Cystitis and pyelonephritis. Illustration showing Escherichia coli bacteria (blue bodies with hair like flagella) attacking the bladder wall (lower frame). Escherichia coli is normally a harmless inhabitant of the gut, although it may cause opportunistic infection if it enters the bladder, producing symptoms of cystitis (pain and frequent desire to urinate). If the cystitis not treated, infection can spread to the kidneys via the ureters (white), causing pyelonephritis. Infection causes swelling of the kidney pelvis and then spreads to the outer areas. These become inflamed, with streaks of pus (yellow). Without antibiotic treatment permanent kidney damage may result.