Diauxie used as a microbiological assay for L-isoleucine or L-valine in Escherichia coli.

FIG. 1. Diauxic lag of cuiltures conitaining valine and isoleucine. All cultures were grown by shakinig (330 rev/ min) at 37 C in medium containing 2.0 g ofNaH2PO4 O H20, 7.0 g ofK2HP04, 2.0 g of (NH4)2SO4, 0.2 g of MgSO4 v 7H20 and I mg of thiamine hydrochloride in I liter of distilled water. Overnight cultures grown in 0.2%o glucose were harvested by centrifugation during exponential phase, resuspended in fresh medium without a carbon source, and shaken for 20 min at 37 C. This procedure took 35 to 40 min. Lactose (0.05%) was then added (time zero), the culture was divided, and additions were made. L-Valine, at 156.3 m,ug/ml, was added to all cultures except the control (-). L-Isoleucine was added as follows: (0) none, (+) 5.2, (0) 10.4, (V) 15.6, (A\) 20.8, and (X) 26.0 m,ug/ml. Cultures (4.0 ml) were grown by shaking (330 rev/min) at 37 C. Optical denisity was read on a Spectroniic-colorimeter 20 at 650 m,u. A denisity of 0.1 corresponds to 61 ,ug (dry weight of cells) per ml.