Microbiology and pathogenesis of acute pyelonephritis, UpToDate, 2009

Urinary tract infections (UTIs) are common, especially in young children and sexually active women. By convention, UTI is defined as an infection either of the lower tract (acute cystitis) or upper tract (acute pyelonephritis and other renal infections). (See "Acute cystitis in women".)
Despite the upper tract involvement in acute pyelonephritis, most episodes are generally considered to be uncomplicated. A complicated urinary tract infection, whether localized to the lower or upper tract, is associated with an underlying condition that increases the risk of failing therapy (such as obstruction, urologic dysfunction, or a multiply-resistant uropathogen).
At the time of presentation, it is difficult to determine whether a patient with pyelonephritis or cystitis has a complicated or uncomplicated infection. However, an uncomplicated urinary tract infection is generally defined as one occurring in a healthy, ambulatory nonpregnant woman; all other urinary tract infections may be considered complicated. Multiple factors are associated with complicated urinary tract infection that generally warrants both a broader spectrum antimicrobial for empiric therapy and, infrequently, consideration of surgical intervention.
Acute pyelonephritis also must be distinguished from chronic pyelonephritis. Chronic pyelonephritis is an uncommon cause of chronic tubulointerstitial disease due to recurrent infection, such as infection in association with a chronically obstructing kidney stone, or vesicoureteral reflux (VUR). VUR in children can cause repeated episodes of acute, often asymptomatic, infection, and produce renal scarring and, in some cases, progressive renal failure. (See "Presentation, diagnosis, and clinical course of vesicoureteral reflux".)
The epidemiology, microbiology, and pathogenesis of acute pyelonephritis will be reviewed here. The clinical manifestations, diagnosis, and treatment of uncomplicated and complicated infections are discussed separately. (See "Clinical manifestations; diagnosis; and treatment of acute pyelonephritis".)... Authorized only