Distribution of luxS and production of autoinducer-2 among gut Bacteroidales
One of the best studied quorum sensing (QS) molecules, autoinducer-2 (AI-2), regulates processes in numerous bacteria. LuxS is an enzyme of the activated methyl cycle which, along with MtnN, converts S-adenosyl-homocysteine to homocysteine, releasing DPD (4,5-dihydroxy-2,3-pentanedione) which is spontaneously converted to AI-2. Many bacteria do not encode MtnN-LuxS and instead encode SahH, which directly converts S-adenosyl-homocysteine to homocysteine without AI-2 production. The genomes of some gut Bacteroidales were shown to contain luxS, however, these reports, as well as reports of the production of AI-2 by gut Bactereoidales have been inconsistent. We performed a comprehensive analysis of the distribution of luxS and sahH in Bacteroidota with an in-depth exploration of gut Bacteroidales. The data suggest that the ancestral Bacteroidota contained sahH, with numerous independent replacements with mtnN-luxS during diversification. In Bacteroidaceae, Parabacteroides, and many Prevotellaceae, mtnN-luxS or sahH are present in the same genetic region, adjacent to yfhO. Using Bacteroides fragilis, which contains sahH, and Bacteroides uniformis and Phocaeicola vulgatus, whose genomes contain mtnN-luxS, we show that luxS-containing strains produce AI-2. Transcriptomic analyses and gnotobiotic mouse experiments using wild-type strains and mtnN-luxS and sahH genetic swaps showed that Bacteroidaceae species do not respond to AI-2 under the conditions tested. However, analyses of 15 human gut metagenomic datasets show that mtnN-luxS containing Bacteroidales species average between 5.1 – 33% of the total bacteria in these various human gut microbiome datasets, and therefore, likely produce substantial amounts of AI-2 sensed by responsive gut symbionts and pathogens. Importance Here, we show that gut Bacteroidales possess one of two pathways to complete the activated methyl cycle, one that produces the QS molecule AI-2. We clarify conflicting data regarding AI-2 production in gut Bacteroidales and show that luxS is present in many gut Bacteroidales species, but lacking in species such as Bacteroides thetaiotaomicron and Bacteroides fragilis. We find that prevalent and abundant human gut Bacteroidales species including Bacteroides uniformis and Phocaeicola vulgatus produce substantial amounts of AI-2. While our data do not show that Bacteroidales sense or respond to AI-2 under the conditions tested, analyses of human gut metagenomic data reveal that AI-2 producing Bacteroidales comprise a large proportion of the gut bacteria of both industrialized and non-industrialized human populations.