The regulation of short chain fatty acid metabolism has been examined. Metabolism of acetoacetate, and short chain fatty acids such as butyrate and valerate, is predicated upon the expression of genes of the ato operon. Acetoacetate induces expression of a CoA transferase (encoded by the atoDA genes) and expression of a thiolase (encoded by the atoB gene). Metabolism of saturated short chain fatty acids requires the activities of the transferase and thiolase and enzymes of 6-oxidation as well. Spontaneous mutant strains were isolated that were either constitutive or that were inducible by valerate or butyrate instead of acetoacetate.