Propionibacterium acidipropionic, which is a Gram-positive, anaerobic bacterium, has been the most used species for propionic acid production with the byproduct of acetic acid. Genetic manipulations were performed on the metabolic pathway that is related to the propionic acid and acetic acid production in P.acidipropionici. Mutants were constructed by inactivating genes associated with the acetate formation pathway to eliminate or reduce acetate formation so that more propionic acid can be produced from sugars. The mutant grew slower in batch fermentation, but produced more propionate and less acetate as compared to the wild type strain. Fermentation with mutants in a fibrous bed bioreactor (FBB) shows that the maximum theoretical propionic acid yield of ~0.54 g/g glucose could be achieved.