Isolation and characterization of mutants from levan-producing Zymomonas mobilis

Levan is a fructose polymer with potential importance as a fructose source and as a thickening agent in food technology. By N-methyl N'-nitro N-nitroso guanidine (NTG) mutagenesis and selection, two mutants, ZML1 and ZML2, of Zymomonas mobilis B4286 overproducing levan (21.6 and 20.0 g/l at 24 h) were isolated. Despite showing similar rates of sucrose hydrolysis (2.38 g/l/h) in fermentation, these mutants exhibited higher rates of levan production (1.72 and 1.68 g/l/h, respectively) than their parent strain (0.69 g/l/h). In contrast, these mutants showed lower rates of ethanol production (1.06 and 1.55 g/l/h respectively) than the parent strain (3.75 g/l/h). The maximum levansucrase activity in the mutants was higher (30.6 and 36 U/mg, respectively, at 24 h) than in their parent strain (12.6 U/mg). Therefore, the increased rates of levan production by the mutants are attributed to their higher levansucrase activity.