*2.1.1 Fimbrial adhesins*

Fimbriae, also known as pili, are thin, filamentous appendages protruding on the bacterial surface and consist of polymerized aggregates of small molecular weight monomers of the fimbrin protein [16]. Characteristically, fimbriae mediate the initial attachment of Gram-negative bacterial pathogens to host cells and surfaces [17]. In *Salmonella*, the initial contact results in relatively weak adherence of the bacteria to intestinal epithelial cells but soon induces *de novo* bacterial protein synthesis which increases the strength and intimacy of the attachment [18]. This process is also accompanied by the development and assembly of a unique secretion apparatus called the Type 3 Secretion System (T3SS) which is required for *Salmonella* to invade epithelial cells [19]. The chromosome of *S. typhimurium* contains 13 fimbrial operons, *afg* (*csg*), *bcf*, *fim*, *lpf*, *pef*, *saf*, *stb*, *stc*, *std*, *stf*, *sth*, *sti*, and *stj* [20–22] (**Table 1** and **Figure 1**). Eight types of fimbriae which have been experimentally investigated [23] are outlined below.
