Oregano essential oil (OEO) has antimicrobial, anti-inflammatory, and antioxidant properties. Thus, OEO is regarded as a promising alternative to antibiotics. In fish, dietary supplementation with OEO improves growth performance, the immunological responses, and pathogens resistance. However, few studies to date have investigate the effects of OEO on the gastrointestinal tract microbiotas of aquatic species. The GI tract supports a vast population of microorganisms that play important roles in host growth, development, and health. We hypothesized that the beneficial effects of OEO on host animals might be closely associated with the host intestinal microbiota. Thus, in this study, we assessed the effects of dietary OEO on both the host immune response and the host gut bacterial community, in order to identify interactions between the host and the gut microbiota.
A total of 300 fish were randomly assigned to one of the four treatments supplemented with OEO at a concentration of 0 (control), 500 (OEO-L), 1500 (OEO-M) or 4500 mg/kg (OEO-H). After eight-week rearing trial, blood and intestinal samples were collected to investigate the immune response and intestinal microbiota. The cumulative survival of fish was recorded for 7 days following A. hydrophila infection. Oral administration of OEO increased the concentration of lysozyme, complement C3 and complement C4 in serum (Fig 1). The 7-d cumulative survival was greater in the OEO- treatment groups (Fig 2). Bacterial community composition in the OEO-H group was distinct from that in the control (Fig 3).