Acacia ataxacanta leaves were screened for possible constituents, and the antimicrobial properties of the leaves were tested against 18 isolates of E. coli O157:H7. The leaves were aseptically collected and crude extracts were prepared by cold maceration in 98% methanol. Phytochemical analysis for bioactive constituents revealed that it contained carbohydrates, cardiac glycosides, steroids, saponins, triterpenes, tannins, flavonoids and alkaloids. Partitioning of the crude extract yielded n-hexane, ethyl acetate and aqueous fractions were evaluated with their minimum Inhibitory Concentration (MIC) and minimum bactericidal concentration (MBC). The various concentrations of the crude extracts (600, 500, 400, 300, 200, 100, 50, 25, 12.5 and 3.15 mg/ml) were tested on the isolates. The crude extract demonstrated antibacterial activity against Escherichia coli O157:H7 isolates with zones of inhibition ranging from 15.67±0.3mm to 18.67± 0.3mm at concentrations of 300 to 600mg/ml. Ethyl acetate fraction was identified as the most potent of the fractions with bacteriostatic activity displayed as wide zone of bacterial growth inhibition of 18 and 13 mm at 200 mg/ml against the isolates. The aqueous fraction produced comparatively lower zones of bacterial growth inhibition of 12 and 15 mm at higher concentrations of 300 to 600mg/ml. The MIC for the extracts was 100, 200 and 300 mg/ml for the crude extract, ethyl acetate and aqueous fractions respectively. It is therefore concluded that A. ataxacantha leaves tested contains bioactive constituents which are of high medicinal value. It is necessary that the extract should be further purified and exploited for toxicological use to confirm its safety in disease therapy, using higher molecular techniques.

Keywords: Photochemical screening, bioactive, E. coli O157:H7 isolates on Acacia ataxacantha leave.