Microbiological Evaluation of Combinations of Extracts of Euphorbia abyssinica and Coleus species for Antibacterial Activity
Antibacterial activity of 50% methanol extracts of Coleus species and Euphorbia abyssinica were investigated singly and in combination against Staphylococcus aureus, Escherichia coli, Salmonella typhi, Pseudomonas aeruginosa, using agar diffusion, macrobroth dilution, Checkerboard and Time kill assays. The inhibition zone diameters (IZDs) produced by the two plant extracts singly ranged from 17mm to 24mm, and those produced by the combination ranged from 17mm to 29mm. In the macrobroth dilution assay, the MICs obtained for the individual plant extracts ranged between 1.95-25mg/ml. In the Time Kill assay of the individual plant extracts, S. aureus was killed by 3.91mg/ml concentration of E. abyssinica and Coleus species alone, (at 1MIC and 2MIC), respectively. The combination killed it at 3.695mg/ml, indicative of synergy. S. typhi was killed by 12.5mg/ml and 6.25mg/ml (1MIC) concentration of E. abyssinica stem-bark and Coleus species, respectively. It was, also, killed by 6.26-, 6.88-, 7.51-, 8.14- and 8.75-mg/ml respective concentrations of different combinations of the two plant extracts. This is, also, indicative of synergy. P. aeruginosa was inhibited by 2MIC (25 mg/ml and 9.38mg/ml) of E. abyssinica stem-bark and Coleus species extracts, respectively, but a combination of the two plant extracts killed at 5.32mg/ml concentration, again indicating synergy. Some of the combinations tested using the checkerboard method showed synergy while others were indifferent or antagonistic after 24hours of exposure. The control drug, Gentamycin, was effective against all the test bacteria. A comparison of two methods revealed that the checkerboard assay was significantly more sensitive than the Time Kill assay (P<0.05).The results authenticate the ethno-medicinal uses of these plants.
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