The skin is a very effective barrier to bacteria so that no bacterium by itself is known to be able to penetrate unbroken skin. Of course, a puncture, cut or scrape in the skin could introduce infectious bacteria. The mucous membranes are more vulnerable to penetration by infectious bacteria but still pose a formidable barrier of mucus and antimicrobial substances. Nonetheless, most infectious agents impinge on the skin or mucous membranes of the oral cavity, respiratory tract, GI tract or urogenital tract, and from these sites most infections occur. The intact surface of the healthy epidermis seems to be rarely if ever penetrated by bacteria.
Perspiration contains lysozyme and other antimicrobial substances.
Many are heavily colonized with bacteria in whose moist secretions they survive. These normal flora are restricted from entry and usually occupy any attachment sites that might otherwise be used by pathogens. The normal flora established on mucous membranes may antagonize non-indigenous species by other means, as well. Nonetheless, some pathogens are able to penetrate the mucous membranes, and this is probably the major site from which pathogens invade. Probably, damage to the epithelial cells caused by toxic products of these bacteria plays a role.