Many CPR bacteria have cell sizes less than 0.5 micrometers in diameter.
• Reduced genomes: Their genomes are often less than 1 megabase in size, lacking many biosynthetic pathways.
• Metabolic dependency: Due to their reduced genomes, CPR bacteria may depend on other organisms for essential nutrients and metabolic functions.
• Environmental diversity: They are found in a wide range of environments, from deep-sea hydrothermal vents to human oral cavities.

Phylogenetic Diversity

The CPR includes several major groups, such as:

• Parcubacteria
• Microgenomates
• Gracilibacteria
• Saccharibacteria (formerly known as TM7)

These groups are often referred to as "superphyla" due to their extensive diversity and the number of lineages they encompass.

Ecological Roles

The ecological roles of CPR bacteria are not fully understood, but they are thought to play important roles in nutrient cycling and microbial community dynamics. Their interactions with host organisms, whether symbiotic or parasitic, may have significant impacts on the health and function of microbial ecosystems.

Research and Implications

The study of CPR bacteria is ongoing, with researchers using advanced techniques such as single-cell genomics and metagenomics to uncover their biology and ecological roles. Understanding CPR bacteria could have implications for microbial ecology, evolutionary biology, and even biotechnology.

Also see

• Microbial dark matter
• Metagenomics
• Symbiosis
• Bacterial phylogeny