Gut microbes supply the human body with energy from dietary polysaccharides through carbohydrate active enzymes, or CAZymes, which are absent in the human genome. These enzymes target polysaccharides from terrestrial plants that dominated diet throughout human evolution. The array of CAZymes in gut microbes is highly diverse, exemplified by the human gut symbiont Bacteroides thetaiotaomicron, which contains 261 glycoside hydrolases and polysaccharide lyases, as well as 208 homologues of susC and susD-genes coding for two outer membrane proteins involved in starch utilization. A fundamental question that, to our knowledge, has yet to be addressed is how this diversity evolved by acquiring new genes from microbes living outside the gut. Here we characterize the first porphyranases from a member of the marine Bacteroidetes, Zobellia galactanivorans, active on the sulphated polysaccharide porphyran from marine red algae of the genus Porphyra. Furthermore, we show that genes coding for these porphyranases, agarases and associated proteins have been transferred to the gut bacterium Bacteroides plebeius isolated from Japanese individuals. Our comparative gut metagenome analyses show that porphyranases and agarases are frequent in the Japanese population and that they are absent in metagenome data from North American individuals. Seaweeds make an important contribution to the daily diet in Japan (14.2 g per person per day), and Porphyra spp. (nori) is the most important nutritional seaweed, traditionally used to prepare sushi. This indicates that seaweeds with associated marine bacteria may have been the route by which these novel CAZymes were acquired in human gut bacteria, and that contact with non-sterile food may be a general factor in CAZyme diversity in human gut microbes.
Cellulose microcrystals with dimensions of ∼5 nm × 150–300 nm were obtained from wheat straw. To evaluate the reinforcing effect of these fillers within a thermoplastic matrix, composites with a weight fraction of cellulose ranging from O to 30 wt% were processed by freeze‐drying and molding a mixture of aqueous suspensions of microcrystals and poly(styrene‐co‐butyl acrylate) latex. It was found that these microcrystals, or whiskers, bring a great reinforcing effect at temperatures higher than the glass transition temperature (Tg) of the matrix and improve the thermal stability of the composite. The relaxed modulus increased continuously with the filler content, and for a film containing 30 wt% of whiskers, it was more than a thousand times higher than that of the matrix. This effect is discussed with regard to theoretical calculations based on a mean field approach (Halpin‐Kardos model). It is concluded that the great reinforcement observed seems to be due not only to the geometry and stiffness of the straw cellulose whiskers but also to the interactions of the microcrystals, their topological arrangement, and the probable formation of whisker clusters within the thermoplastic matrix, the cellulose fillers probably being linked through hydrogen bonds.
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