ISOLATION OF AN ESCHERICHIA-COLI K-12 MUTANT STRAIN ABLE TO FORM BIOFILMS ON INERT SURFACES - INVOLVEMENT OF A NEW OMPR ALLELE THAT INCREASES CURLI EXPRESSION

Classical laboratory strains of Escherichia coil do not spontaneously colonize inert surfaces. However, when maintained in continuous cultur e for evolution studies or industrial processes, these strains usually generate adherent mutants which form a thick biofilm, visible with th e naked eye, on the wall of the culture apparatus. Such a mutant was i solated to identify the genes and morphological structures involved in biofilm formation in the very well characterized E. coli K-12 context . This mutant acquired the ability to colonize hydrophilic (glass) and hydrophobic (polystyrene) surfaces and to form aggregation clumps. A single point mutation, resulting in the replacement of a leucine by an arginine residue at position 43 in the regulatory protein OmpR, was r esponsible for this phenotype. Observations by electron microscopy rev ealed the presence at the surfaces of the mutant bacteria of fibrillar structures looking like the particular fimbriae described by the Olse n group and designated curli (A. Olsen, A. Jonsson, and S. Normark, Na ture 338:652-655, 1989), The production of curli (visualized by Congo red binding) and the expression of the csgA gene encoding curlin synth esis (monitored by coupling a reporter gene to its promoter) were sign ificantly increased in the presence of the ompR allele described in th is work Transduction of knockout mutations in either csgA or ompR caus ed the loss of the adherence properties of several biofilm-forming E. coli strains, including all those which were isolated in this work fro m the wall of a continuous culture apparatus and two clinical strains isolated from patients with catheter-related infections. These results indicate that curli are morphological structures of major importance for inert surface colonization and biofilm formation and demonstrate t hat their synthesis is under the control of the EnvZ-OmpR two-componen t regulatory system.