To better define the role for the TCA cycle during extraintestinal infection, mutants of E. coli and P. mirabilis lacking succinate dehydrogenase; sdhB, fumarate dehydratase (fumarase); fumC, and fumarate reductase; frdA were constructed and tested in competitive infections with wild-type E. coli CFT073 or P. mirabilis HI4320, respectively. The gene discussed is SDHB; the disease is infection.