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.