Pathogenesis of Bacterial Infection

All humans are infected with bacteria (the normal flora) living on their external surfaces (including the skin, gut and lungs). We are constantly also exposed to bacteria (including air, water, soil and food). Normally due to our host defenses most of these bacteria are harmless. In compromised patients, whose defenses are weakened, these bacteria often cause opportunistic infectious diseases when entering the bloodstream (after surgery, catheterization or other treatment modalities). When initiated in the hospital, these infectious diseases are referred to as nosocomial. Some common bacteria found in the normal flora include Staphylococcus aureus, S. epidermidis and Propionibacterium acnes (found on the skin)and Bacteroides and Enterobacteriaceae found in the intestine (the latter in much smaller numbers).