The most common etiologic agent of community-acquired uncomplicated cystitis is which organism?

• A. Enterobacter aerogenes
• B. Escherichia coli
• C. Klebsiella pneumoniae
• D. Proteus vulgaris

Answer: (B)

Escherichia coli is the most common cause of community-acquired uncomplicated cystitis. This bacterium normally lives in the gut, and its UPEC (uropathogenic E. coli) strains have specialized tools that allow them to move from the perineal area into the urinary tract and attach firmly to the bladder lining. Adhesion factors such as fimbriae enable tight binding to urothelial cells, while other virulence traits help the bacteria invade and elicit the inflammatory symptoms of cystitis. Because these strains are well adapted to colonize the urinary tract in otherwise healthy individuals, they account for the vast majority of community-acquired uncomplicated cases.

The other organisms listed are less likely in this setting. Enterobacter species and Klebsiella can cause UTIs but are more commonly associated with hospital-acquired or complicated infections, and Proteus often relates to urease production and stone formation rather than the typical uncomplicated community case. In the community, the pattern most often points to E. coli as the culprit.