4.1 The effect of different dietary protein levels on intestinal microflora diversity
Animal intestines comprise extensive micro-ecological environments, and as organisms rely on their microbial floras to digest and absorb nutrients [13]. The intestine therefore not only provides a suitable living environment for the intestinal flora but also plays a key role in the digestion of food and thus overall health [14]. This means that the number and types of microbes in an intestinal flora play important roles in animal health and growth [2,5]. Diets containing low protein levels can increase the richness and diversity of intestinal floras in finishing pigs. The analysis of differences in bacterial community composition presented here shows that pigs in a diet group with low protein levels had a different gut microflora compared with those in the normal protein level group. Low-protein diets can enhance the intestinal flora balance and benefit overall health, while an excessive protein intake will cause fermentation and produce harmful substances, including ammonia, amines, and phenols. These can adversely influence the intestinal microflora and harm overall health [5,15–16]. In earlier work, Greenhill [17] and Shoaie [18] both showed that the intestinal microflora of fattening pigs is mainly comprised of representatives within the phyla Firmicute and Bacteroides. This means that the ratio between these two groups can be used as an indicator to assess some obesity diseases; the higher the ratio, the easier it is for a pig to become obese. At the same time, Shannon and Simpson indices are usually used to assess intestinal microfloral diversity. These values are usually positively correlated with microbial flora stability and ability to resist pathogenic bacterial infections [19]. Results show that both the diversity and abundance of intestinal microbes in Bamei pigs at the 14% dietary protein level increased while Shannon index values for this group were significantly higher in the jejunum and cecum than for the basal protein level group (16%). These results show that changes in dietary protein level are beneficial to the intestinal health of Bamei pigs and can increase both the richness and diversity of intestinal flora in growing and finishing animals. Previous results published by Rist et al. [20] showed that feeding at different protein levels can influence small intestine microbial composition and promote the reproduction of beneficial bacteria, thereby improving intestinal health. Similarly, Cao et al. [21] studied the effects of dietary protein on the intestinal microbes of weaned piglets; the results of this study showed that increasing dietary protein level also enhanced the number and types of intestinal microbes in weaned piglets as well as their intestinal microenvironments [22]. Fan et al. [19] also showed that when dietary protein level decreased from 16% to 13%, the abundance and diversity of ileal flora increased alongside the proportion of the beneficial Lactobacillus . The proportion of colonic beneficial bacteria Megacoccus also increased significantly. In contrast, when dietary protein level fell to 10%, the diversity of ileum and colon flora in growing and finishing pigs decreased in concert with their growth performance. These results show that changes in dietary protein levels can promote microbial diversity in the intestine of Bamei pigs to a certain extent, but an excessive reduction will influence normal growth performance. Thus, under normal circumstances, a gut microflora changes within a certain physiological range given fluctuations in the external environment and diet [23]. Studies have shown that when dietary protein is reduced from 23% by 4% or more, piglet the performance can compromised. Low-protein diets also maintain pig intestinal health by reducing toxic microbial metabolites and improve microbial diversity [24]. The microbial diversity analysis presented here shows that the communities seen in different protein groups have obvious distributions in the jejunum and cecum. Microorganisms at the 14% level grouped well which indicates that diets containing low protein levels can significantly improve the intestinal microflora of Bamei pigs.