Organization and Structure of Microorganisms

Phylogenetic relationships amongst different cell types


Microbes have different shapes and is of advantage


Microbes are small but this feature is crucial

radius (r) of cell A  = 1um
 radius (r) of cell B = 2um
Surface area (SA) of cell = 4pir2
Volume (V) of cell = 4/3pir3
Ratio of SA to V
Features of bacterial, archaeal and eucarya cells

This section deals with the structure and functions of cells. Cells are of three types as described above (Bacteria, Archaea & Eucarya) and the description below provides similarities and differences amongst these cell types.
Diagramatic representation of cells


Cell walls are external structures that shape and protect cells

1. Bacterial Cell Walls:

Differences Between Gram-positive And Gram-negative Bacterial Cell Walls
 Gram-positive wall
 Gram-negative wall
 Peptidoglycan  Thick layer  Thin layer
Peptidoglycan tetrapeptide  Most contain lysine  All contain diaminopimelate
Peptidoglycan cross linkage  Generally via pentapeptide  Direct bonding 
Teichoic acid  Present  Absent
Teichuronic acid  Present   Absent
Lipoproteins  Absent  Present
LPS  Absent   Present
Outer Membrane  Absent  Present
Periplasmic Space  Absent  Present

2. Archaeal Cell Walls:


3. Eucaryal Cell Walls:


4. Glycocalayx, Capsules, Slimer Layers & S layers:

Cytoplasmic membranes are involved in transport of molecules

(A) Structural & Biochemical Diversity  
Characteristics Bacteria Eucaryotic Archaea 
Protein content High  Low High
Lipid composition Phospholipid Phospholipids Sulfolipids, glycolipids, nonpolar isoprenoid lipids, phospholipids
Lipid structure Straight chain Branched Straight chain
Lipid linkage Ester linked(1)  Ester linked Ether linked (di& tertaethers)
Sterols Absent(2) Present Absent
(1) Aquifex pyrophilus contains phospholipids & ether linked lipids
(2) Cell wall-less bacteria (Mycoplasma, Ureaplasma, Spiroplasma, Anaeroplasma) contain sterols

1. Bacterial cytoplasmic membranes:

(a) Phospholipids: (structure, functions & utility)

  (b) Proteins:  
Location in Membrane
Energy transformation Inside membrane ATPase F1
Transport of molecules Inside membrane HPr
Protein export Inside membrane Docking protein
Association of DNA with membrane Inside membrane DNA binding protein
Transport of molecules Both sides Permease
Chemotaxis Both sides Methylase-accepting chemotaxis proteins
Electron & proton transport Both sides Flavoproteins
Flagellar activity Outside surface M protein (basal body of flagella)
Penicillin-binding proteins Outside surface Cell wall biosynthesis

2. Archaeal cytoplasmic membranes:

(a) Lipids: 3. Eucaryal cytoplasmic membranes:
  • Polyene antibiotics (eg nystatin, candicidin) targets sterols & has more affinity for  ergosterol than cholesterol (more effective against fungi rather than human cells)
  •  B. Transport Across Cytoplasmic membrane 1. Passive Processes:
    Transport does not require energy & include diffusion, osmosis and facilated diffusion

    (a) Diffusion

    (b) Osmosis (c) Facilitated Diffusion 2. Active Energy-linked transport processes
    Require energy for transport and the processes include active transport, group translocation, binding protein transport and cytosis

    (a) Active Transport

    Protonmotive force  (PMF): Sodium-potassium pump:  
    (b) Group translocation- Phosphoenol pyruvate: Phosphotransferase system (PEP:PTS) (c) Binding protein transport (d) Cytosis- Eucaryotic specific transport

    Sites of cellular energy transformations where ATP is generated


    Movement of microbial cells

    1. Flagella / Cilia
    2. Axial Filament
    3. Gas vacuoles
    4. Magnetosomes
    5. Pseudopodia
    6. Chemotaxis, magnetotaxis and phototaxis

    Microbes do not die? Structures for survival

    Ordinary microbes are killed by minor stresses eg chilling, antibiotics, disinfectants but cells with protecive bodies, namely endospores and cysts ressist such stresses. In most cases, the cells that produce endospores and cysts are a part of the soil microflora. Soil heats & dries in summer but is periodically flooded by rain -- harsh fluctuating environment.
    1. Endospores:  
     Present in some genera  Present
     Protective & dispersal function  Reproductive function
     Endospores  Endo- or Exo- spores
     One per cell but C. disporicum=2; C. polypendens=5 Numerous
    Endospore-forming cell
    Time required to kill a suspension in boiling water (100oC)
     B. anthracis    1-2 min (not very heat ressistant)
     C. botulinum   2-6 hours
     C. tetani   1-3 hours
     E. coli & S. aureus (non-endospore formers)  30 minutes at 70o C
      2. Cysts  

    Cellular storage of genetic information

    1. Bacterial & archaeal chromosome  
    2. Plasmids 3. Nucleus & chromosomes of Eucarya cells  

    Information flow in cells: the role of ribosomes


    Storage of materials

    1. Inclusion bodies of bacteria 2. Membrane bound organelles in Eucarya  

    Cell surface structures involved in attachment

    1. Glycocalyx: 2. Fimbrae: 3. Pili:

    Comments and suggestions to: Dr Bharat Patel <>

    [Created 20 Sept 1995]
    [Modified 19 Aug 1997]