A 59-year-old man develops rapidly worsening breathlessness five days after an influenza-like illness. He has a temperature of 39.8°C, respiratory rate of 38/min, blood pressure of 92/56 mmHg and oxygen saturation of 86% despite high-flow oxygen. Imaging shows bilateral consolidation with several small cavities, and he has coughed up blood-streaked sputum. There is no history of bronchiectasis or recent hospital admission. Which one of the following is the most appropriate empiric antimicrobial regimen while cultures are pending?