Keeping up with virus taxonomy: viruses that infect bacteria

Posted on July 18, 2023   by Clare Baker

Way back in the February instalment of Keeping up With Virus Taxonomy, we looked into viruses that infect fungi... So it’s about time we looked into viruses that infect bacteria. Otherwise known as bacteriophages, viruses that infect bacteria are composed of proteins and a DNA or RNA genome that can be very simple, containing four genes, or complex, with hundreds of genes. You can learn more about phages here.

Inoviridae

First up on our list is the family Inoviridae. Virons of the family are long, non-enveloped flexible filaments and share a similar structure, a protein coat spiralling around their DNA. It is this structure that gives many genera of the Inoviridae family their names. Many of the genus belonging to the Inoviridae family have names originating from latin derrivatives of ‘long thin objects’ particularly those associated with weaving.

Hosts of Inoviridae are gram-negative bacteria. The term gram-negative refers to the way in which bacteria respond to Gram’s staining method. Gram-negative bacteria are bacteria which lose the Gram stain colour after treatment with a differentiating agent, usually alcohol. This is an indication of the chemical structure of the bacterial cell — gram-negative bacteria have an outer membrane. This membrane is often responsible for protecting the bacteria from antibiotics.

Corticoviridae

Next up on our list is the family Corticoviridae. Corticoviridae are icosahedral (20 triangular faced) viruses that have double-stranded circular DNA genomes of approximately 10 kilo-base pairs. Corticoviridae is a small family that consists of only two species belonging to one genus. The Corticoviridae virus PM2 is virulent and replicates in their hosts, two known strains of marine bacteria of the genus Pseudoalteromonas.

Plasmaviridae

Plasmaviridae are the third stop of our bacteriophage journey. They are specific viruses that only  infect the Acholeplasma species of bacteria. Currently, the only classified Plasmaviridae virus is Acholeplasma virus L2 of Acholeplasma laidlawii, a temperate bacteriophage. A temperate bacteriophage integrates its genomes into host bacterial DNA and undergoes division without destroying the host.

Virions are slightly pleomorphic, which means there is irregular forms of individual virus particles within the species. Plasmaviridae infect the wall-less bacteria of the Acholeplasma and are released from the host through the cell membrane without breaking it down. Virions are extremely heat sensitive and relatively cold stable.

Cystoviridae

Finally, we come to Cystoviridae, these spiky viruses are spherical and about 85 mm in diameter. They infect Gram-negative bacteria, primarily Pseudomonas syringae bacteria which infect plants. Virions of this family infect bacteria by absorbing into the lipopolysaccharide layer (a major component of the outer membrane) or to pili (an outer, hair like organelle) of the host. Cystoviruses are lytic bacteriophages, they induce cell lysis at the end of the viral reproduction cycle. This is where the membrane of the cell breaks down, destroying the host cell.