BACTERIA CELL STRUCTURE

• Bacteria


  • Omnipresent and essential for the existence of all life on Earth
  • Beneficial and pathogenic
  • Very small (invisible to the naked eye)
• Bacterial cell structure


  • Diverse morphologies and arrangements
  • Sizes range from 1 µm to 5 µm (true range: 0.1 µm – 2.0 cm)

  
• Largest bacterium known


  • Thiomargarita magnifica (2.0 cm long), found in rotting leaves in the mangroves of Guadeloupe (Caribbean)

  
• Bacteria in the tip of a pencil

  • Viewed under scanner electron microscopy (SEM)

  
• Prokaryotes


  Unicellular organisms that lack membranous organelles and true nucleus
• Components of bacterial cell structure


  • Cytoplasm
  • Ribosomes
  • Nucleoid (DNA)
  • Plasma membrane
  • Complex and rigid cell wall
• Additional bacterial cell structures

  • Capsule
  • Flagella
  • Pili/fimbriae
  • Plasmids
  • Endospores

  
• Cytoplasm


  Jelly-like aqueous solution (cytosol) that facilitates major chemical reactions of the cell
• Nucleoid (DNA)


  The bacterial chromosome, mostly single and circular, containing the essential genes for the life of the bacteria
• Plasmids


  Circular DNA molecules that contain additional genes, not essential, providing virulence factors and antibiotic resistance
• Ribosomes


  The cellular protein factory, composed of proteins and rRNA, with two subunits (large 50S and small 30S)
• Components of the bacterial envelope

  • Cytoplasmic or plasma membrane
  • Membrane-associated proteins
  • Cell wall (peptidoglycan layer)
  • Outer membrane (in Gram-negative bacteria)
• Cytoplasmic membrane


  Flexible structure composed of phospholipids and proteins, with hydrophilic outer faces and hydrophobic interior
• Membrane-associated proteins


  • Stabilize the membrane, transport molecules, enable electron transport for bacterial respiration, and act as enzymes
• Cell wall (peptidoglycan layer)

  Provides protection against mechanical damage and osmotic lysis, and enables non-selective transport
• Gram-staining technique


  Allows the characterization of the two main groups of bacteria: Gram-positive (purple) and Gram-negative (pink)

  
• Outer membrane


  Only present in Gram-negative bacteria, containing lipopolysaccharide (endotoxin), porins, and transporter proteins
• Mycobacteria


  • Bacterial envelope contains mycolic acids, thick waxy hydrophobic cell wall, and are acid-fast staining

  
• Mollicutes (Mycoplasma)


  • No cell wall, highly pleomorphic and osmotically unstable, the smallest bacteria described

  
• Capsule


  Glycocalyx, an extracellular polymeric matrix that provides protection, cell adherence, and nutrient reserve
• Flagella


  Anchored to the bacterial cell envelope, multi-protein structure that enables locomotion or bacterial motility

  
• Pili/fimbriae


  Fine, straight, hair-like appendages that act as "adhesins", contributing to adhesion to host tissues and antigenicity
• Endospores


  Cryptobiotic state of dormancy and most durable type of cell, produced by some Gram-positive bacteria to ensure survival during adverse environmental conditions