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.