Project Summary

Role of Curli Fimbriae in Attachment of Shiga toxin-producing and Enterohemorrhagic Escherichia coli (STEC and EHEC) on Raw and Ready-to-eat Beef Products

Principle Investigator(s):
Jinru Chen, Ph.D.
The University of Georgia
Completion Date:
May 2004



Enterohemorrhagic Escherichia coli (EHEC) is a group of important foodborne pathogens that are associated with severe enteric and systematic diseases such as hemorrhagic colitis (HC), hemolytic uremic syndrome (HUS), and thrombotic thrombocytopenic purpura (TTP) (Fey et al., 2000; Karmali, 1989; Sussman, 1997). Outbreaks of EHEC infection have been linked to the consumption of undercooked ground beef as well as ready-to-eat beef products, such as salami (Blanco et al., 1996; CDC, 1995; Duffy et al., 2000). Contamination of beef in the processing environment can occur at any stage from slaughtering to packaging. A survey conducted in the Northwestern regions of the U.S. revealed that the prevalence of EHEC O157:H7 in pre-eviscerated, post-eviscerated, and post-processed beef carcasses was 43% (148 of 341), 18% (59 of 332), and 2% (6 of 330), respectively (Elder et al., 2000). 

EHEC attachment to meat as well as to other surfaces is a complex process that can be influenced by the properties of both EHEC cells and their contact surfaces. Cells of EHEC express curli, a thin, wiry, and aggregative surface appendage (Cookson et al., 2002; Prigent-Combaret et al., 2000; 2001; Vidal et al., 1998), which appears as a ~ 4-12 nm protein fiber under a transmission electron microscope (Chapman et al., 2002; Olsen et al., 1989). Over-expressed curli often forms an interconnecting mesh between EHEC cells. Curli expression in E. coli can be induced by low osmotic condition, nutritional starvation, and lower than optimal growth temperatures (Olsen et al., 1989; 1993). 

Curli enables E. coli to adhere to fibronectin, laminin, and plasminogen (Arnqvist et al., 1992; Gophna et al., 2001; Olsen et al., 1993; Sjobring et al., 1994). These proteins are present in the connective and adipose tissues between the skin and the skeletal tissues of animals. Curli expression in E. coli is controlled by two divergently transcribed operons, csgBA and csgDEFG. The csgA encodes a major subunit of curli protein while the csgB encodes a cell surface protein, which acts as a nucleator for CsgA monomer and is responsible for polymerization of curli fimbriae (Hammar et al., 1995; Bian and Normark, 1997). The CsgD is a transcriptional activator necessary for curli expression, and CsgG is an outer membrane lipoprotein involved in extracellular stabilization of CsgA and CsgB (Loferer et al., 1997). The CsgE and CsgF are chaperone-like and act as a nucleator protein, respectively. Both proteins are required for proper nucleation of CsgA on CsgB (Chapman  et al., 2002).  

The stated objectives for this work were:

  1. To investigate the distribution of the curli subunit gene (csgA) and the expression of curli fimbriae in STEC and EHEC;
  2. To determine the rate of curli expression on beef-based microbiological media at various incubation temperatures;
  3. To identify genetic modifications that convert curli-producing to non curli-producing cells or vice versa; and 
  4. To determine the role of curli in attachment of STEC and EHEC on raw and ready-to-eat beef products  

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