Curli fibers are required for development of biofilm architecture in Escherichia coli K-12 and enhance bacterial adherence to human uroepithelial cells

Sessile bacteria show phenotypical, biochemical, and morphological differences from their planktonic counterparts. Curli, extracellular structures important for biofilm formation, are only produced at temperatures below 30 degrees C in Escherichia coli K-12 strains. In this report, we show that E. coli K-12 can produce curli at 37 degrees C when grown as a biofilm community. The curli-expressing strain formed more biofilms on polyurethane sheets than the curli-deficient strain under growth temperatures of both 25 degrees C and 37 degrees C. Curli are required for the formation of a three-dimensional mature biofilm, with characteristic water channels and pillars of bacteria. Observations by electron microscopy revealed the presence at the surfaces of the curli-deficient mutant in biofilm of flagella and type I pili. A wild-type curli-expressing E. coli strain significantly adhered to several lines of human uroepithelial cells, more so than an isogenic curli-deficient strain. The finding that curli are expressed at 37 degrees C in biofilm and enhance bacterial adherence to mammalian host cells suggests an important role for curli in pathogenesis.