Entry mechanisms of mycobacteria.

Since many mycobacteria are facultative intracellular pathogens, their ability to cause disease involves entry, survival and replication within host cells. Despite the fact that mycobacteria were first associated with disease more than 125 years ago, the first step in the production of an infection, entry into host cells, is not well understood. Mycobacteria have the ability to enter a number of different cell types, but the primary cell type that they are thought to replicate within during human disease is macrophages. Since macrophages have a large number of receptors that are designed for relatively non-specific uptake of foreign particles, there are multiple routes by which nearly any bacteria can be taken up. The outcome of mycobacterial entry into macrophages via different mechanisms is unclear. Although it is thought that mycobacteria may enter macrophages by a mechanism that allows them to avoid lysosomal fusion, it remains possible that mycobacteria enter by more than one mechanism, yet remain viable and replicate intracellularly through modification of the phagosome. In the current discussion we will review mycobacterial research specifically relating to the mechanisms of entry into host cells. Although much progress has been made in our understanding of entry by mycobacteria, we anticipate that clarification of the role of entry in pathogenesis will require further application of newly developed molecular tools to dissect each of the proposed mechanisms.