A 19-year-old woman comes to the student health clinic because of a 2-day history of painful urination. She also has urinary frequency and urgency. This is her third episode of similar symptoms in the past 6 months. She is sexually active with one partner, and symptoms typically begin within 48 hours after intercourse. She has not had fever, chills, or back pain. Her temperature is 37.2°C (99.0°F), pulse is 78/min, and blood pressure is 118/72 mm Hg. Physical examination shows suprapubic tenderness to palpation. There is no costovertebral angle tenderness. Pelvic examination shows no vaginal discharge or cervical motion tenderness. Urinalysis shows:
- Leukocyte esterase: Positive
- Nitrites: Negative
- WBC: 40-80/hpf
- Bacteria: Many
Gram stain of the urine shows gram-positive cocci in clusters.
Which of the following is the most likely causative organism?