AP0404 Bacterial Cell Structure and Function 2024 PDF

AP0404 Bacterial Cell Structure and Function. Dr Vartul Sangal Review: Features of bacteria PROKARYOTES: no nucleus. Organisation: typically unicellular. Diverse metabolism: Heterotrophs/photoautotrophs aerobes/anaerobes. Cell Size: typically ~1-2 m. Internal stru...

AP0404 Bacterial Cell Structure and Function. Dr Vartul Sangal Review: Features of bacteria PROKARYOTES: no nucleus. Organisation: typically unicellular. Diverse metabolism: Heterotrophs/photoautotrophs aerobes/anaerobes. Cell Size: typically ~1-2 m. Internal structure: no membrane-bound organelles. In Eukaryotic cells: organelles are often surrounded by a membrane similar in structure to the cell membrane but with a different composition of protein and phospholipid. General features Cell Shape (aka Morphology) - limited range two most common: spheres rods 1) Spherical Cells cocci (pl.), coccus (sing.) – from Greek - ‘berry’ may aggregate: – chains - ‘streptococci’ – clumps - ‘staphylococci’ – Pairs - diplococci Neisseriae 1) Spherical Cells e.g. Streptococcus pneumoniae 1) Spherical Cells e.g. Staphylococcus aureus 1) Spherical Cells: Division planes and cell arrangements https://www2.nau.edu/~fpm/bio205/morphol-2.html 2) Rods bacilli (plural), bacillus (sing.) – from Latin - ‘stick’ – hence Bacillus genus e.g. Bacillus anthracis But… many other rods exist - so use ROD (not bacillus)  den Blaauwen, 2018 E. coli cells divide perpendicularly whereas Candidatus Thiosymbion oneisti and Candidatus Thiosymbion hypermnestrae cells divide longitudinally. Structures of the bacterial cell Capsule or Mucilage layer ▪ Also called Glycocalyx ▪ Not essential for cell viability ▪ Found in some bacteria ▪ Outside cell wall ▪ When well define -> capsule ▪ When less defined ->Slime layer Structure of capsule or mucilage layer Usually, a single polysaccharide Polysaccharide type can help identify species or strain  Functions of capsule/mucilage 1. ADHERENCE (capsule is sticky) - ‘biofilms’ 2. PROTECTION against water loss 3. PROTECTION against phagocytes (white cells) in pathogenic bacteria 4. PROTECTION against chemicals e.g. disinfectants  Bacterial Cell Wall Structure: major component PEPTIDOGLYCAN or murein complex polymer made of……… think of the name Long (s)GLYCAN chains with repeating sub-units of: (i) N-acetylglucosamine and (ii) N-acetylmuramic acids cross-linked by short PEPTIDE chains  Bacterial Cell Wall major structural component: PEPTIDOGLYCAN The peptidoglycan monomers M G M G are synthesized in the cytosol of the bacterium where they L aa attach to a membrane carrier molecule called bactoprenol. long glycan chains D aa D aa L aa The bactoprenols transport the tetrapeptide L aa D aa peptidoglycan monomers side-chains D aa interpeptide across the cytoplasmic membrane and work with L aa cross-bridge other enzymes to insert the monomers into existing M G M peptidoglycan enabling bacterial growth after the binary fission. aa = amino acid PEPTIDOGLYCAN UNIQUE polymer (unique polysaccharide & peptide components) forms a single molecule surrounding cell very strong, yet permeable target for anti-bacteria attack: penicillin lysozyme Two major types of cell wall Staining reaction Hans Christian Gram (1884) uses two different dyes: bacteria retain only one type of dye due to major differences in cell wall structure (electron microscopy) two groups termed ……………… GRAM-POSITIVE and GRAM-NEGATIVE 1. Crystal violet as the primary stain. 2. Fixing the dye using iodine: to form crystal violet- iodine complex which prevents easy removal of dye. 3. A decolourizer solvent (ethanol) is used to remove the dye. All bacteria will take up crystal violet dye but: I. Gram-positive microorganisms with higher peptidoglycan content: alcohol will dehydrates cell walls with the closure of pores preventing diffusion of violet-iodine complex, and cells will remain purple-blue. II. Gram-negative organisms with higher lipid content: solvent will dissolve the lipid layer and cells will lose the primary stain. The length of decolorization is critical as prolonged exposure can remove all the stains from both types of cells. 4. Use of counterstain (safranin) to give decolorized Gram- negative bacteria pink colour for easier identification. Gram-negative rods E. coli Gram-positive cocci Streptococcus faecalis  The bacterial envelope (wall + membranes) Gram-positive bacteria relatively thick cell wall (>20 nm) high internal osmotic pressure (turgid) high peptidoglycan content (>50%) wall contains other polymers (teichoic acids) typically sensitive to lysozyme and penicillin no further layers outside the cell wall Surface polymers such as teichoic acids play important roles in cell shape determination, regulation of cell division, pathogenesis and antibiotic resistance.  The bacterial envelope (wall + membranes): Gram-negative bacteria relatively thin cell wall (