Describe the three main ways in which epithelia carry out this barrier function, giving details of ...

(i) Mechanical (physical) barriers. Tight junctions between the epithelial cells prevent the penetration of pathogens between the cells to underlying tissues. In addition, there is a flow of air and fluid over epithelial surfaces, which oxygenates and flushes the surface, preventing anaerobic bacterial growth and transient adhesion. On ciliated epithelial surfaces, such as those of the respiratory tract, the formation of a layer of mucus that is kept in continual movement by the beating cilia inhibits colonization and invasion by microorganisms. (ii) Chemical barriers. The epithelium produces a variety of chemical substances that interfere with the adherence of microorganisms to epithelium and with their replication. The skin produces fatty acids in sebaceous glands, which helps to create an acid environment inhibitory to the growth of many bacteria. Lysozyme, an enzyme that inhibits cell-wall formation in bacteria, is secreted in tears, saliva, and sweat. The stomach produces strong hydrochloric acid, creating a highly acidic and formidable environment, which when combined with the stomach enzyme pepsin (an acid protease) poses one of the most inhospitable environments for microbial growth in our bodies. Defensins are antimicrobial peptides secreted by all the protective epithelia. (iii) Microbiological barriers. A microbiota of non-pathogenic commensal microorganisms colonizes many epithelial surfaces and provides an additional barrier to infection. They compete with pathogenic microbes for space and nutrients, and sometimes produce antibacterial proteins that further inhibit attachment to epithelium. For example, Escherichia coli in the large intestine produce colicins, which prevent colonization by other bacteria.

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