ANTIMICROBIAL EFFECTS OF SELECTED HERBAL EXTRACTS ON MULTI-DRUG RESISTANT GRAM-NEGATIVE BACTERIAL STRAINS

Abstract

The rise in antibiotic resistance has resulted in a decreasing number of fully active antimicrobial agents available to treat infections by multi-drug resistant (MDR) bacteria.  This has necessitated a search for new antimicrobial agents. Herbal remedies may offer novel treatment options which elicit little or no transferred resistance if used in optimal concentrations. This study evaluated the antimicrobial properties of ten plants traditionally used as herbal remedies against 27 multi-drug resistant Gram-negative bacterial isolates. The herbal extracts were obtained through  extraction with  organic  (methanol) and inorganic (water) solvents. Susceptibility of the test strains to conventional antibiotics was determined by the disc diffusion  technique. Determination of the Minimum Inhibitory Concentrations (MIC) and the sub-lethal concentrations of the most effective extracts against the MDR strains was done by broth inoculation followed by colony count method.  The effect of sub-lethal extract concentrations was done by a method modified from McMahon et al. (2007). Out of the ten plants, only Warbugia  ugandensis  was active against the MDR strains and its efficacy was significantly different from that of other plant extracts such as  Terminalia brownii, Azaridachta indica, Clausena anisata  and  Strychnos henningsii   (p<0.001).    The root and bark methanol extracts from  W. ugandensis were the most effective with an MIC of 42 µg/ml.  Susceptibility of test strains to conventional antibiotics was not significantly different   before and after habituation to sub-optimal extract concentration (p>0.005). Methanol extracts from the root and bark of W. ugandensis provide potential sources of effective antimicrobial compounds for further development of alternative safe antimicrobial products in form of chemotherapeutic agents or antiseptics.  The optimization and standardization of operation procedures and methods of analysing the efficacy of herbal extracts demands serious consideration.