A prebiotic is a substrate that is selectively utilized by host microorganisms conferring a health benefit. This definition requires that prebiotics 1) reach microorganisms intact, 2) that selected microorganisms use them as substrate and 3) that they mediate health benefits via the microorganisms. Human milk oligosaccharides (HMO) reach the gut intact. They are selective substrates for Lactobacillus and Bifidobacterium strains. Whether HMO reveal health benefits via the microorganisms is actively under investigation.
Since 2017, prebiotics are defined by the The International Scientific Association for Probiotics and Prebiotics (ISAPP) as “a substrate that is selectively utilized by host microorganisms conferring a health benefit.” This statement keeps a microbiota-mediated health benefit, but does not restrict a prebiotic component to be a food or carbohydrate nor are the effects limited to the gastro-intestinal tract (Gibson et al. 2017). This definition still requires that a prebiotic substance
(1) reaches the microorganisms intact,
(2) that selected, not all, microorganisms use the prebiotic candidate as substrate, and
(3) that the beneficial effect to the host is mediated via actions of these microorganisms (La Fata et al. 2017).
Human milk oligosaccharides (HMO) meet the first and second set of requirements, whereas the third set is actively under investigation.
HMO are resistant to digestive enzymes which makes them predominantly indigestible fibres and are unaffected by intestinal pH (Engfer et al. 2000; Gnoth et al. 2000); only 1-2% of HMO are absorbed. Consequently, many HMO are reaching the colon intact (Obermeier et al. 1999) where individual isoforms are candidate prebiotics by serving as nutrient source for specific colonic microbiota species (Gibson et al. 2017).
The HMO fraction as total has been associated with the establishment of a favourable intestinal microbiome (Kunz. 2012). When comparing the gut microbiome of breastfed and non-breastfed infants, different bacterial composition was identified (Harmsen et al. 2000). The predominance of Bifidobacteria species in breastfed infants was attributed to HMO presence (Kunz. 2012). When adding the two industrially generated HMO isoforms 2'‑fucosyllactose and lacto-N-neo-tetraose as mixture to infant formula, a shift of the stool microbiota composition was detected towards a similar composition to that of breastfed infants (Steenhout et al. 2016). Similarly, in vitro studies on cell lines are supporting a HMO-mediated growth advantage for beneficial over pathogenic bacteria (Sela and Mills. 2010). Individual HMO types, including 2'‑fucosyllactose, 3‑fucosyllactose, lacto‑difucotetraose, 3'‑sialyllactose, and 6'‑sialyllactose, promote the growth of Bifidobacteria strains (B. infantis, B. bifidus, B. breve, B. longum), Bacteroides genera, and specific Lactobacillus species (Lactobacillus delbrueckii) (Lewis et al. 2015; Yu et al. 2012). This may be due to the specialised ability of B. breve, B. infantis, and most B. longum species of using fucosylated HMO as growth substrate that these pathogens lack (Sakanaka et al. 2019). In contrast, the same HMO isoforms cannot be used as growth substrate by many pathogens, including Escherichia coli, Clostridium perfringens, Streptococcus agaltictiae, Enterobacter, Acinetobacter baumannii (Ackerman et al. 2018; Underwood et al. 2015; Yu et al. 2013) or Streptococcus aureus, the latter being one of the most frequent causes for bacterial infections in infants (Lin et al. 2017).
Besides directly affecting microbiota growth by serving as substrate of particular species, HMO are indirectly promoting bacterial growth via cross-feeding (Smith et al. 2019). When bacterial strains ferment HMO and other prebiotic substrates in the intestinal lumen the left-overs and bacterial metabolites serve other bacterial strains as growth substrate (Sela and Mills. 2010). This supports the development of a healthy and diverse microbiota ecosystem.
Demonstrating the third set of requirements for a prebiotic classification of HMO – that is the demonstration of health benefits mediated via colonic microorganisms – has been difficult. This is because – until about 2000 – HMO could only be extracted from mother's milk and tested small-scale in laboratory settings. Since then, technological advancements have been making individual HMO isoforms available for large scale clinical studies or commercial productions (Bode et al. 2016) and data from clinical safety and/or efficacy studies have been slowly accumulating (Reverri et al. 2018).
Health benefits associated with HMO include
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