Identification of Type 3 Fimbriae in UropathogenicEscherichia coliReveals a Role in Biofilm Formation

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Abstract
Catheter-associated urinary tract infection (CAUTI) is the most common nosocomial infection in the United States. UropathogenicEscherichia coli(UPEC), the most common cause of CAUTI, can form biofilms on indwelling catheters. Here, we identify and characterize novel factors that affect biofilm formation by UPEC strains that cause CAUTI. Sixty-five CAUTI UPEC isolates were characterized for phenotypic markers of urovirulence, including agglutination and biofilm formation. One isolate,E. coliMS2027, was uniquely proficient at biofilm growth despite the absence of adhesins known to promote this phenotype. Mini-Tn5mutagenesis ofE. coliMS2027 identified several mutants with altered biofilm growth. Mutants containing insertions in genes involved in O antigen synthesis (rmlCandmanB) and capsule synthesis (kpsM) possessed enhanced biofilm phenotypes. Three independent mutants deficient in biofilm growth contained an insertion in a gene locus homologous to the type 3 chaperone-usher class fimbrial genes ofKlebsiella pneumoniae. These type 3 fimbrial genes (mrkABCDF), which were located on a conjugative plasmid, were cloned fromE. coliMS2027 and could complement the biofilm-deficient transconjugants when reintroduced on a plasmid. Primers targeting themrkBchaperone-encoding gene revealed its presence in CAUTI strains ofCitrobacter koseri,Citrobacter freundii,Klebsiella pneumoniae, andKlebsiella oxytoca. All of thesemrkB-positive strains caused type 3 fimbria-specific agglutination of tannic acid-treated red blood cells. This is the first description of type 3 fimbriae inE. coli,C. koseri, andC. freundii. Our data suggest that type 3 fimbriae may contribute to biofilm formation by different gram-negative nosocomial pathogens.