It has been reported that early antibiotic exposure might program later body composition and therefore might be a determinant of obesity risk, which was associated with alteration in gut microbiota composition and metabolites. Since antibiotics have been used to promote growth in animal and have been proposed as therapeutic regimes for malnutrition in humans , we firstly studied the effect of STA on pig growth. It was found that under well-controlled environmental conditions, STA did not significantly improve piglet growth performance during the administration period, which was consistent with other studies [22, 23]. It is generally assumed that growth-promoting mechanism of antibiotics is related to its ability to reduce clinical and subclinical infections under sanitary challenges . A meta-analysis involving more than 900 infants also showed that the positive effects of antibiotics were most prominent in the youngest and most malnourished children, but they were often less dramatic and not statistically significant in children without the disease . A previous report  found that STA affect subsequent performance negatively, and this may be that early STA administration increased their susceptibility to pathogens during the withdrawal period as discussed later. While, in the present study, STA administration didn’t affect subsequent performance during the withdrawal period.
In the present study, STA did not significantly alter body composition and tissue deposition of piglets at the end of the 2-week administration period, which contrasts with the previous report  that STA increased fat mass in young mice after 7-week exposure. This may be attributed to differences in the STA type and dose or the shorter STA administration time, which have not yet been shown to be different in body composition between the groups. During the withdrawal period and the whole period, piglets in the STA group tended to have a higher body lipid deposition than those in the CON group.
It was found that during the administration period, STA treatment increased abundances of several harmful bacteria/pathogenic bacteria (Alloprevotella, Bacteroides, Solobacterium and Sutterella) compared with CON group. Alloprevotella was considered an opportunistic pathogen microorganism that causes infections in the host [27, 28]. It was reported that individuals with the Bacteroides enterotype increased susceptibility to disease . Solobacterium was positively correlated with colorectal cancer . Previous studies showed that rather than directly induce substantial inflammation, Sutterella can degrade IgA to impair the functionality of the intestinal antibacterial immune response [31, 32]. However, during the withdrawal period, STA treatment increased the abundances of several harmful bacteria, for example, Alloprevotella and Oscillibacter and decreased abundances of several beneficial bacteria such as Succinivibrio and Desulfovibrio. For example, Oscillibacter has been reported that promotes metabolic diseases and gut dysbiosis . Succinivibrio was reported to be lower in humans with environmental enteric dysfunction (a causative factor of childhood stunting) . Desulfovibrio is significant in sugar metabolism and is negatively associated with inflammation markers [35, 36]. These results suggested that STA were initially effective in decreasing the abundance of pathogenic bacteria during the administration period, but they were not able to continue the effect during the withdrawal period, leading to the rebound of pathogenic bacteria such as Alloprevotella and the increase of the abundance of other pathogenic bacteria. Evidence has shown that antibiotic administration, especially in early childhood, increases susceptibility to intestinal infections after antibiotic cessation .
Remarkably, we found that STA treated decreased the abundance of Blautia that play a potential protective role against obesity either during STA administration or its subsequent withdrawal period. Blautia, a common acetic acid-producing bacterium, may suppress insulin-mediated fatty deposits in adipocytes and promote the metabolism of unbound lipids and glucose in other tissues by activating the G protein-coupled receptors GPR41 and GPR43, thereby alleviating obesity-related diseases . A previous study found that Blautia is the only intestinal microorganism negatively correlated with visceral fat accumulation, and adiposity biomarker for metabolic disease risk . In a study of differential microbiota between lean and fat line chickens, Blautia was significantly reduced in the latter . Similarly, significant depletion of Blautia was observed in obese children [41, 42].
Metabolomics, an effective method to detect the variant metabolites and biochemical pathways [43, 44], was utilized to further explore gut microbiota metabolism in response to STA administration and its subsequent withdrawal. The PLS-DA model was a clear separation of colonic metabolites between the STA and CON groups either within the administration period or within the withdrawal period, suggesting significant differences in the metabolic profiles due to different treatments. During the administration period, STA significantly altered glycerophospholipid metabolisms, as reflected by the increased concentrations of phosphatidylglycerol (3:0/18:1) and lysophosphatidylethanolamine 22:5. Previous studies showed that glycerophospholipid is vital in the strengthening intestinal barrier [45, 46]. The compounds involved in amino acid metabolisms, such as L-cysteinesulfinic acid, phenylacetylglycine, phenylacetylglutamine and 5-hydroxylysine, were dramatically decreased in the STA group compared to the CON group, which suggests lower nitrogen sources left for the microbial fermentation of the large intestine. A previous report found that antibiotics could upregulate the gene expression of amino acid transporters and receptors in the small intestine and thereby improve the absorption of amino acids . However, during the withdrawal period, piglets in the STA group showed higher amino acid relatives like proline, L-lysine, L-cystathionine and S-adenosylhomocysteine in comparison with the CON group, indicating an increased amount of protein derived substrate for microbial fermentation in the colon, which was consistent with a previous study . This could be harmful to host health due to the possible formation of a range of toxic and harmful products from protein fermentation, such as ammonia, indoxyl sulfate and trimethylamine oxide . Meanwhile, in the present study, STA significantly altered the compounds involved in FAHFAs metabolism, indicating by an enhanced concentrations of FAHFA (16:0/18:0) and FAHFA (18:0/20:2) during the administration period, and a decrease in the concentrations of FAHFA (14:0/22:3), FAHFA (17:1/22:3), FAHFA (18:2/17:2) and FAHFA (20:2/22:3) than CON during the withdrawal period. FAHFAs are a novel class of bioactive lipid, forming by esterification hydroxyl group of a hydroxy fatty acid and the carboxyl group of a fatty acid . Previous studies have shown that palmitic acid esters of hydroxy stearic acids and the family of polyunsaturated FAHFAs have anti-inflammatory and immunomodulatory effects [51, 52]. Besides, several new short-chain FAHFAs (SFAHFAs) of acetic acid or propanoic acid esterified long-chain hydroxy fatty acids tended to be lower in mice fed with a high-fat diet than those fed with a regular diet .Similarly, STA treatment decreased the concentrations of several SFAHFAs like FAHFA (2:0/24:2) and FAHFA (4:0/22:0) during the withdrawal period, which coincided with higher content of body lipid and lower abundance of Blautia, a bacterium with a protective role against obesity. These results suggest that STA plays beneficial roles in gut health during the administration period, but it may exert harmful effects on gut health during the withdrawal period. Simultaneously, most compounds involved in carbohydrate metabolisms like D-arabinose, D-mannitol and coniferin were increased by STA treatment than the CON treatment during withdrawal period, indicating that most carbohydrates can be fermented by gut microbiota in the colon.
The gut microbiota and microbial metabolites are vital for promoting intestinal immunity balance. In this study, we found correlations between the gut microbiota and microbial metabolites. However, whether the effects of STA induced on gut microbiota and microbial metabolites would influence the host immunity and its mechanism was still unknow. In the future, we will further study after STA administration the relationship between gut microbiota, microbial metabolites, corresponding metabolite receptors and host immunity.