Microbial pathogenesis

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  • Harrison's Internal Medicine Chapter 114. Molecular Mechanisms of Microbial Pathogenesis Molecular Mechanisms of Microbial Pathogenesis: Introduction Over the past three decades, molecular studies of the pathogenesis of microorganisms have yielded an explosion of information about the various microbial and host molecules that contribute to the processes of infection and disease.

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  • Numerous virus–target cell interactions have been described, and it is now clear that different viruses can use similar host-cell receptors for entry. The list of certain and likely host receptors for viral pathogens is long. Among the host membrane components that can serve as receptors for viruses are sialic acids, gangliosides, glycosaminoglycans, integrins and other members of the immunoglobulin superfamily, histocompatibility antigens, and regulators and receptors for complement components.

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  • GPI-anchored receptors do not have intracellular signaling domains. Instead, the mammalian Toll-like receptors (TLRs) transduce signals for cellular activation due to LPS binding. It has recently been shown that binding of microbial factors to TLRs to activate signal transduction occurs not on the cell surface, but rather in the phagosome of cells that have internalized the microbe. This interaction is probably due to the release of the microbial surface factor from the cell in the environment of the phagosome, where the liberated factor can bind to its cognate TLRs.

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  • Table 114-1 Examples of Microbial Ligand-Receptor Interactions Microorganism Type of Microbial Ligand Host Receptor Viral Pathogens Influenza virus Hemagglutinin Sialic acid Measles virus Vaccine strain Hemagglutinin CD46/moesin Wild-type strains Hemagglutinin Signaling lymphocytic activation molecule (SLAM) Human herpesvirus type 6 ? CD46 Herpes simplex virus Glycoprotein C Heparan sulfate HIV Surface glycoprotein CD4 and chemokine receptors CXCR4) (CCR5 and Epstein-Barr virus Envelope protein CD21 (=CR2) Adenovirus Fiber protein Coxsackie-adenovirus recepto...

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  • Encounters with Phagocytes Phagocytosis and Inflammation Phagocytosis of microbes is a major innate host defense that limits the growth and spread of pathogens. Phagocytes appear rapidly at sites of infection in conjunction with the initiation of inflammation. Ingestion of microbes by both tissue-fixed macrophages and migrating phagocytes probably accounts for the limited ability of most microbial agents to cause disease.

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  • Encounters with Epithelial Cells Over the past decade, many bacterial pathogens have been shown to enter epithelial cells (Fig. 114-2); the bacteria often use specialized surface structures that bind to receptors, with consequent internalization. However, the exact role and the importance of this process in infection and disease are not well defined for most of these pathogens. Bacterial entry into host epithelial cells is seen as a means for dissemination to adjacent or deeper tissues or as a route to sanctuary to avoid ingestion and killing by professional phagocytes.

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  • Viral Adhesins (See also Chap. 161) All viral pathogens must bind to host cells, enter them, and replicate within them. Viral coat proteins serve as the ligands for cellular entry, and more than one ligand-receptor interaction may be needed; for example, HIV uses its envelope glycoprotein (gp) 120 to enter host cells by binding to both CD4 and one of two receptors for chemokines (designated CCR5 and CXCR4). Similarly, the measles virus H glycoprotein binds to both CD46 and the membrane-organizing protein moesin on host cells.

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  • Invasion Many diseases are caused primarily by pathogens growing in tissue sites that are normally sterile. Pneumococcal pneumonia is mostly attributable to the growth of S. pneumoniae in the lung and the attendant host inflammatory response, although specific factors that enhance this process (e.g., pneumolysin) may be responsible for some of the pathogenic potential of the pneumococcus. Disease that follows bacteremia and invasion of the meninges by meningitisproducing bacteria such as N. meningitidis, H. influenzae, E.

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  • Transmission to New Hosts As part of the pathogenic process, most microbes are shed from the host, often in a form infectious for susceptible individuals. However, the rate of transmissibility may not necessarily be high, even if the disease is severe in the infected individual, as transmissibility and virulence are not linked traits.

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  • Flagella are long appendages attached at either one or both ends of the bacterial cell (polar flagella) or distributed over the entire cell surface (peritrichous flagella). Flagella, like pili, are composed of a polymerized or aggregated basic protein. In flagella, the protein subunits form a tight helical structure and vary serologically with the species. Spirochetes such as T. pallidum and Borrelia burgdorferi have axial filaments similar to flagella running down the long axis of the center of the cell, and they "swim" by rotation around these filaments.

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  • Over the past three decades, molecular studies of the pathogenesis of microorganisms have yielded an explosion of information about the various microbial and host molecules that contribute to the processes of infection and disease. These processes can be classified into several stages: microbial encounter with and entry into the host; microbial growth after entry; avoidance of innate host defenses; tissue invasion and tropism; tissue damage; and transmission to new hosts.

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  • Harrison's Internal Medicine Chapter 132. Infections Caused by Listeria monocytogenes Infections Caused by Listeria monocytogenes: Introduction Listeria monocytogenes is a food-borne pathogen that can cause serious infections, particularly in pregnant women and immunocompromised individuals. A ubiquitous saprophytic environmental bacterium, L. monocytogenes is also a pathogen with a broad host range. Humans are probably accidental hosts for this microorganism. L.

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  • The progression of disease in localized osteomyelitis is characterized by a cycle of microbial invasion, vascular disruption, necrosis and sequestration. The host inflammatory response, discussed in detail below, results in obstruction of small vessels due to coagulopathy and oedema. As a result of this, cortical bone undergoes necrosis and is detached from surrounding live bone, creating an area known as a sequestrum. This provides a fertile environment for further bacterial invasion and progression continues.

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  • The direct effect of probiotics in the lumen include the competition with pathogens for nutrients, production of antimicrobial substances and in particular organic acids competitive inhibition on the receptor sites, change in the composition of mucin hydrolysis of toxins, receptor hydrolysis, and nitric oxide (NO). The indirect effect of probiotics largely depends on the site of interaction between the probiotic and the effectors of the immune response. Thantasha et al.

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  • The AIDS epidemic has resoundingly illustrated this principle: the immunodeficiency of many HIV-infected patients permits the development of life-threatening fungal infections of the lung, blood, and brain. Other than the capsule of C. neoformans, specific fungal antigens involved in tissue invasion are not well characterized. Both fungal and protozoal pathogens undergo morphologic changes to spread within a host. Yeast-cell forms of C. albicans transform into hyphal forms when invading deeper tissues.

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  • Listeria monocytogenes is a food-borne pathogen that can cause serious infections, particularly in pregnant women and immunocompromised individuals. A ubiquitous saprophytic environmental bacterium, L. monocytogenes is also a pathogen with a broad host range. Humans are probably accidental hosts for this microorganism. L. monocytogenes is of interest not only to clinicians but also to basic scientists as a model intracellular pathogen that is used to study basic mechanisms of microbial pathogenesis and host immunity. ...

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  • Propaganda; Australasian Medical Congresses. Committee appointed; Auckland Congress, 1914, Report presented, Nature of Notification recommended; Melbourne Conference, 1922, Review of Legislation, Comments and Recommendations; England, Committee recently appointed to report on Venereal Diseases 5 Section 5.

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  • Pathogenesis of Fever Pyrogens The term pyrogen is used to describe any substance that causes fever. Exogenous pyrogens are derived from outside the patient; most are microbial products, microbial toxins, or whole microorganisms. The classic example of an exogenous pyrogen is the lipopolysaccharide (endotoxin) produced by all gramnegative bacteria. Pyrogenic products of gram-positive organisms include the enterotoxins of Staphylococcus aureus and the group A and B streptococcal toxins, also called superantigens.

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  • Acute Sinusitis Just as the pathogenesis and microbial etiology of acute rhinosinusitis are similar to those of otitis media, so are the principles of diagnosis and treatment. The diagnosis is often empirical, and the less rigorously it is made, the more irrelevant antibiotics are likely to be.

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