A prerequisite for the de- velopment of any bac- terial disease is localiza- tion of the bacteria to a niche that is suitable for growth and pathogenesis. In a mammalian intestine, attachment is critical to avoid displacement from a preferred site by the continu- ous flow of the intestinal con- tents. Attachment is also hin- dered by competition with the multitude of indigenous micro- flora for binding sites on the intestinal epithelium 1 . The initial step in bacterial attachment to the host epithe- lium is usually mediated by fimbriae (Box 1). Fimbriae are proteinaceous appendages of varying lengths and diameters, consisting of a poly- mer of a single subunit tipped or interspersed with adhesive proteins, which protrude from the bacterial cell. The fimbriae are usually arranged peritrichously around the bacterial cell, with hundreds of fimbriae per bac- terium; however, in some systems, the fimbriae are ex- pressed in a polar orientation (Fig. 1). Two of the best-studied enteric pathogens, Escherichia coli and Salmonella enterica, are able to infect a wide range of hosts, but particular strains of either species can often only cause infection in a small vari- ety of potential hosts or colo- nize a particular segment of the intestine. This host and tissue-range specificity is often mediated by different fimbriae and receptors 2,3 . Fimbriae are critical to the pathogenic pro- cess; however, their importance is often overlooked by many researchers in bacterial pathogenesis, who focus on the intracellular aspects of pathogenesis or the later stages of disease. The ability of species of enteric bacteria to recognize and colonize unique niches along the intestine is mainly based on receptor distribution and interpretation of a combination of environmental signals leading to the expression of specific adherence factors. Such elaborate orchestration of events is critical during the initial steps of pathogenesis. R.A. Edwards* is in the Dept of Microbiology, University of Illinois Urbana–Champaign, 601 S. Goodwin Ave, Urbana, IL 61801, USA; J.L. Puente is in the Molecular Microbiology Dept, Instituto de Biotecnología, Universidad Nacional Autónoma de México, Apartado Postal 510-3, Colonia Miraval, Cuernavaca, Morelos, 62250, Mexico. *tel: +1 217 333 2203, fax: +1 217 244 6697, e-mail: edwards2@uiuc.edu Fimbrial expression in enteric bacteria: a critical step in intestinal pathogenesis Robert A. Edwards and José Luis Puente R EVIEWS TRENDS IN MICROBIOLOGY 282 VOL. 6 NO. 7 JULY 1998 Copyright © 1998 Elsevier Science Ltd. All rights reserved. 0966 842X/98/$19.00 PII: S0966-842X(98)01288-8 24 Li, M. et al. (1997) J. Virol. 71, 1984–1991 25 Arvanitakis, L. et al. (1997) Nature 385, 347–350 26 Bais, C. et al. (1998) Nature 391, 86–89 27 Henderson, S. et al. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 8479–8483 28 Sarid, R. et al. (1997) Nat. Med. 3, 293–298 29 Sunil-Chandra, N.P., Efstathiou, S., Arno, J. and Nash, A.A. (1992) J. Gen. Virol. 73, 2347–2356 30 Cardin, R.D., Brooks, J.W., Sarawar, S.R. and Doherty, P.C. (1996) J. Exp. Med. 184, 863–871 31 Sunil-Chandra, N.P., Efstathiou, S. and Nash, A.A. (1992) J. Gen. Virol. 73, 3275–3279 32 Sunil-Chandra, N.P., Efstathiou, S. and Nash, A.A. (1993) Virology 193, 825–833 33 Usherwood, E.J. et al. (1996) J. Gen. Virol. 77, 2819–2825 34 Usherwood, E.J., Stewart, J.P. and Nash, A.A. (1996) J. Virol. 70, 6516–6518 35 Weck, K.E. et al. (1996) J. Virol. 70, 6775–6780 36 Nash, A.A. and Sunil Chandra, N.P. (1994) Curr. Opin. Immunol. 6, 560–563 37 Sunil-Chandra, N.P., Arno, J., Fazakerley, J. and Nash, A.A. (1994) Am. J. Pathol. 145, 818–826 38 Penn, I. (1986) Surg. Gynecol. Obstet. 162, 603–610 39 Doherty, P.C. et al. (1997) Curr. Opin. Immunol. 9, 477–483 40 Rickinson, A.B. and Moss, D.J. (1997) Annu. Rev. Immunol. 15, 405–431 41 Ehtisham, S., Sunil-Chandra, N.P. and Nash, A.A. (1993) J. Virol. 67, 5247–5252 42 Stevenson, P.G. and Doherty, P.C. (1998) J. Virol. 72, 943–949 43 Usherwood, E.J. et al. (1997) J. Gen. Virol. 78, 2025–2030 44 Sarawar, S.R. et al. (1996) J. Virol. 70, 3264–3268 45 Sarawar, S.R. et al. (1997) J. Virol. 71, 3916–3921 46 Dutia, B.M., Clarke, C.J., Allen, D.J. and Nash, A.A. (1997) J. Virol. 71, 4278–4283 47 Weck, K.E. et al. (1997) Nat. Med. 3, 1346–1353 48 Tripp, R.A. et al. (1997) J. Exp. Med. 185, 1641–1650 49 Usherwood, E.J., Ross, A.J., Allen, D.J. and Nash, A.A. (1996) J. Gen. Virol. 77, 627–630 50 Sunil-Chandra, N.P., Efstathiou, S. and Nash, A.A. (1994) Antiviral Chem. Chemother. 5, 290–296 51 Neyts, J. and De-Clercq, E. (1998) Antimicrob. Agents Chemother. 42, 170–172 52 Ensser, A., Pflanz, R. and Fleckenstein, B. (1997) J. Virol. 71, 6517–6525 53 Albrecht, J.C. et al. (1992) J. Virol. 66, 5047–5058 54 Russo, J.J. et al. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 14862–14867 55 Telford, E.A. et al. (1995) J. Mol. Biol. 249, 520–528 56 Baer, R. et al. (1984) Nature 310, 207–211 57 Nicholas, J. et al. (1997) J. Virol. 71, 1963–1974