Isolation of an Escherichia coli strain mutant unable to form biofilm on polystyrene and to adhere to human pneumocyte cells:: involvement of tryptophanase

Escherichia coli adherence to biotic and abiotic surfaces constitutes the first step of infection by promoting colonization and biofilm formation. The aim of this study was to gain a better understanding of the relationship between E. coli adherence to different biotic surfaces and biofilm formation on abiotic surfaces. We isolated mutants defective in A549 pneumocyte cells adherence, fibronectin adherence, and biofilm formation by random transposition mutagenesis and sequential passages over A549 cell monolayers. Among the 97 mutants tested, 80 were decreased in biofilm formation, 8 were decreased in A549 cells adherence, 7 were decreased in their adherence to fibronectin, and 17 had no perturbations in either of the three phenotypes. We observed a correlation between adherence to fibronectin or A549 cells and biofilm formation, indicating that biotic adhesive factors are involved in biofilm formation by E. coli. Molecular analysis of the mutants revealed that a transposon insertion in the tnaA gene encoding for tryptophanase was associated with a decrease in both A549 cells adherence and biofilm formation by E. coli. The complementation of the tnaA mutant with plasmid-located wild-type tnaA restored the tryptophanase activity, epithelial cells adherence, and biofilm formation on polystyrene. The possible mechanism of tryptophanase involvement in E. coli adherence and biofilm formation is discussed.