Acute pyelonephritis. Illustration of small abscesses (yellow) on the surface of a kidney in a case of acute pyelonephritis. Bacteria, the causative agents of acute pyelonephritis, are seen in close-up. 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. The most common causes are Escherichia coli (shown here), Enterococcus sp., Streptococcus sp., or Staphylococcus sp.. Treatment is with antibiotic drugs.