**1. Introduction**

Uropathogenic *Escherichia coli* (UPEC) is a type of extraintestinal pathogenic *E. coli* (ExPEC) responsible for urinary tract infection (UTI). It is reported to be the ExPEC with the greatest medical importance. This is so because UPEC is responsible for most of the UTIs and humans of all ages are affected [1, 2]. These bacteria are associated with both asymptomatic bacteriuria and symptomatic UTIs. UTIs are categorized based on the parts of the body which the infections occur. These are cystitis which occurs in the bladder and pyelonephritis which occurs in the kidney [3–6]. UPEC strains have a lot of virulence factors which are responsible for the pathogenicity associated with symptomatic UTIs [7, 8]. The virulence factors are classified into two categories: (i) bacterial cell surface virulence factors and (ii) bacteria secreted virulence factors [9–11]. Many of virulence-associated genes can be found on pathogenicity islands (PAIs) [12, 13]. Though the mechanisms of asymptomatic bacteriuria are still not clear, studies have reported that UPEC becomes nonadherent and nonhemolytic resulting to asymptomatic bacteriuria [14–16]. Thus, this chapter will elucidate on the important UPEC virulence factors which are responsible for UTIs.
