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Category: Immunology; Clinical Microbiology
Overview of Bacterial Pathogens, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555816872/9781555815141_Chap12-1.gif /docserver/preview/fulltext/10.1128/9781555816872/9781555815141_Chap12-2.gifAbstract:
Pathogenic bacteria are capable of inflicting damage to the infected host, thereby causing disease. Importantly, the different body habitats have distinct physical-chemical properties as to pH and abundance of molecular oxygen and water, which allow the presence and growth of specific types of bacteria. Treponema pallidum subspecies pallidum, the bacterium which causes syphilis and Streptococcus pyogenes (Group A streptococci), which is associated with suppurative skin and mucosal infections, necrotizing fascitis, scarlet fever, and sequelae such as glomerulonephritis and rheumatic fever, are examples of extracellular obligate human pathogens. Most virulence factors only act locally at the site of infection, whilst others, are also transported to distal parts of the body, where they subvert host cell functions. Importantly, bacteria growing in biofilms are able to partially evade the immune response and are resistant to antibiotic treatment. The causes and mechanisms of genetic variation account for the evolution of both commensal and pathogenic bacteria. The difference lies in the fact that in pathogens virulence factors are acquired. However, sometimes it is not easy to distinguish a true virulence factor from a feature that just increases the bacterial fitness without really harming the host.
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The gram-negative cell wall. In gram-negative microorganisms the cell wall is composed of two lipid membranes, the inner (IM) and the outer membrane (OM). The two membranes are separated by a peptidoglycan (PG) layer, the periplasm. The exterior leaflet of the outer membrane contains lipopolysaccharide (LPS) and porins, which mediate the passage of solutes (e.g., ions and sugars), from the extracellular milieu to the periplasm. Membrane and membrane-associated proteins are present in both the inner and the outer membrane. In pathogenic gram-negative bacteria, virulence determinants such as secretion systems are associated with the cell wall, as exemplified by the Type 3 secretion system (T3SS). The basis of the T3SS spans the entire cell wall, whereas the needle protrudes into the extracellular milieu and is able to form a connection with the host cell plasma membrane. This architecture allows the translocation of effectors across the cell wall from the bacterial cytoplasm to the host cell.
The gram-positive cell wall. In gram-positive microorganisms, the cell wall is composed of a lipid membrane that is surrounded by a thick peptidoglycan (PG) layer. The peptidoglycan layer is interwoven with teichoic and lipoteichoic acid, which is bound to the lipid membrane. Proteins are both associated with the lipid membrane and with the peptidoglycan layer. In pathogenic gram-positive bacteria, virulence factors such as adhesins are found in the cell wall, as exemplified by the presence of a pilus. Pili are covalently linked to peptidoglycan and extend into the extracellular milieu.
The main virulence mechanisms that allow the establishment of an infection are illustrated. Please note that the list of examples is not meant to be exhaustive.