Type 1 Fimbriae, Curli, and Antigen 43: Adhesion, Colonization, and Biofilm Formation

This review is primarily concerned with the first step in biofilm formation, namely, bacterial attachment to surfaces. It describes three examples of bacterial adhesins, each of which belongs to a different subgroup and follows different strategies for surface presentation and adhesin exposure. These are type 1 fimbriae, very long stiff rodlike organelles; curli, amorphous fluffy coat structures; and finally antigen 43, short outer membrane structures with a simple assembly system. Their role as adhesins, their structure and biosynthesis, and their role in biofilm formation are described in detail in the review. The FimH protein presented by type 1 fimbriae seems to be a highly versatile adhesin fulfilling a diverse spectrum of roles ranging from pellicle and biofilm formation to being a bona fide virulence factor in uropathogenic E. coli (UPEC) strains, where it plays important roles in the manifestation of cystitis. Curli formation promotes two fundamental processes associated with biofilm formation: initial adhesion and cell-to-cell aggregation. A role for curli in the colonization of inert surfaces has been demonstrated. Severe sepsis and septic shock are frequently caused by gram-negative bacteria, and several factors suggest a significant role for curli during E. coli sepsis. The protection provided by Ag43-mediated aggregation was underlined in a series of experiments addressing the role of Ag43 in protection against oxidizing agents. Type 1 fimbriae, curli, and Ag43 are structurally different bacterial surface structures and follow completely different strategies for surface display and assembly.