2-5. Action of lysozyme. Lysozyme targets the peptidoglycan in bacterial cell walls of mainly Gram-positive bacteria and hydrolyzes the link between N-acetylmuramic acid and N-acetyl-D-glucosamine. It can also degrade the peptidoglycan of Gram-negative bacteria if the outer membrane is first disrupted by bile salts.
Defenses of Mucosal Surfaces
The respiratory, gastrointestinal, and urogenital tracts are topologically inside the body, but they are exposed constantly to the outer environment and foreign materials. Unlike the many dead layers found in skin, internal surface areas (called mucosal epithelia) are comprised of only one epithelial layer. The role of mucosal epithelial cells is absorption or secretion, so they are continuously bathed in fluids, having a temperature of around 37°C and a pH of 7.0 to 7.4. These warm, neutral, and moist conditions are ideal for growth of bacteria. To protect from bacterial colonization, these vulnerable epithelia have evolved a formidable array of chemical and physical barriers (see Table 2-2).
Mucosal cells are regularly replaced, with old cells sloughed off into the lumen. In fact, mucosal cells are one of the fastest dividing populations of cells in the body. Thus, bacteria that do manage to reach and colonize a mucosal surface are constantly eliminated from the mucosal surface and can only remain in the area if they can grow rapidly enough to colonize newly produced cells. Chemical and other innate defenses help reduce the growth rates of bacteria sufficiently to allow ejection of mucus blobs and sloughing of mucosal cells to clear the bacteria from the area.
An important protection of many internal epithelia is mucus. Mucus is a mixture of secreted glycoproteins (mucin) produced by goblet cells, a specialized glandular, modified columnar epithelial cell type incorporated into the epithelial layer. (The basic structure of glycoproteins is reviewed in Box 3-1). Mucus has a viscous, slimy consistency, which allows it to act as a lubricant. It also plays a protective role because it traps bacteria and prevents them from reaching the surface of the epithelial cells. Mucus is constantly being produced, and bacteria trapped in the mucus blobs are shed from the site. In the gastrointestinal and urinary tracts, peristalsis and the rapid flow of liquids through the area removes the mucus blobs along with the lumen contents. Indeed, infections can occur when this flow is disrupted, as may occur in the case of bedridden patients with urethral catheterization, which keeps the urethral entrance open and does not allow for flushing action, instead causing a slow, constant drain.
Another protective role of mucus is to bind proteins that have antibacterial activity (Table 2-2). Lysozyme is a major host defense protein found in tears, saliva, milk, and mucus and is also secreted by Paneth cells found at the bottom of crypts along the small intestine. Lysozyme is an enzyme that targets the peptidoglycan of the bacterial cell wall and hydrolyzes the linkage between N-acetylmuramic acid and N-acetyl-D-glucosamine (Figure 2-5). It is most effective against the cell walls of Gram-positive bacteria, but it can also digest and weaken the cell walls of Gram-negative bacteria, especially if membrane-disrupting substances, such as the detergent-like bile salts found in the intestine, first breach the outer membrane. Phospholipase is an enzyme found in tears and secreted by Paneth cells that degrades the cytoplasmic membrane to lyse bacterial cells. Lactoferrin is an iron-binding, secreted protein found in mucus that sequesters iron and deprives bacteria of this essential nutrient.
Lactoperoxidase is another protein with antibacterial activity that is found in secretory fluids (e.g., milk, tears, saliva, and airway mucus). Lactoperoxidase is a heme-containing peroxidase that uses hydrogen peroxide (H2O2) as an oxidant to generate highly reactive oxygen species that kill bacteria. An important substrate is thiocyanate (SCN-), which is converted into hypothiocyanite (OSCN-), a potent bactericidal reagent. It is believed that exposed thiol groups (-SH) of enzymes and other proteins on the bacterial membrane surface are the primary targets of these reactive oxidants, which oxidize the thiols and thereby disrupt the normal function of the bacterial surface proteins.
Paneth cells also secrete toxic antimicrobial peptides (Figure 2-6), such as defensins (known as cryptdins in mice), cathelicidins, and histatins, which contain highly cationic (basic) regions that enable them to interact with negatively charged phospholipids and interact with or insert into bacterial cell membranes. These defensins kill bacteria by forming channels or holes in their membranes and depolarizing the cell by collapsing the proton-motive force (the potential energy stored as an electrochemical gradient across a membrane) that is essential for bacterial survival. This type of activity is responsible for the effectiveness of one of the first antibiotics, gramicidin, which is a pore-forming protein that kills bacteria. Defensins and other antimicrobial peptides have been found in the mouth, on the tongue, on skin, in the vagina, in the lungs, and in the crypts of the small and large intestines.
Figure 2-6. Three types of bacterial membrane permeabilization by gut antimicrobial peptides. Several antimicrobial peptides (AMPs) secreted by epithelial and Paneth cells in the intestine kill bacteria by forming pores in membranes. (A) α-defensin is expressed as an inactive propeptide that is processed and activated by proteases (MMP7 in mice and trypsin in humans). (B) REG3α is expressed as an inactive propeptide that is processed by proteolytic cleavage. The activated REG3α then binds to bacterial peptidoglycan and oligomerizes to form a hexameric pore in bacterial membranes. (C) Cathelicidins, such as LL-37, first bind to bacterial membranes via electrostatic interactions, then form α-helical structures in the presence of lipids and insert into the bacterial membrane to form pores. Adapted from Mukherjee S, Hooper LV. 2015. Immunity 42:28–39, with permission.
In the mouth, defensins may be the reason why infections of the tongue are so rare and why animals lick wounds. In the crypts of the intestinal mucosa, they are presumably protecting the intestinal stem cells, which divide constantly to replenish the cells of the intestinal mucosa. These peptides probably have some antibacterial effects that protect these locations from bacteria. It is worth keeping in mind, however, that the membrane-disrupting activity of these peptides can be inhibited by physiological salt concentrations or by serum. Consequently, their most important antibacterial activities may be exerted largely in another location where they are found: vacuoles inside phagocytic cells that engulf and kill bacteria.
Special Defenses of the Gastrointestinal Tract
Different regions of the gastrointestinal tract (Figure 2-7) have special antibacterial features that serve as barriers to pathogens. The lumen of the stomach has an extremely acidic environment (pH ∼2). It was previously thought that most bacteria could not survive there, but scientists have found DNA from 128 different species of bacteria occupying the stomach. The only bacterium inhabiting this harsh environment that has been studied in detail is Helicobacter pylori, which causes gastritis, gastric ulcers, and even gastric cancer. In fact, H. pylori does not actually live in the lumen of the stomach, but rather in the mucin layer that covers and protects the stomach lining. Cells in the stomach lining secrete carbonate, which buffers the mucin layer to near neutral pH. H. pylori has the ability to protect itself long enough to reach the mucin layer and thus is not killed as readily as many bacteria are when they are exposed to the highly acidic environment of the stomach.
Figure 2-7. The gastrointestinal tract. Different types and quantities of bacteria, mostly strict or facultative anaerobes, colonize along the gastrointestinal tract as the environments change with pH, bile, mucus content, digestive enzymes, and transit time. Data from Sartor
