Conversion of Citrate to Extracellular Glutamate by Penicillin-treated Resting Cells of Corynebacterium glutamicum

Triggering of glutamate excretion by penicillin is thought to occur by increasing cell permeability. It seemed odd that glucose-grown resting cells, after penicillin treatment, would not convert citrate to extracellular glutamate especially since citrate had been reported to be a substrate for the glutamate fermentation. Citrate was not even taken up by such cells. Upon addition of at least 2 percent glucose, citrate was converted to extracellular glutamate. Both glucose and citrate were used simultaneously and citrate metabolism continued even after sugar was exhausted. It was suspected that glucose was required as energy source for induction of a citrate-transport system. Resting cells pregrown in glucose plus citrate, were indeed found to take up citrate and convert it to extracellular glutamate even in the absence of sugar. In line with the induction hypothesis, chloramphenicol inhibited the metabolism of citrate by glucose-grown resting cells but had no such effect on the citrate-adapted cells. The antibiotic did not inhibit glucose utilization by citrate-adapted or unadapted resting cells.