The effect of deletion of trp operon and tna operon on the Escherichia coil biofilm formation was investigated in order to elucidate the role of L-tryptophan metabolism in biofilm formation. tip operon deletion mutants Delta trpC, Delta trpD and Delta trpE deficient in L-tryptophan biosynthesis showed higher biofilm formation. In addition, Delta tnaC with increased L-tryptophan degradation activity showed higher biofilm formation. On the contrary, Delta tnaA deletion mutant which lost L-tryptophan degradation activity showed low biofilm formation. From these results, it was suggested that decrease of intracellular L-tryptophan level induced biofilm formation and increase of L-tryptophan repressed biofilm formation. So the effect of the addition of L-tryptophan to the medium on the E. coil biofilm formation was investigated. L-Tryptophan addition at starting culture decreased biofilm formation and furthermore L-tryptophan addition after 16 h culture induced the degradation of preformed biofilm. From the above results, it was suggested that maintenance of high intracellular L-tryptophan concentration prevents E. coil biofilm formation and elevation of intracellular L-tryptophan concentration triggers degradation of matured biofilm. (C) 2012 Elsevier Inc. All rights reserved.