Abstract: Mutant microorganisms comprising Lactobacillus fermentum Lex.sup.+ which are obtained from Lactobacillus fermentum produce lysine in a significantly greater quantity than the wildtype microorganism. The microorganism is added to a sourdough starter to produce bread of increased nutritive content, such as flat bread. Freeze-dried cultures of the microorganism may be added to cereal grains such as wheat in bulk to increase the basic nutritive protein quality of the wheat, whereby foodstuffs produced from the cereal grains have increased protein values. Cultures of the microorganisms in admixture with yeast may be used as a bread starter.