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PspA, autolysin, and PsaA have all been shown to elicit protective immune responses in mice ( 12, 22, 23). PspA may also interfere with complement fixation ( 19, 20) and may play a role in colonization ( 21). C3 protease is able to inactivate native C3 in serum. pneumoniae has pneumococcal surface protein A (PspA) ( 8, 9), pneumococcal surface protein C (PspC) ( 10, 11), pneumococcal surface adhesin A (PsaA) ( 12), autolysin ( 13), immunoglobulin A1 (IgA1) protease ( 14, 15), C3 protease ( 16), and other less well-characterized proteins ( 17, 18). The cell wall and outer layer of gram-positive bacteria are the locations of a number of cell-surface proteins involved in a variety of functions, including, but not limited to, adherence, enzyme activity against host substrates, nutrient transport, and interference with complement deposition. They are thought to differ only in their mode of attachment to the bacterial surface ( 7). In pneumococci, the overall PS structures of lipoteichoic acid (also called F-antigen) and cell wall teichoic acid are very similar. In gram-positive bacteria, teichoic acid is often linked to lipid molecules, and this is then called lipoteichoic acids, which is attached to the cytoplasmic membrane and extends out through the cell wall ( 6). In the pneumococcus, the teichoic acid is C-polysaccharide (C-PS), which is relatively invariant among pneumococcal strains ( 4, 5). Cell wall PS often has a phosphate group in the repeating unit, and it is then called a teichoic acid. The cell wall PS of gram-positive bacteria varies among different bacteria, and the structure of the cell wall PS has been used to distinguish many different species of streptococci (i.e., groups A, B, C, and so forth) ( 2, 3). In addition to peptidoglycan, many gram-positive bacteria have PS associated with their cell walls, and this cell wall PS often extends into the capsular area. The thick cell wall of the gram-positive bacteria may be responsible for their resistance to complement-mediated lysis. The cell walls help protect the bacteria from the extremes of the environment (especially differences in osmolarity). 2- to 4-nm) cell wall layer and can thus retain the Gram’s stain better. In comparison with gram-negative bacteria, gram-positive organisms have a thicker (20- to 30-nm vs. The peptidoglycan polymerization is performed by enzymes, many of which are also referred to as penicillin-binding proteins. The cell wall surrounds the cytoplasmic membrane and is made of peptidoglycan, which is a highly cross-linked polymer of amino sugars ( N-acetyl glucosamine and muramic acid) and amino acids. Many membrane proteins are transport proteins, and many physical connections between the cell wall and the membrane can be visualized by electron microscopy.Ī cell wall is found in all of the pathogenic bacteria of both tribes, with the exception of Mollicute organisms (which include mycoplasmas). The lipid bilayer is composed mainly of phospholipid and does not contain sterols. This membrane is an osmotic barrier and forms a barrier for most molecules. The inset in B shows lipopolysaccharide anchored to the outer leaflet of the outer membrane.Īll bacteria have a cytoplasmic membrane, which is a phospholipid bilayer containing various proteins. pneumoniae is often called C-polysaccharide. Schematic representation of the surfaces of Streptococcus pneumoniae ( A) and Neisseria meningitidis ( B) as examples of gram-positive and gram-negative bacteria, respectively. It is also important to note that the capsule, O-antigens, and cell wall are not contiguous shields but are open enough to be permeable to secreted products and nutrients, as well as some immunological factors (e.g., antibodies and complement) ( Fig. Thus, there are molecules other than peptidoglycan and capsular polysaccharide (PS) in the cell wall and the outer layer, respectively. Molecules anchored in the cytoplasmic membrane or cell wall may extend into or through other layers. Although these layers are described later in detail, it is important to note that these definitions are operational and that, in reality, the layers are not entirely distinct. Three layers are commonly recognized: cytoplasmic membrane, cell wall, and outer layer. To illustrate the surface of bacteria in the two tribes, the surface structures of Streptococcus pneumoniae and Neisseria meningitidis are shown in Fig. Moon H. Nahm, Michael A. Apicella, David E. BrilesĮxtracellular, as well as intracellular, pathogenic bacteria can be divided into two major tribes (gram-negative and gram-positive) on the basis of their staining characteristics with Gram’s stain. Chapter 41 Immunity to Extracellular BacteriaīACTERIAL SURFACE STRUCTURE OF GRAM-POSITIVE AND GRAM-NEGATIVE BACTERIA
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