10.22376/ijpbs.2019.10.1.p1-12 Volume 7 Issue 1 2016 (January - March) Mutagenesis has been used to generate highly productive and stress tolerant variants of microbes. In this study a random mutagenesis using a chemical mutagen, ethidium bromide, was carried out on Lactic Acid Bacteria (LAB) isolated from Ethiopian traditionally fermented cow milk. Twenty colonies of LAB were isolated and out of which two isolates (B6S01 and A6B08) with least mean pH scores were selected and characterized to genus level. After being mutagenized with four different concentrations of ethidium bromide (0.5, 1.0, 1.5 and 2.0 g.L lessThan sup greaterThan -1 lessThan /sup greaterThan ), 80 survivors were selected and screened. Accordingly, four mutants with the least mean pH scores were selected and characterized. The two wild type isolates, (B6S01 and A6B08) with mean pH 4.50 ± 0.03 and 4.52 ± 0.02 respectively, were identified as lessThan i greaterThan Lactobacillus spp lessThan /i greaterThan . and lessThan i greaterThan Lactococcus spp lessThan /i greaterThan respectively. The mutagenesis affected the sugar fermentation pattern and acidification potential of the mutants. The mutant types MAS03 and MAB18 were now able to ferment arabinose and fructose, and galactose respectively. Lower mean pH values were associated with ethidium bromide at 0.5 g lessThan sup greaterThan -1 lessThan /sup greaterThan L and 1.5 gL lessThan sup greaterThan -1 lessThan /sup greaterThan concentration significantly (p lessThan 0.01). However, ethidium bromide at 2.0 gL lessThan sup greaterThan -1 lessThan /sup greaterThan concentration resulted the least pH value, 4.20 ± 0.03, on the mutant MAB37. The effectiveness of the mutagenesis was demonstrated by better acidification potential and unique array of sugar fermentation. Utilization of these identified mutants in dairy processing could reduce production cost and enhance productivity. SEWUNET ABERA, MELAKU ALEMU AND AMEHA KEBEDE Acidification; Characterization; Ethidium Bromide; Lactic Acid Bacteria; Mutagenesis. Screening 183-192