tetrathionate, tetracycline, and vitamin B12 were from Sigma Chemical Co.; St. Louis, MO. Isopropyl-3-D-thiogalactopyranoside (IPTG) was from Diagnostic Chemical Ltd.; Charlottetown, Canada. Glucose, formaldehyde (r,s), mono and dibasic potassium phosphate, ammonium sodium phosphate tetrahydrate, pyruvate, and succinate, were from Fisher Scientific; Pittsburgh, PA. Glutaraldehyde was from Tousimis; Rockville, MD. Uranyl acetate was from E.M. Sciences; Ft. Washington, PA. Osmium tetroxide was from Tedpella Inc.; Redding, CA. Powdered milk was from Nestle; Glendale, CA. Yeast extract, MacConkey agar base, agar, Luria-Bertani (LB) agar and LB broth were from Difco Laboratories; Detroit, Michigan. Bacterial Strains, Media, and Growth Conditions

     The bacterial strains used in this study are listed in Table 2-1. The complex

medium used was LB broth. The minimal medium used was NCE, prepared according to Vogel and Bonner (Vogel and Bonner 1956) and supplemented with 1 mM MgSO4. Agar was added to a final concentration of 1.5% to make solid media. Carbon sources were used at the following concentrations: fumarate, 0.32%; glucose, 0.2% (aerobically) and 0.4% (anaerobically); glycerol, 0.4%; PD, 0.2 or 0.4%; pyruvate, 0.44%; and succinate, 1.0%. Tetrathionate was used at 10 mM, CNB12 was used at 200 ng/ml, tetracycline at 10 [tg/ml, and ampicillin at 100 [tg/ml. Overnight cultures were grown aerobically at 370C, with shaking at 275 RPM in LB broth supplemented with the appropriate antibiotics. MacConkey/PD/B12 indicator plates were composed of MacConkey agar base supplemented with 1% PD and 200 ng B12 per ml. Aldehyde indicator plates were prepared according to the method of Conway et al. (Conway et al.