Location, Location, Location: Five Facts about Tissue Tropism and Pathogenesis

Infectious disease burden remains high [1]. An improved knowledge of host–microorganism interactions is key to delineating disease progression and developing new control and treatment modalities. Microorganisms may be pathogenic (able to cause disease) or non-pathogenic (unable to cause disease). Disease severity is influenced by host and pathogen factors; in particular, pathogens may express different virulence factors that modulate disease severity and progression. Within this context, the interaction of the pathogen with organ and tissue niches is especially important. Tropism refers to the ability of a given pathogen to infect a specific location. Organ or tissue tropism reflects the ability of a given pathogen to infect a specific organ or sets of organs. Some pathogens are broadly tropic, infecting all or most organs, while others are restricted to a given tissue or even to certain tissue niches. From this point of view, the ability of a pathogen to infect specific organs may vary over the course of the disease and could be active, pathogen-mediated, or passive, requiring, for example, a prior skin break or vector bite. Within tissue niches, intracellular pathogens may also preferentially infect specific organelles or intracellular sites. This article will focus on tissue tropism and its relationship to pathogenesis with examples from Staphylococcus aureus bacteria, Trypanosoma brucei protozoan parasites, and the influenza virus.