The source of pathogens is the endogenous flora of the patient’s skin, mucous membranes, or hollow gastrointestinal viscera. These organisms are E. coli, Klebsiella sp., Enterobacter, B. fragilis, Peptostreptococci, Clostridia etc. Another source of pathogens can be a distant focus. Exogenous sources include the operating room personnel and environment (including air), and all tools, instruments and materials brought to the surgical field. These bacteria are primarily aerobes (staphylococci and streptococci).