Succession of the pig gut microbiota
Animals are thought to be bacteria-free prior to birth. However, during the birthing process animals are exposed to a variety of bacteria in the vagina and from fecal contamination on the dam or in the environment. The concept of microbial succession in animals is an ecological principle that has been long recognized. The composition of the gut microbiota is not static and shifts over time. There is a succession of microbes over time that culminates in a “climax” community, which is more stable (Palmer et al., 2007). Many factors contribute to the succession process including the physiological changes that occur in the gut as it transitions to an Sulfo-NHS-Biotin environment. The consumption of solid foods is another major factor that triggers a shift toward a bacterial assemblage characteristic of the adult microbiota (Palmer et al., 2007). A recent study of pig fecal microbial shifts during the weaning transition has contributed to our understanding of microbial transitions that occur in this physiologically stressful time for animals (Pajarillo et al., 2014a). In that study, the fecal microbiota of 15 commercial pigs was measured during the weaning transition using pyrosequencing of the V1–V3 (pre-weaning at 4 weeks of age and post-weaning at 6 weeks of age) (Table 1). At the phylum level gut microbial communities during the pre-weaning period were primarily comprised of the phyla Firmicutes (54%), Bacteroidetes (38.7%), Proteobacteria (4.2%), Spirochaetes (0.7%) and Tenericutes (0.2%) (Fig. 1). In comparison, at the post-weaning period the compositions of the fecal microbiota of these pigs, while containing the same major phyla, show marked changes in the relative proportion of each phylum: Bacteroidetes (59.6%), Firmicutes (35.8%), Spirochaetes (2.0%), Proteobacteria (1%), and Tenericutes (1%). Overall, Firmicutes and Bacteroidetes were the most abundant phyla in fecal microbiota of piglets accounting for more than 90% of the fecal bacterial community at both pre-weaning and post-weaning periods. Firmicutes were most abundant in pre-weaning piglets, shifting gradually to Bacteroidetes after weaning (Pajarillo et al., 2014a). At the genus level, Bacteroides, Blautia, Dorea, Escherichia and Fusobacterium were abundant before weaning. After weaning Prevotella and Clostridium became more abundant while there was a decrease in Bacteroides (Pajarillo et al., 2014a). It was speculated that the greater abundance of Bacteroides during the pre-weaning period might be due to their ability to utilize monosaccharides and oligosaccharides present in sow\’s milk, and the increased abundance of Prevotella during the post-weaning was due to their ability to degrade hemiculluloses such as xylans in plant-based feed (Hayashi et al., 2007; Lamendella et al., 2011). It has also been suggested that microbial shifts during weaning transition might be due to a combination of multiple factors such as chemical composition of the diet, stress resulting from the weaning process, and other physiological factors (Pajarillo et al., 2014a).
After weaning, shifts in the composition of the gut microbiota continued until market age (22 weeks of age). Firmicutes and Bacteroidetes were also the most abundant phyla in fecal microbiota of the growing-finishing pigs using pyrosequencing of the V3 region of the 16S rRNA gene (Table 1 and Fig. 1) (Kim et al., 2011, 2012). Kim et al. (2011) described the fecal microbial succession from a total of 20 commercial growing-finishing pigs from two different farms. They sampled pigs starting when they were 10 weeks of age and then at three-week intervals until the pigs were 22 weeks of age. In that study, the fecal microbiota was comprised of major five phyla, Firmicutes, Bacteroidetes, Proteobacteria, Actinobacteria, and Spirochaetes regardless of age. Firmicutes and Bacteroidetes accounted for approximately 90% of all bacteria present between 10 and 22 weeks of age. The proportion of bacteria in the phylum Firmicutes increased over time while the proportion of bacteria in the phylum Bacteroidetes decreased (Fig. 1). From a genus level perspective, the predominant genus was Prevotella which is in the phylum Bacteroidetes. Prevotella represented up to 30% of all classifiable bacteria when the pigs were 10 weeks of age. However, by the time these pigs were 22 weeks of age, Prevotella accounted for only 3.5–4.0% of the bacteria. As the levels of Prevotella decreased, there was a pronounced increase in Anaerobacter (in the phylum Firmicutes). Among the 15 most abundant genera Anaerobacter, Sporacetigenium, Oscillibacter, and Sarcina increased as pigs aged, whereas Prevotella, Lactobacillus, Megasphaera, Faecalibacterium and Dialister decreased. A comparison of individual pigs revealed some variation between animals but groups of animals of the same age were more similar to each other compared to pigs of different ages (Kim et al., 2011). Correlated with this observation there was a tight clustering of microbial OTUs between pigs at the same age as measured by principal coordinate analysis. Overall the results from Kim et al. (2011) indicated that microbial ecosystems in each pig continued to change and converged toward a profile characteristic climax community of the GIT of adult pigs as the pigs aged (Kim et al., 2011).
Succession of the pig gut microbiota