Some bacterial pathogens cause disease while remaining on the mucosal surfaces, but many others breach the anatomical barriers. By traversing the epithelial cell barrier and accessing the nutrient-rich tissue, these invading microbes can enjoy an exclusive source of nutrients, multiplying without competition.
Skin is the most difficult anatomical barrier for a microbe to penetrate. Bacterial pathogens that invade via this route rely on trauma of some sort that destroys the integrity of the skin. Staphylococcus aureus, for example, a common cause of wound infections, accesses tissues via the lesion. Yersinia pestis, the causative agent of plague, exploits fleas, relying on them to
- Actin filaments rearranged, causing a loss of microvilli
19.6 Invasion—Breaching the Anatomical Barriers 467
inject the microbe through the skin via a flea bite. ■ plague, p. 723
Invasion of mucous membranes is the most common route of entry for most pathogens, but the invasive processes are complex and inherently difficult to study. In vitro techniques that employ bacteria added to eukary-otic cells growing in tissue culture give important insights, but they do not necessarily demonstrate what actually happens in the complex environment of a mucous membrane. Studies involving mice or other animals can often be used to confirm the results of tissue culture, but some pathogens behave quite differently in non-human hosts. It appears, however, that there are two general mechanisms used for invasion—directed uptake by an epithelial cell and exploitation of antigen-sampling processes.
Host epithelial cell
Some pathogens induce non-phagocytic epithelial cells to use the process of endocy-tosis to take up the bacterial cells. The pathogen first attaches to a cell, then triggers that cell to engulf the bacterium. In many cases, specialized proteins that cause changes in the host cell's cytoskeleton are delivered by the type III secretion systems to induce the uptake. The directed disruption of the cytoskeleton causes obvious morphological disturbances in the cell membrane called ruffling (figure 19.5).
The bacteria appear to sink into the ruffles, leading to their eventual engulfment. In vitro studies show that Salmonella species can induce their own uptake by intestinal epithelial cells. Shigella species do the same, but they enter the mucosal epithelial cells at their base; they must first pass through a cell called an M cell to access the basement membrane side of epithelial cells. The mechanism they use to do this will be described in the next section.
Exploitation of Antigen-Sampling Processes
Recall that components of the immune system are located at strategic sites so that antigens that enter the body or pass through the intestine can be sampled. Some bacteria exploit the sampling process in order to access deeper tissues.
Several intestinal pathogens gain access to tissue by way of the M cells of the Peyer's patches in the intestine. Recall that M cells are specialized cells that function as a conduit between the intestinal lumen and the lymphoid tissues of the Peyer's patches. They routinely sample intestinal contents and deliver them to macrophages in the lymphoid tissues beneath; this process of transferring material from one side of the cell to the other is called transcytosis. Most microbes transcytosed by M cells are destroyed by the macrophages that receive them, but pathogens have evolved mechanisms to avoid this demise. Shigella species use M cells to traverse the epithelial barrier; once on the other side, the macrophages ingest them, but the bacteria are able to survive the phagocytic process, eventually escaping by inducing apoptosis in the phagocytic cells (figure 19.6). Shigella cells then adhere to specific receptors at the base of the epithelial cells and then induce these non-phagocytic cells to engulf them.
■ survival within the phagocyte, p. 471 ■ Peyer's patch, p. 397
(a) Shigella or other bacteria cross the mucous membrane into tissues by passing through M cells. m cell intestinal Space
(b) Macrophages engulf bacteria
Bacteria move from cell to cell propelled by actin filaments.
(c) Pathogen (bacteria) released from macrophages enters host cells by endocytosis.
Some pathogens invade by means of alveolar macrophages, which engulf material that enters the lungs. Mycobacterium tuberculosis produces surface proteins that facilitate their uptake by the alveolar macrophages that reside in the lung. While this might seem disadvantageous, it actually allows the organism to avoid a process that could otherwise lead to macrophage activation. Mycobacterium cells can survive within macrophages that have not been activated.
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