Yes, bacteria can grow in a thermos if food is not stored properly. Most bacteria grow rapidly between 4°C and 60°C (40°F and 140°F). If food is stored in a thermos at temperatures within this range for over 2 hours, harmful bacteria could multiply, making the food unsafe to eat. To avoid this, ensure that hot foods are packed at or above 74°C (165°F) and cold foods at or below 4°C (40°F). Additionally, moisture and food residue can breed bacteria. Always clean your thermos thoroughly immediately after use, and air-dry fully.