Effect of antibiotic treatment on microbial composition and expression of antimicrobial peptides and cytokines in the chick cecum

The aim of this study was to confirm whether the expression of innate immune molecules in the chick cecum is altered in association with changes in the composition of the intestinal microbiome that are regulated by treatment with antibiotics. Broiler chicks were administered with antibiotics (penicillin and streptomycin) daily, and the composition of the microbiota, expression of innate immune molecules, and localization of antimicrobial peptides in the chick cecum were examined at day 7 and day 14 using real-time PCR and immunohistochemistry. The oral administration of antibiotics caused an increase in the relative frequency of the Enterobacteriaceae family and a decrease in some gram-negative (Barnesiellaceae) and gram-positive bacterial (Clostridiaceae and Erysipelotrichaceae) families. The gene expression levels of immune molecules, including 4 Toll-like receptors (TLR) (TLR 2, 4, 5, and 21), inflammation-related cytokines (IL-1β, TGFβ3, TGFβ4, and IL-8), and antimicrobial peptides (avian β-defensins and cathelicidins) showed a tendency to decrease with antibiotic treatment at day 7. However, expression levels of TLR21 and some cytokines (IL-1β, TGFβ3, and IL-8) were higher in the cecum or cecal tonsils of the antibiotic-treated group than in those of the control at day 14. The immunoreactive avian β-defensin 2 and cathelicidin 1 proteins were localized in the leukocyte-like cells in the lamina propria, and they were aggregated in the form of small islands. We conclude that the expression of innate immune molecules, including TLR, inflammation-related cytokines, and antimicrobial peptides, in the cecum are altered in association with changes in the density or composition of the luminal microbiota during the early phase of life in chicks.