**2. Etiology of UTI**

UTIs are mainly caused by bacteria, although fungi and some viruses have also been implicated. Among bacteria, Gram-negative bacteria of the *Enterobacteriaceae* family, including *E. coli, Klebsiella, Enterobacter, Proteus* species, *etc*., are mostly involved. However, some Gram-positive organisms, principally *Staphylococcus aureus, Staphylococcus saprophyticus* and *Streptococcus agalactiae*, also play a role especially among young women. *E. coli* is the dominant causative agent in all patient groups, causing 80–90% of all UTIs [5]. Consequently, *E. coli* serves as a model pathogen for studying UTI pathogenesis.

*Escherichia coli* is a normal constituent of the intestinal microbiota of humans and animals [9, 10]. The distinctive *E. coli* strains that cause most UTIs have been designated uropathogenic *E. coli* (UPEC). They possess diverse virulence-associated factors (VFs) that assist them in attaching to, invading, and injuring the host, and include adhesins, toxins, siderophores, protective polysaccharide coatings, invasins, and serum resistance-associated proteins. The presence and numbers of such VFs predicts *in vivo* virulence [11].
