Fig. 3.
Exogenous and endogenous routes of infection and evolving approaches to surgical site infection prevention. Traditional conceptions of healthcare-associated infection are hospital-centered, emphasizing the role of “exogenous” infection with bacteria newly acquired through contact with the hospital environment (A). Under this model, infection occurs when pathogens are transmitted from nosocomial reservoirs (surfaces, hands, ventilation systems, instruments) to the patient through contact that occurs in the course of clinical care. Exogenous sources are frequently implicated in common-source outbreaks, which attract significant attention; however, bacterial genetic analyses now demonstrate that, in routine clinical circumstances, the vast majority of healthcare-associated infections are “endogenous,” arising from the patient microbiome rather than the hospital environment (B). Under this complementary model, bacteria colonizing the patient before contact with the healthcare system become pathogens when procedures (1), exposures (2), and stresses (3) that occur in the hospital disrupt normal regulation of the microbiome. The evolution of surgical site infection prevention strategies over time (C) can be conceptualized through this perspective. Fio2, fraction of inspired oxygen; IV, intravenous.