Background and Aim: As an alternative to natural and chemically synthesized direct-acting bactericides, there has been an increase in the use of plant extracts, which possess a set of phytochemicals with potential for microbial disease control; this is due to the spectrum of secondary metabolites present in extracts, which include phenolic compounds, quinones, flavonoids, alkaloids, terpenoids, and polyacetylenes. The biologically active substances within plant extracts, which perform protective functions for plant tissues, can have ambiguous effects on the animal body. Therefore, the aim of this study was to assess the ability of gamma-octalactone, isolated from Eucalyptus viminalis extract, to inhibit various LuxI/LuxR quorum-sensing (QS) systems in bacteria, and to evaluate its effect on broiler chickens.

Materials and Methods: Phytochemical analysis of E. viminalis extract was performed. The ability of gamma-octalactone to inhibit QS was evaluated using four different LuxI/LuxR bacterial test systems. In vivo assessments were performed on one hundred and twenty 7-day-old broiler chickens (Arbor Acres cross), split into four groups of 30 chickens: 1. Control group: Basic diet (BD); 2. experimental Group I: BD + gamma-octalactone at a dosage of 0.05 ml/kg live weight/day; 3. experimental Group II: BD + gamma-octalactone at a dosage of 0.1 ml/kg live weight/day; and 4. experimental Group III: BD + gamma-octalactone at a dosage of 0.2 ml/kg live weight/day. Hematological blood parameters were assessed using an automatic hematological analyzer (URIT-2900 Vet Plus, URIT Medial Electronic Co., China) and an automatic biochemical analyzer (CS-T240, Dirui Industrial Co., Ltd., China). Statistical analyses were performed using SPSS Statistics Version 20 (IBM); averages (M), standard deviations (σ), and standard deviation errors (m) were calculated. Results with p≤0.05 were considered significant.

Results: Based on the phytochemical analysis results, libraries of compounds with putative QS inhibitory properties were compiled. Gamma-octalactone exhibited a pronounced inhibitory effect on the LuxI/LuxR QS systems, characterized by EC₅₀ values of 0.15-0.4 mM. In the in vivo portion of this study, broiler chicken live weights increased in all experimental groups, with the most significant increase in Group III (14.0%), in relation to the control group. Blood serum from the experimental group chickens had significantly higher levels of triglycerides and uric acid (p≤0.05), in comparison to the control group chickens. With respect to blood serum enzyme activity and antioxidant status indicators, the experimental group chickens had a higher level of gamma-glutamyl transferase, an enzyme associated with amino acid metabolism, than those in the control group; this increase was especially pronounced in Group III, with 37.0% increase (p≤0.05). Superoxide dismutase and catalase levels were higher in the experimental groups than the control group, corresponding to increases of 30.4-56.2% (p≤0.05), 33.3-83.3%, and 27.9-45.5% (p≤0.05) in Groups I, II, and III (p≤0.05), respectively. Morphological blood parameters did not display significant changes due to gamma-octalactone.

Conclusion: According to the results of this in vivo study in broiler chickens, gamma-octalactone, isolated from E. viminalis leaf extract and supplied at a dosage of 0.2 ml/kg live weight/day, led to an increase in the activity of blood plasma digestive enzymes, increased live weight, and had a positive effect on lipid metabolism and antioxidant status.

Keywords: broiler chickens, Eucalyptus viminalis leaf extract, gamma-octalactone, productivity, quorum sensing.