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Essential Oil extract from Ocimum basilicum (Labiatae; basil) was investigated for its in vitro antibacterial properties against an emerging human food-borne pathogen, Shiga Toxin Producing E. coli O 157:H7. Agar disc diffusion method and time-kill assay in a batch culture were employed. The results showed that essential oil of basil was effective to inhibit the growth of pathogenic bacteria tested. The diameter of inhibition zone was 15.25±0.58 mm. In a batch culture, four oil concentrations (4, 2, 1, 0.5, 0.25 and 0.125 mg/ml) were initially tested to determine the concentration of basil oil that could inhibit growth of E. coli O157:H7. It was found that the oil at 4, 2, 1 and 0.5 mg/ml killed the bacteria after 1 h, while at 0.25 mg/ml the number of bacteria was only reduced. Two oil concentrations, 0.25 mg/ml and 0.5 mg/ml, were chosen to study its effect on E. coli O157:H7 at different growth phases, i.e. exponential, late exponential and stationary phase in a batch culture. Oil at both concentrations (0.25 mg/ml and 0.5 mg/ml) could not completely kill the cells at the exponential phase, and upon prolonged incubation, resistant cells emerged. But basil oil at 0.5 mg/ml could completely killed cells at the late exponential and stationary phases suggesting that the oil at 0.25 mg/ml was bacteriostatic, while at 0.5 mg/ml was bactericidal against E. coli O157:H7.

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