Wild-type K-12 strain NCM3722 of E. coli45 (link) was cultured in Gutnick minimal complete medium46 (link) with 4g/L of glucose, glycerol, or acetate as the carbon source. Growth of cells was as previously reported26 (link). Preparation of filter cultures is described in Supplementary Methods online. For 13C-glucose cultures, uniformly 13C-glucose (> 99% 13C from Cambridge Isotope Laboratories) was used for the overnight culture and the liquid and filter cultures, resulting in at least 10 doublings on uniformly 13C-glucose media.