Aliivibrio-More genetically diverse than initially thought

The genus Aliivibrio contains the newly classified members of the Vibrio fischeri species group. The new genus, Aliivibrio contains following species; the afore mentioned, along with A. logei, A.salmonicida (as mentioned in a previous blog post by Hjerde et al., 2008) and A.wodanis. The separation of these species is down to phylogenetic and phenotypic differences. (Ast et al., 2009)

However, although the difference between the species has been shown to be strong, the phylogenetic analyses conducted on Aliivibrio are limited. The taxonomy of the group is reportedly very ambiguous and it is suggested that a number of strains are incorrectly identified. For example, some bacterial symbionts associated with bioluminescent squid were identified as belonging to A.logei, however was done so using methods that didn’t differentiate between different species of Aliivibrio. As a result, large levels of unknown genetic diversity may be present due to incorrect identification. (Ast et al., 2009)

Ast et al., (2009) isolated strains from squid and fish, particularly from their light organs or from seawater. The strains included several previously identified and newly isolated strains. Samples were homogenized, plated and incubated overnight. Whole genomic DNA was isolated and PCR carried out. 16s rRNA genes were aligned and analyzed phylogenetically to assess similarities and differences between strains.

Analysis revealed the following. They found Aliivibrio to be distinct from Vibrio, Photobacterium and other Vibrionaceae. Furthermore, A.fischerei, A.salmonicida, A.logei and A.wodanis are distinct clades. It was also found that some of the previously classified strains had been done so incorrectly and that there is therefore undiscovered diversity within the genus. For example, some strains showed yellow-shifted luminescence and so didn’t belong to A.fischerei as was previously thought. Several strains have been identified previously as A.logei instead of A.wodanis. They may even be members of a new clade. Within the group A.fischerei there appears to be 2 strongly supported clades.

Overall, I feel this study by Ast et al., (2009) gives a hugely important conclusion. This being that it is of the utmost importance to base any taxonomic conclusions on the examination of type strains. If this isn’t the case, then there is always the potential for mis-identification. The use of multi-gene phylogenetic analysis for strain identification, and the use of numerous strains is emphasized so as to avoid putting too much emphasis on phenotypic characteristics. I feel this is a very important conclusion to make, as mis-identification is obviously far from ideal. It emphasizes the need for correct identification that (for obvious reasons!) is a very important aspect of taxonomy. I therefore feel that this study has obvious benefits in the grand scheme of things, showing type strains as a very important aspect in taxonomy and the benefits they have in correctly identifying an organism’s correct phylogeny.

Reference:

Ast, J., Urbanczyk, H., Dunlap, P. (2009). Multi-gene analysis reveals previously unrecognized phylogenetic diversity in Aliivibrio. Systematic and Applied Microbiology. 32, 379-386