Bacteria cell structure

• Microscopy


  • mm (micron) = 1/1000mm or 10-6 m
  • hm (nanometre) = 1/1000000mm or10-9 m

  
• Light microscope


  • Max. magnification ~ 1000X
  • Provides information about size, shape, staining properties

  
• Electron microscope


  • Uses e- in vacuum instead of light & magnets instead of lenses
  • Transmission electron microscope resolving power ca. 1 hm (viruses 10-200 hm)
  • Scanning EM - resolving power ~ 10 hm but get 3D picture of cells
• Confocal Microscopy


  • Visualisation of biofilms or planktonic cultures
  • When combined with software, biofilm parameters can be determined
  • Stains allow determination of viable (Green) and non-viable (Red) cells

  
• Staining


  • Bacteria are basophillic (have a negative charge) at physiological pH
  • Stain readily with basic dyes (positive charged) e.g. crystal violet, methylene blue
• Gram stain


  1. Stain with aqueous soln of crystal violet

  2. Apply iodine soln - CV-I2 complex in cell wall

  3. Differential decolourisation with organic solvent (ethanol) - Gram positive bacteria do not decolourise readily, Gram negative bacteria decolourise

  4. Counterstain with dye of contrasting colour - safranin
• S. aureus


  • Gram positive
• E. coli


  • Gram negative
• Bacterial Cell Sizes


  • Cocci (spherical cells ~ 0.5-2 mm)
  • Bacilli (cylindrical cells 3-10 mm x 1 mm)
  • Long & slender (filamentous) - taper at both ends (fusiform)
  • Short & thick (cocco-bacilli)
  • Some spiral shaped
  • Viruses - picacornaviruses (0.03 mm) - poxvirus (~0.3 mm)
  • Pathogenic fungi – yeasts (eukaryotes) - (2-5 mm) Candida albicans

  
• Colonial Morphology


  • Colony - arises from single cell when numbers reach ca. 106
  • Grown in or on surface - medium solidified with agar
  • Described by size, shape, pigmentation, transparency topography etc.
• Capsules (or slime layers)

  • Produced outside Cell Wall
  • Vary greatly in thickness and adherence - loose = slime layer
  • Usually polysaccharides
  • Related to virulence
  • Streptococcus mutans produces a capsule in the presence of sucrose
• Flagellae


  • Provide random or coordinated motion - chemotaxis
• Fimbrae (Pili)


  • 100's hair like appendages/cell
  • Thinner, shorter, less rigid than flagella
  • Involved in surface attachment
  • Specialized - transfer of DNA to "female cell" - allows genetic variation in population
• Peptidoglycan


  • Different cell wall structures exist between Gram +ive and Gram -ive
  • Biosynthesis is complex
  • Enzymes & antibiotics can interfere - cell death
  • Penicillin (suicide inhibitor) inhibits X linking peptide chains by transpeptidase
• Peptidoglycan (murein)


  • Backbone made of glycosaminoglycans (NAM & NAG) joined by b 1-4) linkage
  • + tetrapeptide cross linked to NAM
  • Peptide X bridges- covalent bonding
  • Lysozyme attacks b 1-4 linkage between NAG and NAM
• Gram +ive Cell Wall


  • Teichoic acids - linear polymers of either glycerol or ribitol (10-50 units)
  • Teichoic acids - linked by phosphodiester bonds to NAM of peptidoglycan
  • Lipoteichoic acid - linked to cell membrane glycolipid
  • Function - involved in cells -ive charge & therefore attracts & allows passage of cations (ie. Positively charged ions)
• Gram -ive Cell Wall


  • Thinner but more complex than Gram +ive
  • 3 components outside PG: Lipoprotein (LP), Outer Membrane (OM), Lipopolysaccharide (LPS)
• Lipopolysaccharide


  • Lipid A - attached to OM via hydrophobic bonds, Fatty acid varies with species
  • Polysaccharide Core - present in all Gram -ive, major cell surface antigen (O antigen) - determined by terminal repeat units
  • LPS Lipid A is toxic (endotoxin) - released upon cell lysis and may invoke an acute inflammatory response
• Cytoplasmic Structures


  • Ribosomes - protein synthesis - ~12,000/E. coli cell - polysomes
  • Nuclear Body - discrete areas of nuclear material
• Granules


  • Cellular storage materials e.g. Intracellular polysaccharide
• Endospores


  • 3 Gram +ive genera capable of forming spores; most common are Bacillus (aerobic rods) and Clostridium (anaerobic rods)
  • Resistant to heat, desiccation & chemical agents
  • Under favorable conditions spore germinates into vegetative cell