One of the main challenges that has arisen due to the accelerated development of the aquaculture industry is the emergence of antibiotic -resistant pathogens. And this issue is aggravated by the limited FDA approved chemotherapeutics available for aquaculture. A primary approach to address this challenge and mitigate infectious diseases is the use of functional feeds that has bactericidal/bacteriostatic properties and/or immunostimulate the host. In this context, numerous investigations have been carried out testing phytocompounds supplemented to the feed. Their antimicrobial, immunostimulant, and anti-stress antioxidant features can interact with the host, and ultimately act as growth promoters. Nevertheless, the use of these class of compouds is challenging when the phytocompounds can widely vary depending on the extraction method, climate conditions when growing the plants, soil composition, or phenotypic differences in the intrinsic variation between the variety of plants.
The present study focused on analyzing the inhibitory activity of main active compounds (carvacrol, thymol, and citral ) and the guava extract against pathogenic fish bacteria such as Aeromonas hydrophila, Edwarsiella ictaluri , and E. piscicida . This evaluation was carried out using the test sensitivity by diffusion disc, the microdilution method in culture broth for antimicrobial activity, and the evaluation of bacterial biofilm formation and inhibition through the gentian violet assay. In the diffusion disc test, the carvacrol and thymol extracts had significant differences compared to a control (without exposure to the extracts) in all strains evaluated. Additionally, E. piscicida had higher diameters of zones of inhibition (ZOI) being significantly different with the pure guava extract.
In the pilot tests of bacterial growth against serial dilutions of the extract, significant reductions in the growth curve have been observed. The minimum inhibitory concentrations (MIC), and minimum bactericidal concentrations (MBC) will be determined, and these concentrations will be further evaluated during the bacterial biofilm formation. In conclusion, the phytocompounds tested presented promising inhibitory activity against common pathogenic bacteria for catfish, and future studies evaluating these compounds as feed additives for channel and hybrid catfish in feeding trials are encouraged.