Photobacterium profundum

Photobacterium profundum is a deep sea Gammaproteobacterium, belonging to the family Vibrionaceae and genus Photobacterium. Like other members of this genus, P. profundum is a marine organism and has two circular chromosomes. P. profundum is a gram-negative rod with potential growth at temperatures from 0 °C to 20 °C and pressures from 0.1 MPa to 70 MPa depending on the strain. It has a requirement for salt, is able to metabolise a wide range of simple and complex carbohydrates and has two flagella systems. Cells are rod shape, 2-4μm long and 0.8-1.0μm wide, with a single unsheathed flagella. This bacterium was originally isolated in 1986 from the Sulu Sea and there are currently 4 cultured wild-type strains of P. profundum, (strains SS9, 3TCK, DJS4 and 1230). P. profundum strain SS9 has optimal growth at 15 °C and 28 MPa making it both a psychrophile and a piezophile. P. profundum strain 3TCK, isolates from San Diego Bay, grows optimally at 9 °C and 0.1 MPa and P. profundum strain DSJ4, isolated from the Ryukya trench off of Japan at a depth of 5110m, grows optimally at 10°c and 10MPa. Based on 16S rRNA sequence P. profundum is closely related to the Vibrio genus, the most prominent species being the human pathogen Vibrio cholerae.

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In strain SS9 it has been shown that several stress response genes are up regulated in response to atmospheric pressure, these include htpG, dnaK, dnaJ, and groEL. The types and abundance of fatty acid chains in the cell membrane also respond to changes in pressure and temperature. At low temperature and high pressure strain SS9 increases the abundance of mono- and polyunsaturated fatty acids. This has the effect of increasing membrane fluidity by reducing packing of the fatty acid chains. The outer membrane protein OmpH has been shown to be up regulated at elevated pressures, the opposite is true for the outer membrane protein OmpL which is up regulated in response to low pressures.

In 2005 Vezzi et al published the genome sequence for P. profundum strain SS9. The genome of P. profundum consists of a 4.1-Mbp circular chromosome, a 2.2-Mbp minor circular chromosome, as well as a 80-kbp circular plasmid. Strain SS9 has 14 ribosomal RNA (rRNA) genes on chromosome 1, and 1 on chromosome 2; this is the largest number of rRNA genes found in a bacterium. Chromosome 1 consists largely of genes which are essential for growth where as chromosome 2 appears to be a large plasmid, which, on an evolutionary time scale, has be gained several transposable elements. With in the genome of P. profundum there are a number of open reading frames (ORF) which are not found in other members of the Vibrionaceae family.

This work was followed by another paper that year by Campanaro et al which detailed microarray work comparing gene expression at sub-optimal, optimal and supra-optimal temperatures and pressure for strains SS9, 3TCK and DSJ4. Campanaro et al showed that there are 544 ORF’s divergent or missing from the 3TCK genome and 562 ORF’s divergent or missing from the DSJ4 chromosomes when compared to that of SS9.