Antibacterial and norfloxacin potentiation activities of Ocimum americanum L. against methicillin resistant Staphylococcus aureus

Staphylococcus species are among the most common resistant bacteria associated with the major cause of human ailments. The crude methanol extract from Ocimum americanum (OA) leaf was tested alone or in combination with norfloxacin (NOR) against strains of Staphylococcus aureus using the broth microdilution assay. The cytotoxicity of the OA extract was also evaluated using the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) reagent assay on a HepG2 hepatocarcinomal cell line. While the plant extract exhibited a mild to poor antibacterial activity against our panel of bacteria, the antibiotic activity of norfloxacin at one-quarter MIC was enhanced by 2-4 fold in the presence of one-half MIC of OA extract against SA-1199B that over expresses the NorA efflux pump and MRSA-274829. These positive interactions were confirmed using a time-killing test; the combination therapy remarkably reduced the bacterial count of SA1199B and MRSA274829 ranging from a 6.0-4.2-log10-CFU/mL, after 24 h incubation. The OA extract strongly depleted DPPH* (IC50: 146.5 μg/mL), LOI (152 μg/mL), PGI (47.6 μg/mL) and FRAP (122.75 μmolFe(II)/g) possibly due to its richness in phenolic compounds. Furthermore, the OA extract showed a non-toxic effect on the HepG2 cells having an IC50 value of 378.0 μg/mL. These findings therefore support the folkloric use of Ocimum americanum at least in part for the treatment of infectious and free radical stress-related diseases.