In VitroAntibacterial and Time-Kill Evaluation of theErythrina caffraThunb. Extract against Bacteria Associated with Diarrhoea

Abstract

The antibacterial activities of stem bark ethanolic extract ofErythrina caffraThunb. against bacteria in diarrhoea was determinedin vitroby the agar diffusion and dilution, macrobroth dilution, and time-kill assay methods. The result showed that the extract produced inhibition zones ranging between15±1.0 mm and23±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 625μg/mL, the MICs ofMicrococcus luteus,Proteus vulgarisCSIR 0030,Enterococcus faecalisKZN, andStaphylococcus aureusOK3were 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 averagelogreduction of the viable cell count ranged between0.916log 10and1.851log 10 cfu/mL on incubating the bacteria for 4 h at the MICs, while the reduction ranged between0.183log 10and1.105log 10 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 −0.264log 10and0.961log 10 cfu/mL, while the averagelogreduction ranged between −3.968log 10and −0.425log 10 cfu/mL after 8 h of incubation withMicrococcus luteus,Proteus vulgarisCSIR 0030, andStaphylococcus aureusOK3being the most highly affected bacteria. The result showed that the extract exhibited broader-spectrum antibacterial activity and justifies the use ofErythrina caffrain the folkloric medicine for treating gastrointestinal infections in South Africa.