A 52-year-old man with diabetes mellitus presents to the emergency department with fever (39.2°C), hypotension (BP 88/54 mmHg), tachycardia (HR 118/min), and altered mental status. Blood cultures are drawn. He meets criteria for septic shock. After fluid resuscitation and source control measures, which antibiotic regimen is the first-line empirical choice for community-acquired sepsis in this patient?