Introducing the Chromid

Harrison, PW, et al. 2010. Introducing the bacterial ‘chromid’: not a chromosome, not a plasmid. Trends in microbiology.18:4.

Although it is common to think that a scientist’s job is to make new discoveries, equally or perhaps more important is a scientist’s ability to communicate. A brilliant discovery does the world no good if it can’t be explained to other scientists or the populous at large. Defining terminology in a useful and biologically meaningful way is therefore an important aspect of biological pursuits. In a recent article Harrison et al. recognize this necessity in proposing a new term: the bacterial “chromid.” Typically bacterial genomes consist of one circular plasmid, but may also contain smaller replicons as well. These replicons may be standard plasmids or (usually) larger entities that contain plasmid-type replication machinery but also core genes that are essential to bacterial growth and survival. Currently these larger replicons are classified as second chromosomes due to their necessity for a functional cell. Here the authors argue that “chromid” rather than second chromosome is a more apt classification, as they are distinct from plasmids and the chromosome in important ways.

Here the authors defined chromids with the following criteria: “i. chromids have plasmid-type maintenance and replication systems; ii. Chromids have a nucleotide composition close to that of the chromosome; iii. Chromids carry core genes that are found on the chromosome in other species.” Chromids therefore share certain commonalities between plasmids and chromosomes, but have combined aspects of each in a consistent-enough manner, with enough differences from each as to form their own class of replicon. This mélange of chromosome and plasmid could have important consequences for bacterial evolution. Indeed, chromids can be differentiated at the genus level by the core genes they encode, indicating specific phylogentic and evolutionary histories.

As about one in ten sequenced bacterial strains have replicons that fit the definition of a chromid, the authors argue that it is important to have this term in order to clearly communicate about these replicons and their importance in bacterial evolution and adaptation. With new sequencing technologies more and more bacteria are being sequenced and so this clarity of communication will also become increasingly important in the future.

Julie Hughes

University of Idaho