Bacterial structure

• Bacterial anatomy


  • Cell
  • Cell wall / outer membrane (if present)
  • Cytoplasmic membrane
  • Flagellum
  • Granular inclusion
  • Ribosomes
  • Cytoplasm
  • Capsule
  • Nucleoid
  • Mesosome
  • Pili
  • Capsule/Glycocalyx/Slime layer
• Capsule/Glycocalyx/Slime layer


  • Gelatinous layer covering the entire bacterium
  • Polysacharride Structures surrounding the outside of the cell envelope
  • Exception: The capsule of Bacillus anthracis is composed of polymerized D-glutamic acid
  • Helps in the formation of biofilms on inert surfaces such as catheters, teeth and heart valves
• Significance of capsules


  • Inhibits ingestion and killing by phagocytes
  • Protects the cells from lysozyme
  • Growth in a biofilm prevents access of host cells or antibiotics
  • Prevent cell from drying out
  • Used as antigens in certain vaccines e.g pneumococcal vaccine
• Cell wall


  • Main component is peptidoglycan (murein)
  • The thickness of peptidoglycan distinguishes gram positive from gram negative bacterial
  • Overlapping N-acetyl glucosamine (NAG) & N-acetyl muramic acid (NAM)
  • Present in almost all bacteria, except Mycoplasma and ureaplasma
• Cell wall of gram positive bacteria


  • Thick peptidoglycan layer (50-90% of cell wall material)
  • Teichoic acids: wall teichoic acid and lipoteichoic
  • Lipoteichoic acid
  • Polysaccharides and proteins
• Teichoic acids


  • Control of cell division, cell morphology, and contribute to cellular adhesion
• Lipoteichoic acid


  • Anchor cell wall to cell membrane
  • For epithelial cell adhesion
• Polysaccharides and proteins


  • Protect peptidoglycan layer from action of agents such enzymes
  • Promote colonization by sticking the bacteria to the surface of host cells
• Cell wall of Gram negative bacteria


  • Thin peptidoglycan layer comprising 5-10% of cell wall material
  • Have an additional outer membrane
  • Have a periplasmic space- contains digestive enzymes and other transport proteins
  • Contains porin protein - specifically allow transport of solutes in and out of the cell
• Cell wall of Gram negative bacteria (cont.)

  • Lipoprotein anchors the outer membrane to peptidoglycan
  • Outer membrane protects the cell from proteolytic enzymes
  • Lipopolysaccharide (LPS)
• Lipopolysaccharide (LPS)


  • Present in almost all gram -ve bacilli
  • Major component is Lipid A- endotoxin (responsible for endotoxic activities - fever, hypotension, septicemia)
• Acid-fast bacteria


  • The cell envelopes of Mycobacteria are more complex than other bacterial
  • Composed of Mycolic acid (thick waxy membranous layer outside the peptidoglycan layer)
• Significance of cell wall

  • Maintains cell shape
  • Protects bacteria from osmotic lysis
  • Determines reactivity to Gram stain
  • Site of action of certain antimicrobial agents (E.g. Penicillins, Cephalosporins)
  • Enhances pathogenicity
• Plasma/Cytoplasmic/Cell membrane


  • Separates cell wall from cytoplasm
  • Acts as a semi-permeable membrane
  • Composed of lipoproteins with small amounts of carbohydrates
  • Generally do not contain sterols (except for Mycoplasma)
• Functions of plasma/cytoplasmic/cell membrane


  • Active transport of molecules into the cell
  • Synthesis of precursors of the cell wall
  • Secretes enzymes and toxins
• Pili


  • Hair-like projections on the surface of the cell
  • Shorter and straighter than flagella
  • Composed of protein – pilin
  • Mostly on gram negative bacteria
• Types of pili


  • Fimbriae/ Common pili - cover the cell surface, for attachment
  • Sex pili - longer than common pili, involved in conjugation
• Flagella


  • Long, filamentous surface appendages
  • For bacterial motility
  • Composed of the protein 'flagellin'
  • May serve as antigenic determinants (e.g. the H antigens of Gram-negative enteric bacteria)
• Ribosomes


  • They are composed of RNA and proteins
  • Site of protein synthesis
  • Site of activity of antimicrobials that disrupt protein synthesis
  • 70S in size with 50S and 30S subunits
• Ribosome (70S) is composed of a large subunit (50S) and a small subunit (30S)
• Nucleoid


  • Area of cytoplasm in which DNA is located
  • Bacterial DNA consists of a single, circular double-stranded DNA
  • Lacks nuclear membrane (called nucleoid)
  • Contains genetic material that codes for all genetic information expressed by the cell
• Plasmids


  • Extrachromosomal DNA molecules
  • Easily passed from bacterium to bacterium through sex pili
  • Free or integrated into the chromosome
  • May encode genes of antibiotic resistance and pathogenesis factors (e.g. enzymes and toxins)
• Inclusion/nutrient granules


  • Composed of volutin, lipid and polysaccharide
  • Stain characteristically with certain dyes
  • Example; Volutin granules are seen in Corynebacterium spp
  • Serve as storage area for nutrients and energy for cell metabolism
• Mesosomes


  • Appear as convoluted indentations (invaginations) in the cytoplasmic membrane
  • Are sites of respiratory enzyme activity
  • Coordinate nuclear & cytoplasmic division during binary fission
• Spores


  • Round, oval, or elongated structures
  • Formed inside the parent cell (Endospores)
  • Formed when conditions for vegetative growth are not favourable
  • They exhibit no metabolic activity
  • Resistant to heat, radiation and drying and can remain dormant for hundreds of years
  • Formed by bacteria like Clostridia, Bacillus
• Bacterial physiology


  Study of how bacteria function including such processes as nutrition, growth, reproduction and locomotion
• Bacterial reproduction


  1. Cell replicates its DNA

  2. Nucleoid- The cytoplasmic membrane elongates, separating DNA molecules

  3. Cross wall for membrane invaginates

  4. Cross wall forms completely

  5. Daughter cells
• Temperature classification


  • Psychrophiles - low temp 10-20 °C
  • Mesophiles - 20-40°C
  • Thermophiles – temp. > 40°C
• Bacterial oxygen use


  • Strict (obligate) aerobes - require O, for growth e.g. Pseudomonas aeruginosa
  • Strict (obligate) anaerobes - grow in the absence of O, e.g. Bacteroides fragilis
  • Facultative anaerobes - do not require O₂ for growth but grow better in its presence e.g. Staphylococcus species
  • Capnophilic - thrive in the presence of high concentrations of CO2, or which require the presence of CO₂ to survive
  • Microaerophilic - Grow well in low concentrations of oxygen and higher carbon dioxide concentrations; ex. Campylobacter
• Other growth requirements


  • Macronutrients: carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, potassium, calcium, magnesium, iron
  • Micronutrients: manganese, zinc, cobalt, nickel, copper
  • Growth factors: Amino acids, Purines and pyrimidines, Vitamins
• pH classification


  • Neutrophiles (5 to 8)
  • Acidophiles (below 5.5)
  • Alkaliphiles (above 8.5)
• Transport mechanisms


  • Passive diffusion – for CO₂, O₂ and H₂O
  • Facilitated diffusion: protein carrier in the cell membrane shuttles molecules of a substance from one side of the membrane to the other
  • Active transport
  • Group translocation
• Generation time


  The time required for a bacterium to give rise to 2 daughter cells under optimum conditions
• Examples of generation times

  • Escherichia coli & other medically important bacteria - 20 mins
  • Tubercle bacilli – 20 hrs
  • Leprae bacilli – 20 days
• Bacterial growth curve


  • Lag phase
  • Exponential/Logarithmic phase
  • Stationary phase
  • Decline/ Death phase
• Lag phase


  • The bacteria are adapting to the new environment
  • No cell division
  • Vigorous metabolic activity
  • Cells may increase in size during this time, but simply do not undergo binary fission
• Exponential/Logarithmic phase


  • Cells start dividing and their number increases exponentially
  • Constant, maximal growth rate
  • Increased rate of metabolism
• Stationary phase


  • The death rate equals the growth rate
  • Cell division stops due to depletion of nutrients & accumulation of toxic products
  • Spore formation
• Decline/ Death phase


  • Loss of viability-cells die due to toxic products
  • Loss of selective permeability. Fluid gets into the cells causing cell lysis