fermentative or enzymatic production of 2 - keto - l - gulonic acid from 2 , 5 - diketo - d - gluconic acid using living or processed mutants , being defective in metabolizing 5 - keto - d - gluconic acid and incapable of producing 2 - keto - d - gluconic acid . the mutant is derived from 2 - keto - l - gluconic acid producing microorganisms of genus corynebacterium . the production is carried out in the presence of nitrates and / or hydrogen donors in a preliminarily sterilized fermentation broth in which a 2 , 5 - diketo - d - gluconic acid producing microorganism of genus gluconobacter or erwinia has been cultivated .