Introduction
Chicken meat is a major protein source throughout the world [
1] and the improvement of chicken production performance is necessary to meet increasing demand. Market weight is a key growth trait in chickens [
2] that can be scientifically altered. High market weight can increase the turnover in the chicken production pen, reduce labor costs and bring direct economic benefits to farmers. In spite of numerous studies focused on unraveling the mechanisms for market weight increases, this complex economic trait has yet to be fully understood [
3,
4,
5].
The gut microbiome influences host metabolism [
6], immune responsiveness [
7] as well as feeding behavior [
8]. These processes are potential mechanisms through which the gut microbiome affects chicken growth traits. For example, the presence of
Lachnospiraceae in the cecum was found to be related to high growth performance (body weight) for chickens while the presence of
Escherichia had the opposite effect [
9]. Cecal microbiome possessing
Microbacterium and
Sphingomonas in Turpan gamecock progeny×White Leghorn chickens were significantly correlated with high body weight
while Slackia was enriched in the ceca of low-weight chickens [
10]
. In addition, high Lactobacilli abundance in the chicken jejunum was beneficial to growth while Comamonas enrichment produced a negative outcome on growth rates [
11]
. The addition of exogenous Bacillus subtilis and Bacillus licheniformis to chicken feed also promoted growth performance [
12,
13]. Together, studies such as these have indicated that gut microbes are an important factor affecting growth performance in chickens. However, the conclusions have not been completely consistent in determining which microbes produce the greatest effect on chicken growth performance.
Relationships between the gut microbiome and growth performance in chickens have been also linked to gender [
14], induction of inflammatory factors [
11,
15]and regulation of fat metabolism [
10]. Modern omics technologies and especially metabolomics have linked gut microbes to regulation of metabolite production that can profoundly alter host physiological functions and thereby affect host phenotype [
6,
16,
17].These technologies have provided a new perspective for the analysis of the complex economic characteristics of livestock and poultry and is becoming an important means of analyzing complex traits of agricultural animals. Based on this, the mechanism of the formation and regulation of some complex traits in livestock and poultry has been revealed [
18]. For example,
Prevotella are a key bacterial genus that affect pig intramuscular fat deposition via production of lipopolysaccharide, branched-chain amino acids (BCAA) and arachidonic acids. This has clarified the mechanism of fat deposition in pigs [
19]. Xue et al. combined rumen microbiome and metabolome methods to explore the formation mechanism of milk protein in cows and high
Prevotella abundance in the rumen altered amino acid metabolism resulting in increased milk protein content. In contrast, enrichment of methanogens in the rumen was not conducive to increasing the milk protein content [
20]. Therefore, the technology of integrating gut microbiome and metabolomics is helpful for the identification of key microbes related to growth traits and the analysis of their metabolic mechanisms in chickens [
21,
22].
The Guizhou yellow chicken is a breed of yellow-feathered broiler chicken with excellent meat quality and good flavor currently being cultivated in Guizhou Province, China. This important local breed was obtained by crossing Guizhou Weining ♀ and Golden Plymouth Rock ♂ [
23] . However, this chicken does not have the high growth performance of commercial breeds. Therefore, in the current study we combined microbial 16S rDNA gene sequencing and untargeted metabolomics to explore the effect of gut microbiome on the growth performance of Guizhou yellow chickens to identify key microbes associated with market weight and preliminary explore the possible metabolic mechanisms for improved growth performance. Our results can provide new insights into the analysis of chicken growth performance and provide research references for the discovery of growth-promoting probiotics.
4. Discussion
Growth performance is an important economic trait for chickens and the gut microbiota have been proven to play an important role in the life activities of the host. However, clear correlations between the gut microbiome and chicken growth performance have not been fully revealed. Therefore, we employed 16S rDNA sequencing to characterize the cecal microbiota profile of Guizhou yellow chickens and explored the relationship between cecal microbiota and body weight. We found that high-growth chickens exhibited distinct cecal microbiota composition characteristics compared with low-growth chickens as did serum metabolites. These results suggested that cecal microbiota dysbiosis may be an underlying reason for poor growth performance in chickens.
The composition of the gut microbiota for individual animals also varies by location within the animal and the diversity and abundance of microbiota in the cecum is the highest. Therefore, our research used representative cecal microbiota from local chickens for further research [
30,
31,
32]. By comparing the α-diversity of cecal microbiota with in high-weight and low-weight chickens, we found that there was no significant difference between the two groups and was consistent with another report [
33]. The similarity of cecal microbiota in high-weight and low-weight chickens was analyzed using PCoA. The results of β-diversity analysis displayed an aggregation effect for each group indicating common cecal microbiota characteristics in the same weight group. At the phylum level, abundance of the Verrucomicrobiota was significantly higher in the cecum of HC while Desulfobacterota was significantly less while more abundant in the LC group. These results were inconsistent with a previous study using chickens in which the phyla Bacteroidetes was more abundant in low-weight chickens and Firmicutes was more abundant in low-weight chickens [
33]. This difference points out important relationships between breed, rearing methods and feed for the particular animals.
At the genus level, we identified
Lachnoclostridium,
Alistipes,
Negativibacillus,
Sellimonas and
Ruminococcus torques group as characteristic bacteria associated with high body weight in chickens while
Phascolarctobacterium was the key cecal genus in the low body weight chickens. This indicates an important role of these bacteria in the formation of growth traits. In particular,
Lachnoclostridium was a newly defined genus that can ferment polysaccharides to produce short-chain fatty acids including butyric and acetic acids. These have anti-inflammatory effects and can promote the growth of intestinal epithelial cells and the enhancement of intestinal barrier function.
Lachnoclostridium is also linked to host nutrient absorption and its reduced abundance will lead to the downregulation of functional pathways such as protein processing and nutrient transport in the host [
34,
35,
36]. In mice, a reduced
Lachnoclostridium abundance was associated with decreased body weight [
37]. These beneficial effects of
Lachnoclostridium on the host might be the reason for its enrichment in the cecum of high-weight chickens.
Alistipes is an obligate anaerobic Gram-negative bacterium (Bacteroidetes) that can produce acetic and propionic acids. A reduction in
Alistipes abundance is linked to a reduction in the levels of short-chain fatty acids. In addition,
Alistipes finegoldii was specifically proven to promote the growth of broiler chickens [
38,
39,
40].
Negativibacillus is a Gram-negative Firmicute and its abundance in the mouse gut was positively correlated with body weight gain [
41].
Sellimonas is an obligate anaerobic, non-motile Gram-positive first isolated from human feces in 2016 [
42].
Ruminococcus torques is an important microbe in the gut of patients with inflammatory bowel disease [
43]. It has a confirmed role in digesting resistant starch and stabilizing the intestinal barrier. Nevertheless, the functions of different species of the
Ruminococcus genus differed widely between reports. We found that the
Ruminococcus torques group was significantly positively correlated with pantothenic acid. A specific cause and effect relationship between growth performance and pantothenic acid metabolism by this group will be the subject of subsequent experiments.
Our serum metabolome analyses identified several vitamin metabolism-related pathways that may be related to body weight including pantothenate and CoA biosynthesis that were enriched in the serum of the high body weight group. In contrast, the riboflavin metabolism pathway enriched in the low body weight group which highlights the importance of vitamin metabolic pathways for the regulation of growth traits in chicken. In the differential metabolites, pantothenic acid (Vitamin B5) and menadione (Vitamin K3) were significantly higher in the serum of high-weight chickens. Pantothenic acid is involved in the metabolism of sugar, fat and protein in both humans and animals [
44]. Lack of pantothenic acid will cause growth retardation in poultry [
45]. Menadione is the fully reduced form of vitamin K that possesses had strong antioxidant effects in addition to its conventional function related to coagulation as a cofactor for glutamyl carboxylase that is also required for the function of the bone matrix protein osteocalcin required for bone remodeling and growth [
46]. Oxidative damage adversely affects growth performance in animals [
47] and menadione enrichment in high body weight group might be directly linked to this trait but also requires further experimentation.
Interestingly, we also established an association between gut microbiota and serum metabolites. In particular, pantothenic acid and menadione were enriched in high-weight chickens and were significantly associated with a variety of high-weight-associated bacteria. For example, pantothenic acid was significantly positively correlated with
Ruminococcus torques group and menadione was significantly positively correlated with
Lachnoclostridium,
Alistipes,
Negativibacillus and
Sellimonas. Metabolites enriched in low-weight chickens were primarily negatively correlated with these bacteria. These results indicated that
Lachnoclostridium and other characteristic bacteria of high-weight chickens may have beneficial effects on the body by promoting the secretion or absorption of metabolites.
Phascolarctobacterium was an important bacteria that we found to be enriched in the cecum of low-weight chickens.
Phascolarctobacterium is an obligate anaerobic originally isolated from koala feces [
48]. Another study also found that
Phascolarctobacterium abundance was higher in low feed conversion chickens and was related to a low nutrient absorption capacity of the host. However, the study did not elucidate a detailed mechanism [
49]. An addition study found that the abundance of
Phascolarctobacterium in the gut increases when chickens are exposed to high temperatures for extended periods and this exposure also results in elevated levels of heat shock proteins and related inflammatory gene expression [
50]. Heat stress altered the structure and function of enzymes in the chicken body, reduced the pH of the blood and caused metabolic acidosis. These were negative influences on chicken growth [
51,
52] and intestinal inflammation also decreases nutrient absorption causing body weight to drop [
53,
54,
55]. The presence of
Phascolarctobacterium was correlated with the induction of inflammation under the action of heat stress and other harmful factors in chickens. These results are consistent with our results where
Phascolarctobacterium was enriched in the cecum of low-weight chickens.
In our study, we identified the cecal microbes associated with body weight and preliminarily explored the metabolic mechanism of cecal microbiota affecting growth performance in chickens. However, our research also has some limitations that should be addressed or avoided in subsequent research. Firstly, our research was a small sample, single center, cross-sectional study. Secondly, we only analyzed the cecal microbiota and serum metabolites at time of sacrifice and did not collect the samples at different growth stages. Therefore, we need to further study the causal mechanism between gut microbiota and growth traits through larger sample sizes, use a multi-center design and apply the innovative research techniques of integrated omics technology to provide a research basis for revealing the mechanism of chicken growth traits as related to the cecal microbiome.