INHIBITION OF BACTERIA ISOLATED FROM NINH THUAN GRAPES BY ORGANIC ACIDS
Keywords:minimal inhibitory concentration, minimal bactericidal concentration, Ninh Thuan grapes, organic acids.
Ninh Thuan grapes are famous specialty of Vietnam. After harvesting, they are very susceptible to damage if not preserved in time. This study aimed to find the minimal inhibitory concentration and minimal bactericidal concentrations of organic acids, such as lactic acid, citric acid, and ascorbic acid towards seven bacteria previously isolated from Ninh thuan grapes (Empedobacter brevis, Citrobacter sp. Enterobacterium ludwigii, Bacillus cereus, Flavobacterium sp., Pseudomonas oryzihabitans and Bacillus thuringiensis) in order to apply in pretreatment of grapes prior to storage. The Minimal Inhibitory Concentration (MIC) was determined by the binary dilution method. 0.1 ml of each dilution of test acids with initial concentration of 3 % was mixed with 0.1 ml of bacteria (c.a. 5.105 CFU/ml), in 96-culturing-well plate. Innoculated plate was incubated for 24 hours at temperature of 37 °C. Optical density was measured at 620 nm wavelength by a Microplate reader device. For Minimal Bactericidal Concentration (MBC) determination, 0.1 ml of bacteria (c.a.5.105 CFU/ml) was mixed with 0.4 ml liquid medium of Tryptone Glucose Agar (TGA) and of 0.5 ml acid at above concentrations, culturing within 24 hours at temperature of 37 °C. After 24 hours, they were dropped onto TGA agar plates, and cultured for 24 hours at 37 °C. The results showed that, the MIC and MBC of citric acid toward Empedobacter brevis were 0.12 mg/ml and 0.12 mg/ml; Citrobacter sp. were 0.9 mg/ml and 3.8 mg/ml; Enterobacterium ludwigii were 0.45 mg/ml and 1.9 mg/ml; Bacillus cereus were 0.9 mg/ml and 0.9 mg/ml; Flavobacterium sp. were 0.12 mg/ml and 0.45 mg/ml; Pseudomonas oryzihabitans were 0.12 mg/ml and 0.45 mg/ml and Bacillus thuringiensis were 0.12 mg/ml and 0.9 mg/ml, respectively. Similarly, MIC and MBC of lactic acid toward: Empedobacter brevis were 0.12 mg/ml and 0.23 mg/ml;Citrobacter sp. were 0.9 mg/ml and 1.9 mg/ml; Enterobacterium ludwigii were 0.45 mg/ml and 0.9 mg/ml; Bacillus cereus were 0.23 mg/ml and 0.45 mg/ml; Flavobacterium sp. were 0.12 mg/ml and 1.9 mg/ml; Pseudomonas oryzihabitans were 0.23 mg/ml and 1.9 mg/ml and Bacillus thuringiensis were 0.12 mg/ml and 1.9 mg/ml, respectively. Ascorbic acid was found to be inefficient for use as antimicrobial agent against isolated bacteria. The above results suggested that citric and lactic acids could be used at maximal concentrations of 3.8 mg/ml and 1.9 mg/ml, respectively to suppress bacteria from grapes. This finding would contribute to develop method for pretreatment of grapes in fresh grape preservation techniques.
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