# Pathogenesis

Pathogenesis

The most common route of  infection is ascending UTI; contamination of  the vaginal and periurethral area with uropathogenic organisms originating from the gastrointestinal tract leads to adherence and migration of  bacteria into the urethra and bladder. Once in the bladder, adherence of  the bacteria to the urothelium triggers a process of  bacterial inter nalisation into the urothelial cell and the subsequent formation of intracellular bacterial communities (IBCs) and quiescent intracellular reservoirs (QIRs), which may remain viable for months and act as a source of rUTI. These IBCs and QIRs act in a similar way to a biofilm, protecting bacteria from the host immune response and from the action of  antimicrobial agents. Less common routes of  infection include haematogenous spread (seen with Staphylococcus aureus and fungal infections) or direct infection from retroperitoneal abscess or inflammatory bowel disease. The commonest organisms implicated in uncomplicated UTI are Escherichia coli (85%), Staphylococcus saprophyticus, Enterococcus faecalis, Proteus and Klebsiella . Complicated UTIs are caused by E. coli (50%), enterococci , S. aureus and monas . Successful infection depends on the bacterial virulence rel ative to the host defence mechanisms. The common bacterial virulence factors and host defence mechanisms are shown in Table 83.13 .