A 79-year-old woman with a previous stroke develops fever, cough and a new right lower-zone infiltrate 36 hours after vomiting during a meal. She is alert, has a respiratory rate of 24/min, blood pressure of 132/74 mmHg and oxygen saturation of 95% on room air. She has purulent sputum but no pleural effusion, lung abscess, severe periodontal disease or bowel obstruction. Which one of the following is the most appropriate initial antibiotic treatment?