2.8. Antibacterial Activity Tests

The antibacterial activities of all of the compounds were determined for three Gram-positive bacterial strains (S. aureus ATCC 25923, L. monocytogenes ATCC 19115, and B. cereus ATCC 10876) and three Gram-negative bacterial strains (E. coli ATCC 25922, S. typhimurium ATCC 14028, and K. pneumoniae ATCC BAA-2146). Standard procedures involving the microdilution technique recommended by the Clinical and Laboratory Standards Institute (CLSI) were used to determine the antimicrobial susceptibility [24]. The minimum inhibitory concentrations (MICs) were evaluated in a concentration range from 2000 to 3.9 μg·mL⁻¹. The negative control was Mueller–Hinton broth (MHB) with no bacteria, and the positive control was MHB with only bacteria. All determinations were performed in triplicate. Ciprofloxacin (range between 4.0 and 0.008 μg·mL⁻¹) and AgNO₃ (range between 500 and 100 μg·mL⁻¹) were used as standard bactericides. To exclude the possibility that resistance could be induced by ciprofloxacin and AgNO₃, the antimicrobial susceptibility of isolates was tested by the broth microdilution method on the antibacterial agent-free medium. An additional study was performed to test the effect of the solvent in the biological screening, and DMSO was found to have no activity against any of the tested strains.