Novel quantitative assays for estimating the antimicrobial activity of fresh garlic juice

Novel agar diffusion and broth dilution assays were developed for quantitat ively estimating the antimicrobial activity of fresh garlic juice. Bacteria found to be inhibited by garlic juice in agar diffusion assay included two gram-positive and five gram-negative species. Leuconostoc mesenteroides wa s not inhibited. Escherichia coli B-103 (HB 101, with pJH101, ampicillin re sistant, 100 mug ml(-1)) was inhibited and chosen as the standard culture f or quantitative assays. The agar diffusion assay was based on the slope rat io method, where the slope of dose response for garlic juice was divided by the slope of dose response for methylmethane thiosulfonate (MMTSO2). Juice from fresh garlic varied in activity between 1.76 and 2.31 mug of MMTSO2 p er mg of garlic juice. The activity of juice decreased during 11 months of storage of garlic cloves at 5 degreesC from 2.31 to less than 0.1 mug of MM TSO2 per mg of juice. The broth dilution assay also used the E. coli B-103 culture, which permitted selective enumeration of this bacterium when 100 m ug ml(-1) of ampicillin was incorporated into the enumerating agar. Selecti ve enumeration was essential since the garlic juice was not sterile and, th us, contained natural flora. Growth of E. coli was unaffected by 0.1%, dela yed by 0.25%, and completely inhibited at 0.5 and 2% garlic juice in broth during 24 h of incubation at 37 degreesC. The minimum inhibition concentrat ion of garlic juice by broth dilution assay was, thus, estimated to be 0.5% , which is equivalent to 3.46 mug of MMTSO2 per mg of garlic juice by the a gar diffusion assay.