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Bacteria and the bacterial cell wall Lecture 2 0 Aims of the Lecture • • • Review the molecules and structures of the cell envelope of bacterial cells Describe the roles played by the major components. Discuss briefly the features and structures of the cell well that are targets for current antibacterial drugs. Bacterial morphology The size, size shape and arrangement of bacteria, bacteria and other microbes, microbes is the result of their genes and thus is a defining characteristic called morphology. The most common bacterial shapes are the rod (bacilli) and spherical (cocci) shaped bacteria. 1 1 Enterococcus sp. A coccoid bacterium SEM x27,235 © Dennis Kunkel (www.denniskunkel.com) 2 Bacillus anthracis A rod-shaped bacterium which causes anthrax SEM x24,200 © Dennis Kunkel (www.denniskunkel.com) 3 2 Bordetella holmesii Coccobacillus bacterium: causes respiratory infections SEM x35,200 , © Dennis Kunkel (www.denniskunkel.com) 4 Spirulina sp. A filamentous cyanobacterium b t i SEM x2,080 © Dennis Kunkel (www.denniskunkel.com) 5 3 Vibrio cholerae A “curved rod” (vibrio) shaped prokaryote SEM x15,575 © Dennis Kunkel (www.denniskunkel.com) 6 Staphylococcus aureus A coccoid b t i bacterium SEM x23,000 © Dennis Kunkel (www.denniskunkel.com) 7 4 Group Association • Group association describes how microbial cells, in either liquid or on solid medium, are arranged in relation to one another. • They are helpful in classification (identification). • Bacteria may exist mainly as single cells or as common grouping such as chains (e.g. streptococci) or clusters of cells (e.g. staphylococci), or uneven clusters, pairs, tetrads, octads etc. • They may exist as masses embedded within a large capsule. • There are square bacteria bacteria, star-shaped bacteria bacteria, stalked bacteria bacteria, budding bacteria that grow in net-like arrangements and many other morphologies. • These range of morphologies are also found in fungi ranging from the single cell yeasts to the filamentous moulds and dermatophytes. 8 Bacteria and the Gram Stain • Bacteria are prokaryotic (i.e. no membrane enclosing a nucleus) • They have a cell wall in addition to cell membrane (together making the cell envelope). envelope) • Many bacteria can be assigned to one of two major groups, based on the Gram-staining reaction. The differences in cell-wall structure and composition account for the differential Gram reaction. • The cell wall is both a target for antibacterial drugs and a defence against antibacterial drugs, i.e. a resistance factor. 9 5 Cell Wall Ultrastructure Differences in cell-wall structure and composition account for the differential Gram staining process. We need to know something about the composition and organisation ("ultrastructure") of the cell wall to understand why some antibacterial drugs work and others do not.. 10 Cell Wall Structure • A complex cell wall is present in prokaryotes (and not eukaryotic mammalian cells). • The outer membrane in GNB is more complex than the inner cytoplasmic membrane and presents an extra barrier that drugs must to pass through to get into bacterial cells. GNB are therefore often resistant to drugs that GPB are susceptible to. 11 6 Cell Wall Components Gram-positive • Major component (~50%) is peptidoglycan • No N lilipid id and d often ft no protein t i presentt • Accessory polymers (teichoic acid and/or teichuronic acid) are covalently linked to peptidoglycan • Relatively thick and featureless (under electron microscope) Gram-negative • Cell envelope consists of a pair of membranes (cytoplasmic and outer) with a thin, intermediate layer of peptidoglycan • Outer membrane contains lipopolysaccharide (LPS) as well as lipids and proteins. • The protruding polysaccharide in LPS makes the bacteria appear fuzzy under an electron microscope. 12 Peptidoglycan: a polysaccharide core Lysozymes are glycosidases specific to the bond from MurNAc to GlcNAc. They are commonly found in egg white and present in some secretions (tears) as part of the body's defence. GPB are attacked and killed directly, but GNB are usually resistant (outer membrane prevents access to the peptidoglycan). 13 7 Peptidoglycan: glycan chain cross-linked with peptides Autolysins are glycosidases (for bond from MurNAc to GlcNAc), amidases (for bond from MurNAc to the peptide) and peptidases (breaking bonds in the peptide or the bridge) which are produced by bacteria to “remodel” the cell wall as the bacteria 14 grows and divides. The Peptide Side Chain 15 8 Biosynthesis of peptidoglycan 16 17 9 18 The Outer Membrane in GNB Lipoproteins anchor the outer membrane to the peptidoglycan (not all of them are attached). The lipopolysaccharide is important for the functions and properties of the outer membrane. It is essential for structural integrity and viability of the 19 bacteria and makes the outer membrane more rigid. 10 Lipopolysaccharide LPS contributes to the structural and functional properties of the OM and is present in almost all GNB. LPS is an endotoxin [cell-bound]. The Lipid A portion of LPS is responsible for the endotoxic activities. Free, monomeric LPS, is released from the OM naturally or through antibiotic action and interacts with receptors on macrophages and other blood cells, causing the release of inflammatory agents. 20 11