of milk, after lactose and fat, and up to 200 different structural forms have been identified in human milk (Wu et al., 2010, 2011). Since HMOs are resistant to digestion by the infant and pass into the colon, Donovan described them as the fiber of human milk, a fact that she said hasn’t really been appreciated until recently. In the colon, they potentially function in a variety of ways, including as substrates for fermentation and the production of short-chain fatty acids. They can also serve as prebiotics for beneficial bacteria. Donovan referred to David Mills’s very elegant data showing that Bifidobacterium infantis metabolizes specific HMOs (see the previous section for a more detailed description of work conducted in the Mills laboratory and by Bruce German).

HMO composition is influenced partly by secretor status of the mother and whether she has the 2-fucosyltransferase gene; non-secretor mothers do not produce 2’-fucosyllactose (2’-FL), which is one of the primary HMOs in the milk of secretor mothers. Therefore, Donovan and others are exploring potential predictive associations between HMO composition and infant gut microbiota. Systematic evaluation of the impact of HMO on infant development, however, has been limited by the lack of sufficient quantities of pure HMOs to conduct animal or human feeding studies. However, in the near future, this limitation will be overcome through improved synthetic approaches, opening avenues of investigation into the biology of HMOs. Additionally, the availability of noninvasive methods of assessing outcomes in human infants (Chapkin et al., 2010; Schwartz et al., 2012) and high-throughput methods for measuring HMOs (Wu et al., 2010, 2011) and the infant microbiome (Schwartz et al., 2012) will facilitate our understanding of the role of HMOs in host-microbe interactions in the developing infant (Donovan et al., 2012).

THE RESISTOME AS A DRIVER OF THE MICROBIOME5

Food is not the only major driver of the microbiome. So too is the way we raise food, Ellen Silbergeld stated. Most food animals are grown very intensively, including through the use of animal feeds that contain antibiotics. Food and Drug Administration (FDA) data indicate that 80 percent of total antimicrobial production in the United States in 2009 was for use in animal feed.6 Silbergeld stressed that the use of antibiotics in animal feed is not for veterinary medical purposes; rather, antibiotics are added to feed

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5 This section summarizes the presentation of Ellen Silbergeld.

6 These values were calculated by the Center for a Livable Future based on data provided by the Food and Drug Administration. For more information, read the posting on its website: http://www.livablefutureblog.com/2010/12/new-fda-numbers-reveal-food-animals-consume-lion%E2%80%99s-share-of-antibiotics (accessed September 19, 2012).



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