Acute calculous cholecystitis may progress in a variety of ways from mild cases treatable with (or even without) oral antibiotics to severe cases complicated by bile peritonitis that require emergency surgical or radiological intervention. A sample of bile should always be sent for microbial cultures to identify aerobic and anaerobic bacterial organisms. Empirically selected broad spectrum antibiotic therapy (with a defined duration, dosage and administration route) should be prescribed according to the severity of the cholecystitis, an associated history of recent antibiotic therapy, and local bacterial susceptibility patterns. As soon as causative organisms have been identified, antibiotic therapy should be adjusted to a narrower spectrum antimicrobial agent based on the specific micro-organism(s) and the results of sensitivity testing.