Background and Aim: The resistance of susceptible fish populations and the adaptive potential of heterogeneous biofilms, which cause multiple antibacterial resistance and long-term persistence of microorganisms, mediate the development and outcome of the infectious process. The study of the fish immunological parameters in interaction with biofilm-forming bacteria is of practical importance for assessing the stability of the homeostasis of the fish. This study aimed to determine the immunobiological parameters of Cyprinus carpio Linnaeus when interacting with biofilm-forming bacteria Aeromonas hydrophila.

Materials and Methods: Clinically healthy fish C. carpio L. (Linnaeus, 1758) of both sexes, aged 4 years, and weighing 1.0–1.5 kg (n = 10), were used in this study. The fish were taken from the pond of the VNIIR experimental base in the period of 2020–2022. The standard method was employed to determine the phagocytic activity of blood cells, the total redox activity of neutrophils, and the bactericidal activity of blood serum.

Results: After 24–48 h of cultivation in nutrient broth, the implementation of the processes of intercellular communication of bacteria had common patterns of formation of the heterogeneous structure of biofilms. Moreover, analyzing the optical density indices (density, D), it was observed that A. hydrophila was a strong producer of biofilms, as the optical density of the sample (density of sample, Ds) exceeded the optical density of the control (density of control, Dc) by more than 4 times (D = 0.464 ± 0.07). The ratio of the average number of microorganisms attached to the surface of one erythrocyte (average adhesion index) and the percentage (%) of erythrocytes having bacteria on their surface (adhesion coefficient [AC]) was 14.05 ± 0.72, and the adhesion index, AI was ≥4.00, indicating A. hydrophila to be highly adhesive. In addition, the AC of erythrocytes having bacteria on the surface was 14.05% ± 0.72%. A direct correlation was established (R² = 0.94) between the AC (14.05% ± 0.11%–13.29% ± 0.08%) and the phagocytic index (11.3% ± 0.29%–32.0% ± 0.8%). The indicators of spontaneous nitro blue tetrazolium were 103.20 ± 11.70 when estimating the total redox activity of neutrophils. The optical density increased to 182.10 ± 21.12 with the addition of 20.0 μL of A. hydrophila bacteria (1 billion/mL) and the activity of neutrophils also increased.

Conclusion: Among the markers of homeostasis stability, immunological indicators most fully reflect the mechanisms of initiation, development, and outcome of the infectious process mediated by the interaction of adhesive molecules of multicellular eukaryotes and adhesives of infectious disease pathogens. The research will contribute to further understanding the potential mechanism of quorum-sensing molecules and the search for new anti-adhesive drugs that reduce the formation of biofilms.

Keywords: adhesion, Aeromonas hydrophila, biofilm, Cyprinus carpio Linnaeus, optical density.