Cystitis is most commonly caused by

(E. coli): Ref: 173-S* The organisms found in urine in cystitis are the E. coli, streptococci, staphylococci and B. proteus, the tubercle bacillus, and occasionally pseudomonas pyocyanea, Gonococcal cystitis is relatively rare, and almost invariably follows instrumentation.* The organism which is found most frequently is the E. coli* As the result of antibiotic treatments Pseudomonas, pyocyanea sometimes become the dominant infecting organism because of its resistance to antibiotics relative to the other infecting organisms.