Differences in the natural-abundance carbon stable isotopic compositions between products from aerobic cultures of Escherichia coli K-12 were measured. Respired CO2 was 3.4%o depleted in 13C relative to the glucose used as the carbon source, whereas the acetate was 12.3%o enriched in 13C. The acetate 13C enrichment was solely in the carboxyl group. Even though the total cellular carbon was only 0.6%o depleted in '3C, intracellular components exhibited a significant isotopic heterogeneity. The protein and lipid fractions were-1.1 and-2.7%o, respectively. Aspartic and glutamic acids were-1.6 and +2.7%o, respectively, yet citrate was isotopically identical to the glucose. Probable sites of carbon isotopic fractionation include the enzyme, phosphotransacetylase, and the Krebs cycle.

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