Phocaeicola plebeius

Phocaeicola plebeius, previously known as Bacteroides plebeius, is a microbe found in the human gut which was originally identified in Japanese populations. It has low abundance or not present at all in the gut microbiome of Western populations. This is due to the fact that its abundance is closely correlated to the consumption of nori (Porphyra seaweed) as P. plebeius facilitates its digestion. Otherwise, human gut cannot process Porphyra seaweed on its own.

 Digestion of Porphyra seaweed 
Porphyra is a genus of red seaweed whose cell wall is composed of porphyran and agarans – two structurally related sulphated galactans. These two polysaccharides have characteristic alternating β-D-galactose and α-L-galactose units. To hydrolyse these molecules enzymes GH16 β-porphyranase and GH16 agarase are needed, respectively.

P. plebeius have the genes required to express these enzymes, which do not usually occur in Phocaeicola species. It is believed that P. plebeius have acquired a cluster of sugar-processing genes from a marine bacterium Zobellia galactanivorans in the process called horizontal gene transfer. A total of 11 genes have been transferred including Bp1689 (GH16 β-porphyranase), Bp1670 (GH16 agarase) as well as genes encoding for two GH86 agarases and a sulfatase. The order of the genes in the cluster is similar to the one in Z. galactanivorans and the sequence homology between the two is 48-69%, which proves the suggestion.

 Impact on colon cancer 
A recent study on mice showed that P. plebeius can suppress colitis-associated colon cancer development by promoting anti-inflammatory immune responses. The results of the study suggest that P. plebeius colonization can alter bacterial community to promote abundance of species with anti-inflammatory functions as well as produce certain metabolites that inhibit the development of cancer.

References

Category:Bacteria