In Vitro Antibacterial and Time-Kill Evaluation of the Erythrina caffra Thunb. Extract against Bacteria Associated with Diarrhoea

The antibacterial activities of stem bark ethanolic extract of Erythrina caffra Thunb. against bacteria in diarrhoea was determined in vitro by the agar diffusion and dilution, macrobroth dilution, and time-kill assay methods. The result showed that the extract produced inhibition zones ranging between 15 ± 1.0 mm and 23 ± 1.0 mm, and the bacteria were susceptible at concentrations ranging between ≤100 and ≤1000 μg/mL. While the MICs of the extract ranged between 39.1 and 625 μg/mL, and the MBCs ranged between 78.1 and 625 μg/mL, the MICs of Micrococcus luteus, Proteus vulgaris CSIR 0030, Enterococcus faecalis KZN, and Staphylococcus aureus OK₃ were less than 100 μg/mL, and the mechanisms of antibiosis indicated that the crude ethanolic extract was highly bactericidal against the entire test bacteria isolates. In the time-kill assay, the average log reduction of the viable cell count ranged between 0.916 log₁₀ and 1.851 log₁₀ cfu/mL on incubating the bacteria for 4 h at the MICs, while the reduction ranged between 0.183 log₁₀ and 1.105 log₁₀ cfu/mL after 8 h of incubation. Incubating the bacteria for 4 h at 2 × MICs resulted in the reduction of the viable cell count to between -0.264 log₁₀ and 0.961 log₁₀ cfu/mL, while the average log reduction ranged between -3.968 log₁₀ and -0.425 log₁₀ cfu/mL after 8 h of incubation with Micrococcus luteus, Proteus vulgaris CSIR 0030, and Staphylococcus aureus OK₃ being the most highly affected bacteria. The result showed that the extract exhibited broader-spectrum antibacterial activity and justifies the use of Erythrina caffra in the folkloric medicine for treating gastrointestinal infections in South Africa.