VetBact

Introduction

The term genome refers to the complete set of genetic material in a cell. Genome size is usually expressed in Mb or Mbp (= mega base pairs). Thus, 1 Mbp = 1 000 000 bp. The size of the bacterial genome varies between 0.5 and 10 Mbp. Bacterial genomes comprise the chromosome(s) and possibly also plasmid(s). Bacteria are haploid, unlike higher organisms, which are diploid. Gametes of higher organisms are, however, also haploid. The complete genome sequences have been determined for about 30 000 bacterial strains and there are approximately 20 000 published genome projects for bacteria, where the sequence fragments have not been aligned into one single sequence (a so-called contig) .

Chromosome

The chromosome represents the primary genetic material, which is essential for the bacterium. Bacteria in general have a circular chromosome, but there are exceptions (see below).

Plasmid

Plasmids (see also Genome above) are secondary genetic material, not always essential for the bacterium. Bacteria which carry plasmids generally have one or more circular plasmids, but there are also exceptions. Plasmids generally constitute up to 10% of the genome and they are replicated independently of the chromosome.

Exceptions

• Some species within the genera Brucella, Burkholderia, Leptospira och Vibrio have two cirkular chromosomes
• Species within the genus Borrelia have one linear chromosome (about 1 Mbp), which is relatively small.
• Species within the genus Borrelia also have linear plasmids and these are essential for the bacterium.
• Species within the genus Streptomyces have one linear chromosome, which is relatively big (about 10 Mbp).

Updated: 2020-09-23.