A 32-year-old G2P1woman at 34 weeks' gestation presents to the labor and delivery floor with the chief complaint of regular contractions, bloody show, and a gush of fluids. A 2.3 kg (5 lb 1 oz) boy is delivered by spontaneous vaginal delivery without further complication 1 hour after presentation. Twenty-four hours later, the infant has developed irritability, fever, and respiratory distress. He is diagnosed with sepsis secondary to pneumonia. The mother has no complaints other than anxiety regarding the condition of her child. She denies rigors, chills, sweats, nausea, or vomiting. The mother's pulse is 60/min, blood pressure is 125/80 mm Hg, and temperature is 37°C (98.6°F). Physical examination reveals lungs that are clear to auscultation bilaterally, and no murmurs, rubs, or gallops are present on cardiac examination. The suprapubic region is not tender to palpation. Vaginal and cervical examination reveals no significant tears or bleeds.Which prenatal test would have provided the most useful information in preventing this condition: