The effects of rosmarinic acid and antibiotics on the intestinal morphology, function, and cecal microbiota of broiler chickens

While both rosmarinic acid (RA) and antibiotics (Abx) demonstrate growth-promoting effects, their mechanisms of action differ significantly. This study comparatively evaluated the effects of RA and Abx on the intestinal morphology, function, and cecal microbiota in broilers. 180 one-day-old broilers were divided into three groups and fed a basal diet or basic diet supplemented with RA (100 mg/kg) or Abx (100 mg/kg, composed of vancomycin, metronidazole, kanamycin, and ampicillin in a 0.5:1:1:1 ratio). The experimental birds were housed in single-layer cages at a density of 20 birds/m2, with ad libitum access to both feed and water. On day 21, chickens were euthanized with subsequent collection of blood, intestinal tissues, and cecal digesta for analyses. Data were analyzed by one-way ANOVA, except for 16S rRNA sequencing data which were analyzed by Kruskal-Wallis test. The results showed that both RA and Abx supplementation significantly increased body weight, average daily gain, and feed intake (P < 0.05). Both treatments significantly upregulated ileal tight junction protein expression and enhanced nutrient absorption capacity (P < 0.05). However, Abx treatment showed negative effects including reduced jejunal and ileal lengths (P < 0.05), impaired immune organs (P < 0.05), and suppressed ileal immunoglobulins (sIgA/IgM) (P < 0.05). However, RA demonstrated beneficial effects by enhancing both ileal and cecal morphology (P < 0.05). Notably, Abx treatment significantly increased the abundance of drug-resistant bacteria such as Escherichia_Shigella, Enterobacter, Clostridia_vadinBB60, and Ligilactobacillus (P < 0.05). In contrast, RA treatment significantly increased the abundance of beneficial bacterial genera including Lachnospiraceae, Faecalibacterium, Lachnoclostridium, Oscillibacter, and Eubacterium_hallii_group (P < 0.05), while the abundance of these genera were significantly decreased by Abx treatment (P < 0.01). Furthermore, RA treatment significantly increased the concentrations of acetate, isobutyrate, and isovalerate (P < 0.05), whereas Abx treatment significantly decreased the concentrations of all short-chain fatty acids (P < 0.01). These results demonstrated that RA offered superior benefits for gut health without the negative impacts observed in Abx treatment.