My old Nan...

 Sialic acids are a family of structurally distinct nine carbon amino sugars which are present in mucous regions of the human body. Previous studies have shown that many pathogenic bacteria can utilize these sialic acids in a number of ways. However there has been very little research on pathogens utilizing them as a carbon source. This study examines three genes needed for the catabolism of sialic acids (nanA, nanE and nanK) known as the Nan cluster. It examines the distribution and evolution of the Nan cluster among bacteria and determines whether bacteria containing the Nan cluster can utilize sialic acids as a sole carbon source. Although they looked at many bacteria I will mainly concentrate on the vibrios.

 To examine the distribution of the Nan cluster among bacteria DNA sequences were obtained from GenBank. Phylogenetic trees of NanA, NanE and NanK were constructed to examine its evolutionary history. They also examined the growth of four Vibrio spp. on minimal media supplemented with sialic acid.

 The Nan cluster was present in 46 of the approx. 1900 bacterial genomes examined. Only three Vibrio spp. contained the cluster (Vibrio cholerae, Vibrio vulnificus and Vibrio fisheri). The cluster was found in all tested strains of V.vulnificus and V.fisheri however was only found in pathogenic strains of V.cholerae. Phylogenetic analysis of nanA suggests that V.cholerae and V.vulnificus obtained the gene from commensal gut bacteria possibly through horizontal gene transfer and was very closely related to eukaryotes. Phylogenetic analysis of nanE and nanK suggests that V.cholerae and V.vulnificus evolved the genes separately from V.fisheri. In the growth experiment only and all vibrios containing the Nan cluster were able to grow on the sialic acid media.  

 The results strongly suggest that pathogenic vibrios have the ability to catabolise sialic acid and use it as a sole carbon source. It also gives tremendous insight into how marine bacteria have evolved to compete in the hostile human gut. I found it interesting that Vibrio parahaemolyticus cannot catabolise sialic acid yet V.fischeri can. It is however a bit annoying that the paper doesn’t mention all the vibrios tested, only the Nan positive species. The high correlation between the Nan cluster and pathogens suggests that genes needed for survival in the human body could be potential genetic marker for virulence.

Almagro-Moreno, S. and Boyd, E. (2009) Insights into the evolution of sialic acid catabolism among bacteria. BMC Evolutionary Biology. 9, 118.