Efeito dos óleos essenciais de Citrus latifolia e Citrus sinensis sobre Aeromonas patogênicas isoladas de tilápia-do-Nilo (Oreochromis Niloticus)

Abstract

Freshwater fish farming in Brazil has seen significant growth in production volume over the last decade. The farming of tilapia (Oreochromis niloticus) stands out due to zootechnical, sensory, and productive factors that have favored its cultivation. However, this activity has been threatened by several bacterial diseases that can affect animals at different stages of life, causing economic losses to fish farmers. Although the use of antibiotics is common to treat bacterial diseases in fish farms, their incorrect use can make microorganisms resistant, causing damage to human health and environmental pollution. An alternative would be the use of herbal medicines, based on essential oils, which can lyse the cell wall, destroying or preventing the growth of pathogenic microorganisms. The objective of this study was to evaluate the potential use of essential oils (EO) from Citrus latifolia (Tahiti lime) and Citrus sinensis (pear orange) as prophylactic or curative measures against bacteria of the genus Aeromonas. In the first phase, water was collected from the Malcozinhado reservoir and fish mucus to prospect for bacteria related to O. niloticus pathogens. The isolates were subjected to biochemical tests, expression of extracellular enzymes related to pathogenicity, susceptibility to antibiotics, and genotypic identification of the strains. The second phase consisted of the extraction of essential oils from Tahiti lime and pear orange, followed by the study of the chromatographic profile and toxicity. The essential oils were subjected to in vitro tests of bactericidal activity through a disc diffusion assay and determination of minimum inhibitory and bactericidal concentrations. The third phase included an in vivo test where tilapia were fed commercial feed supplemented with lime essential oil at concentrations of 800, 1,600, and 3,200 µg/mL for 30 days. After this period, the animals were subjected to a challenge test against A. veronii biovar veronii, followed by hematological and histological analyses. The results revealed the presence of bacteria of the genus Aeromonas, which expressed many factors related to virulence, such as extracellular enzymes and antibiotic resistance profiles. The major compound in orange EO is D-limonene, while in lemon it is γ-terpinene. The analyses indicate greater antimicrobial activity in pear orange EO compared to Tahiti lemon EO. In the in vivo test, there was no statistically significant difference in growth between the treated tilapia groups and the control, but histological images confirm that the animals in the treated groups had a greater number of intestinal villi, indicative of greater capacity for nutrient absorption. Despite the limitations, the results strongly suggest that lemon essential oil is an attractive candidate for improving the treatment of aeromonosis in Nile tilapia and promoting growth in captive fish.