Acute pyelonephritis caused by Acinetobacter baumannii bacteria. Illustration of small abscesses (yellow) on the surface of a kidney in a case of acute pyelonephritis. Pyelonephritis may be either acute or chronic. The acute form is usually caused when bacteria causing cystitis spread up the ureters from the bladder. It causes fever and back pain. Acinetobacter baumannii are small Gram-negative rod-shaped bacteria commonly associated with opportunistic nosocomial (hospital-acquired) infections of different location, including urinary tract infections and acute pyelonephritis.