we will further explore in chapter 15, antibiotics that result in lysis of the bacterium might cause further harm by causing release of membrane components such as the endotoxin LPS.
Fortunately, some new methods for rapid bacterial identification have been approved by the FDA and others are in late-stage development. These methods rely on highly sensitive technologies, such as mass spectroscopy and PCR-coupled high-throughput sequencing, which enable determination of protein or DNA content from samples taken directly from blood or cultured from blood for only a few hours. The protein or DNA profiles obtained are then quickly compared to library databases of hundreds of standard profiles from known bacterial pathogens. These new methods make identification of pathogens possible in less than a day, and sometimes in just a few hours, instead of the several days required by standard microbiological testing. Other research efforts are focused on determining biomarkers indicative of different causes of sepsis to guide appropriate antibiotic administration and shock treatments. We will discuss more of these methods in chapter 9.
Other Innate Defenses of the Body—Nutritional Immunity
Another innate defense that is present in blood and helps to hamper spread of the invading microbes is a group of proteins that sequester metals away from bacteria that enter the body. Transition metals, such as iron, manganese, and zinc, are essential to the survival of invading bacteria. By restricting metal availability through a group of metal-binding proteins as part of the innate defenses, the body is able to prevent the growth of invading microbes. This limitation of essential nutrient transition metals, termed nutritional immunity, is gaining recognition as a potent innate defense against pathogens.
We have already discussed the role of lactoferrin as a secreted iron-binding protein of surface defenses (milk, tears, saliva, and nasal secretions) that hampers microbial growth and colonization outside the body. Like lactoferrin, transferrin sequesters iron and prevents iron acquisition by microbes that enter the blood. Transferrin is a glycoprotein produced by the liver that circulates through blood and lymph and binds extremely tightly to the ferric form of iron (Fe3+), controlling the level of free iron available. Transferrin bound with iron is recognized by a cell surface receptor that transports the Fe3+-transferrin complex into recycling endosomes, where the iron is sequestered and removed from the transferrin, and the transferrin is then recycled to the surface, where it can scavenge more iron. In this way, the levels of iron in blood drop to an even lower level than normal, which severely limits the growth of most bacteria. Transferrin and lactoferrin, along with their receptors, are also thought to mediate cellular uptake of other divalent metal ions, such as manganese, zinc, and copper, albeit to a lesser extent.
Manganese, like iron, is required for many essential enzymes in bacteria. Given its importance for bacterial growth, the host has also devised strategies to restrict bacterial invaders from acquiring manganese. In phagosomes, the divalent-metal ion transporter NRAMP1 is an integral membrane protein that depletes manganese, iron, and other essential metals from the phagosomal compartment to effectively starve the engulfed bacteria. Genetic mutations in the gene encoding NRAMP1 are associated with susceptibility to tuberculosis, leprosy, inflammatory bowel disease, and autoimmune diseases.
Calprotectin is an abundant cytoplasmic protein that is constitutively expressed in neutrophils, but it can also be expressed in other cell types during infection. Extracellular concentrations of calprotectin can exceed 1 mg/ml at sites of infection. Calprotectin acts as a dimer of two proteins (S100A8 and S100A9) that bind and sequester the essential nutrients manganese and zinc with high affinity. The sequestration of these metal ions by the host inhibits pathogen growth and can render the invading bacteria more sensitive to the oxidative burst produced by phagocytic cells. Calcium binding enhances the affinity of calprotectin for manganese and zinc. This suggests that calcium functions as a molecular switch, keeping the protein in a low affinity state within the host cell cytoplasm, where calcium concentrations are very low, but allowing calprotectin to convert into a high affinity state in the calcium-replete extracellular medium, where it can chelate and sequester manganese and zinc once it is mobilized and released from the cells.
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