1 INTRODUCTION
The animal intestine is the main place where nutrients are digested. A body provides microorganisms with the nutrients they need for growth via the intestine; microorganisms in intestines decompose food materials and produce smaller molecules that the host organism can digest and absorb. Microorganisms in the guts of animals are therefore mutually beneficial [1,2]; a large number of microorganisms live in animal intestines, reaching a total such that the number of microbial cells exceeds other components [3]. Microorganisms influence host growth, development, nutrient absorption capacity, and intestinal immunity and play a vital role in health [2]. In earlier work, Kim et al. [4] showed that intestinal microorganisms exert a certain promotional effect on animal body growth and development and participate in digestive immune functions. Intestinal microbial diversity therefore exerts an important impact on microbial flora stability. In other words, the higher the diversity, the more stable a flora will be and so the healthier the animal gut. The microbial ecosystem of a pig gastrointestinal tract is influenced by numerous factors but changes in dietary structure are thought to be amongst the most important. The amount and quality of protein in a diet exerts a significant influence on microbial community structure in an intestine [5]; indeed, an intestinal flora seems to be sensitive to source changes in dietary protein. In one example, the use of highly digestible protein sources has been shown to reduce fermentation and the growth of potentially pathogenic species [5]. Similarly, Fan Peixin et al. [6] showed that reducing dietary protein levels by just 3% can increase ileal microbial diversity in fattening pigs and, indeed, when this level is reduced by another 3%, microbial diversity is significantly reduced. In another study, Chen found that restricting 15% of dietary protein can increase the ratio of beneficial microorganisms to harmful bacteria and optimize the structure of the intestinal microbial community [7]. It is clear that dietary protein level plays a very important role as a component of pig production. A correct level not only provides the body with the nutritional effects of amino acids (AA) necessary for endogenous protein synthesis but also participates in regulating feed intake, lipids, and glucose metabolism to maintain growth, development, biological proliferation, digestive enzymes, and hormone secretion in the intestine [8-10].
The demand for protein feed is very large throughout pig production. Indeed, in order to reduce heavy nitrogen emissions into the environment, protein levels should be reduced to promote the sustainable development of this industry [11–12]. Protein levels fed to Bamei pigs were reduced in this study by around 2% and 4% compared with the base protein level of 16%, while essential AA were supplemented. The effects of dietary protein levels on the intestinal microflora of Bamei pigs was studied in order to provide a theoretical basis for further enhancing the production of this species.