Effect of Breast and Formula Feeding on Gut Microbiota Shaping in Newborns

Abstract

The gastrointestinal microbiota is a complex and dynamic ecosystem consisting of several hundreds of different microbes, mainly bacteria (1011–12 bacteria/g of colonic content, forming 60% of total fecal mass; Eckburg et al., 2005; O’Hara and Shanahan, 2006). Total number of bacteria exceeds 10 times the number of human cells, and the collection of microbial genome (microbiome) contains 100 times more genes than the human genome (Vael and Desager, 2009). Gut microbiota influence the growth and differentiation of gut epithelial cells, and play pivotal nutritive, metabolic, immunological, and protective functions (O’Hara and Shanahan, 2006). Its deregulation is involved in the pathogenesis of immunological, cardiovascular, and metabolic diseases (Hammer, 2011; Maslowski and MacKay, 2011; Harris et al., 2012). The investigation on microbiota composition started in 1900 (Tissier, 1900) and has been performed by culturing methods since the recent advent of DNA sequence-based methods, that, thanks to their ability to identify a large number of species that cannot be cultivated, have allowed a more complete and rapid assessment of the gastrointestinal ecosystem (Palmer et al., 2007; Adlerberth and Wold, 2009). On the basis of 16S ribosomial – RNA encoding gene, more than 7000 distinct phylotypes have been detected in the human distal gut (Vael and Desager, 2009), with high inter-individual and age variability, but belonging to a limited number of broad taxonomic divisions (mainly the anaerobes Bacteroides, Eubacterium, Clostridium; Hayashi et al., 2002; Eckburg et al., 2005; Zoetendal et al., 2008). In a very recent study, Arumugam et al. (2011), by combining fecal metagenomes of individuals from different countries, identified three different enterotypes (with the prevalence of Bacteroides, Prevotella, and Ruminococcus species, respectively) that are not nation or continent specific, and showed that intestinal microbiota variation is stratified, not continuous, indicating further the existence of a limited number of well-balanced host-microbial symbiotic states. These enterotypes do not seem to differ in functional richness and apparently do not correlate with nationality, gender, age, or body mass index; at the same time, they seem to characterize and be quite stable in individuals, so that they can be restored after perturbations. Gut microbiota composition and concentration physiologically varies throughout the gastrointestinal tract (increasing gradient from the stomach to the colon and characteristic gut-compartment distribution of microflora) and life stages, progressing from the newborn sterility to the extremely variable and dense colonization of adult gut, under the influence of various internal host-related and external factors (Mackie et al., 1999; Palmer et al., 2007).