An outflow obstruction of urine can lead to incomplete emptying and urinary stasis, which causes bacteria to multiply without being flushed out. Urinary tract obstruction caused by something such as a kidney stone can also lead to acute pyelonephritis. UTIs are more common in females than in males due to shorter urethras, hormonal changes, and close distance to the anus. Bacteria will first attach to urethral mucosal epithelial cells and will then travel to the bladder via the urethra either through instrumentation or urinary tract infections which occur more frequently in females. Ascending infection happens through several steps. Most patients will get acute pyelonephritis through ascending infection. Hematogenous spread is less common and usually occurs in patients with ureteral obstructions or immunocompromised and debilitated patients. Bacteria can reach the kidneys in 2 ways: hematogenous spread and through ascending infection from the lower urinary tract. In most patients, the infecting organism will come from their fecal flora. Other gram-negative bacteria which cause acute pyelonephritis include Proteus, Klebsiella, and Enterobacter. The main cause of acute pyelonephritis is gram-negative bacteria, the most common being Escherichia coli.
