Odor emission from chicken excreta is produced mainly by the microbial degradation the substrates in the cecum of broilers, which is a serious environmental problem in broilers industry. Strategies to mitigate emissions are needed. Among the strategies to reduce odor and gas emissions in the broiler industry, dietary supplementation of feed additives including SBO are becoming an accepted strategy because of their beneficial effects against intestinal pathogenic microbes and reduces the production of odor compounds. The aim of this study was to evaluate the effects of different levels of dietary SBO and CHL on odor-causing compounds in excreta and the composition of cecum microbiota of broilers.
At the end of the experiment, birds fed SBO has indole reduced by 21–80% and impairment of skatole by 33–64% compared with negative treatment. The value of skatole was decreased 22% between 3.5SBO treatment and antibiotic treatment. Also, 3.5 g/kg SBO supplementation decreased the pH, increased acetate in excreta compared with both control treatments, while 3.5 g/kg and 5.0 g/kg SBO had higher formate than that both control treatments. The produced short chain fat acids from fermented indigestible residues by the abundant microorganisms present in the ceca. Oligosaccharides are able to enter into the ceca in generous amounts and being fermented there by microflora [24]. Our former study has shown similar results [3], when supplementing 1.25 g/kg SBO in broiler decreased indole, skatole and the pH in broiler excreta. Additionally, it has been observed that SBO could decreased the concentration of indole and skatole, and pH value by intestinal microbiota of broilers in vitro [18].
Overall, in this study, the results demonstrated that the dietary supplementation with SBO might be a useful strategy to attenuate the production of odor in broilers. SBO supplementation decreased the concentration of indole, skatole and pH, and enhance the concentration of part of organic acid of excreta in broilers. The results of correlation analysis further indicated that the odor compounds were associated with intestinal microbial community. Broilers fed SBO and CHL had some alterations in the alpha and beta diversity, though non-significant, which indices of the cecal microbiota and these modifications promoted relevant changes of the microbial community structure.
In the early animal trials and in vitro studies, we used the molecular technique of denaturing gradient gel electrophoresis revealed a shift in the microbiota composition [3, 8, 18]. However, this technique lack the depth and precision to reveal specific changes in bacterial composition. Subsequently, the method next-generation sequencing of the bacterial 16S rRNA gene could offer more information on specific changes in certain bacterial populations. However, there is scarce reports about using high-throughput sequencing study the changes on microbial communities of cecum by supplementation SBO in broilers. The approach of high-throughput sequencing provided a much accurate and in-depth estimation of cecal microbial diversity in broilers. Therefore, the results of present study provided the information of the ecology of cecal bacterial communities. In line with the previously reports on cecal microbiota of broilers [1, 30], it was dominated by Bacteroidetes and Firmicutes at the phylum level, regardless of diet. The metabolism favours the fermentation of cellulose and stach etc in the cecum, results in higher microbial diversity and dominance of the saccarolytic and anaerobic order of Bacteroides and Clostridiales [1]. The most abundant genera detected in this study were unclassified, Bacteroides, Ruminococcus, Phascolacrctobacterium, Rikenella, Oscillospira, Faecalibacterium, and the unclassifed bacteria was the most. The results were quite similar with those obtained by Wen et al. and Zhu [1, 30]. Bjerrum et al. [30] reported that bacterial strains closely related to Faecalibacterium prausnitzii were dominant in the ceca.
Based on the phylogenetic diversity of bacterial communities and number of OTUs, SBO and CHL did not change the chicken cecal bacterial community membership, but the treatments significantly altered relative abundance of certain taxa in the cecum. This observation is in accordance with the study of Zhu et al. [30] who reported that SBO addition (0.6%) altered the chicken cecal microbiota. When added frucotooligosaccharide in broiler diets, the total number of anaerobes and Lactobacilli in the ceca increased while the number of E. Coli decreased [32]. Similar results were observed by Baurhoo et al by mannanoligosaccharide [33]. In present study, Bacteroides, Bilophila and Escherichia were significantly decreased in the cecum by SBO (Fig. 4). While Ruminococcus (Fig. 4), Lachnospiraceae and Coriobacteriaceae (Additional file 4) were enriched, Rikenella (Fig. 4) appeared to be diminished by chlortetracycline. These results are consistent with earlier observations that in-feed antibiotic preferentially enriched butyrate-producing bacteria [19], SBO inhibited the growth of pathogenic microbes, and reduced the production of odor compounds [3, 14, 30]. Previous studies have found that supplementation of broiler diets with a mixture of chlortetracycline and other antibiotic increased both Ruminococcaceae and Lachnospiraceae [34]. It is known that both Lachnospiraceae and Ruminococcaceae produce butyrate [19, 35, 36].
The most dramatic effect of SBO supplementation is differential reduction of the members of four most dominant bacterial families (Lachnospiraceae, Bacteroidaceae, Desulfovibrionaceae,and Enterobacteriaceae) in the chicken cecum (Additional file 4). SBO increased the population of a group of lactic acid bacteria in vitro, genera Lactobacillus, Pediococcus, Weissella, and Leuconostoc in the cecal contents of young broiler chickens [17]. In broilers, supplementation of 1.25 g/kg SBO showed a higher cecal bacterial diversity, along with lower excreta indole and skatole production, it also increased the excreta total VFA concentration and decreased the pH value, when compared with that of the broilers fed the control diet [3]. Dietary supplementation with SBO increased the diversity of intestinal microflora and elevated the numbers of some presumably beneficial intestinal bacteria, (eg, Bifidobacterium sp, Faecalibacterium prausnitzii, Fusobacterium prausnitzii, and Roseburia), also increased the concentration of short-chain fatty acid in the intestinal lumen, and reduced the numbers of bacteria with pathogenic potential (eg, Escherichia coli, Clostridium, and Streptococcus) and the concentration of several protein-derived catabolites (eg, isobutyrate, isovalerate, and ammonia) in piglets[15]. In contrast, Ma et al. [16] described that intragastric administration of 4.0 g/kg body weight SBO significantly enhanced the proliferation of Bifidobacteria and lactic bacteria, and increased numbers of Enterococci and decreased numbers of Clostridium perfringens in the fecal contents of mouse.
Two lactic acid bacterial genera including Lactobacillus and Enterococcus were differentially regulated by SBO among the abundant OTUs in the cecum, however, no statistical differences were observed on the relative abundance of these bacterial genera between treatments, which may be related with the adding levels of SBO (Additional file 3). Lactic acid bacteria provide a myriad of beneficial effects to the host, and are widely used as probiotics in animal production [37, 38]. It is interesting to note that Lactobacillus was upregulated by 0.5, 2.0, and 3.5 g/kg SBO, while the Enterococcus was obviously reduced in response to 0.5, 2.0, and 3.5 g/kg SBO. An upregulation in the Lactobacillus abundance is consistent with earlier observations that SBO administration was associated with population of the Lactobacillus species [17, 39]. The lactate concentration in excreta was significantly positive correlation with Lactobacillus in the cecum (Table 4). Both Lactobacillus spp. and acetate production in the cecum of chickens by xylo-oligosaccharides treated diets have been shown to be increased, may promote intestinal health [40].The abundance of Enterococcus was reduced by lower levels of SBO (Additional file 3). Enterobacter aerogenes has been showed to be responsible for the degradation of L-tryptophan to skatole in cecum of broilers [18]. A lone Proteobacteria (Escherichia) was also reduced by all levels of SBO and CHL, and significantly positive correlation with the levels of skatole in excreta, which is in agreement of earlier reports of a downregulation of Escherichia in response to in-feed SBO[16, 18]. Escherichia is a typical harmful bacterium in the intestinal tract that is involved in the degradation of tryptophan to indole [41]. These results collectively suggest that SBO has a strong tendency improve the intestinal microbial balance [3] by favouring a quick proliferation of beneficial strains and inhibiting the growth of pathogenic microbes [7] and then contributes to reduce the production of odor compounds [18, 42].
Our results show that a higher positive correlation for acetate versus Faecalibacterium, which is a bacterial stain known to ferment mono-oligosaccharides and oligosaccharides into butyrate and lactate [31]. The intestinal metabolite skatole is generated by decarboxylation of indoleacetic acid by Bacteroides spp. and Clostridium spp. [43]. Indole formation is converted from tryptophan via the action of the enzyme tryptophanase, which is expressed in many Gram-positive bacterial species including Escherichia coli, Clostridium spp. and Bacteroides spp. [44–46]. Our results show that a positive correlation for indole versus Bilophila, skatole versus Bacteroides, Bilophila and Escherichia, which is in agreement of earlier report of Uncultured Lachnospiraceae bacterium and Bacteroides sp. are associated with the production of major odor-causing compounds in the excreta of broilers [3].
Indole and skatole are mainly produced by the microbial degradation of several substrates of cecum of broilers. The present study demonstrated that supplementation of broilers dietary SBO significantly reduced the production of odor compounds in excreta, and this effect was associated with cecal microbiota composition, in comparison to its un-supplemented control counterpart, and these partly similar to those of CHL individuals.