Antibacterial Activity of Cannabidiol Against Oxytetracycline-Resistant Streptococcus agalactiae Isolated from Nile Tilapia (Oreochromis niloticus)

Streptococcosis caused by oxytetracycline (OTC)-resistant Streptococcus agalactiae is a major constraint in Nile tilapia aquaculture, where antibiotic options are increasingly limited. This study evaluated the in vitro antibacterial activity of cannabidiol against eight OTC-resistant S. agalactiae isolates confirmed by qPCR, capsule typing, and tet(M)/tet(O) detection; all isolates carried tet(M) and five carried tet(O). In this exploratory comparison, cannabidiol produced a larger mean inhibition zone than OTC (20.9 ± 2.2 mm vs. 15.1 ± 0.9 mm); however, because the disks were tested at different concentrations (50 μg vs. 30 μg), this difference should not be interpreted as a validated measure of relative potency. We therefore used MIC and MBC values, obtained across equivalent concentration ranges for both compounds, as the primary basis for comparison. Because cannabidiol has no validated clinical breakpoints, we interpreted the results using established criteria for plant-derived antimicrobials. MIC and MBC values for CBD (4–32 and 8–32 μg mL−1, respectively) were lower than those for OTC. Growth kinetics showed concentration-dependent inhibition, with CBD at 64 μg mL−1 producing sustained suppression comparable to the inhibition control. These findings provide preliminary in vitro evidence supporting its consideration as an antimicrobial option for OTC-resistant streptococcosis in tilapia aquaculture.