A 24-year-old woman in her third trimester of pregnancy presents with urinary frequency and burning for the past few days. She denies fever, or chills. She has mild suprapubic tenderness, and a urine dipstick is positive for WBC, protein, and a small amount of blood. Culture produces greater than 100,000 colonies of gram-negative bacilli. Which attribute of this uropathogenic organism is most strongly associated with its virulence?