Concepts and mechanisms: crossing host barriers

The human body is bordered by the skin and mucosa, which are the cellular barriers that define the frontier between the internal milieu and the external nonsterile environment. Additional cellular barriers, such as the placental and the blood-brain barriers, define protected niches within the host. In addition to their physiological roles, these host barriers provide both physical and immune defense against microbial infection. Yet, many pathogens have evolved elaborated mechanisms to target this line of defense, resulting in a microbial invasion of cells constitutive of host barriers, disruption of barrier integrity, and systemic dissemination and invasion of deeper tissues. Here we review representative examples of microbial interactions with human barriers, including the intestinal, placental, and blood-brain barriers, and discuss how these microbes adhere to, invade, breach, or compromise these barriers.