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Anatomy of Bacterial Cell

  1. Cell Wall
    1. Structure
      1. Complex rigid structure which gives bacteria shape
      2. 10-20 nm in thickness, gram negative thinner
      3. Strength due to peptidoglycan/mucopeptide/ murein
      4. Peptidoglycan :
    2. Backbone : n-acetylglucosamine & n-acetyl muramic acid
  2. Tetrapeptide side chain
  3. Pentapeptide cross bridges   
3. Gram-positive cell wall
  1. ~ 80nm thick
  2. Composed mostly of several layers of peptidoglycan
  3. Also contains teichoic acid  (cell wall TA’s, cell membrane TA’s/lipoteichoic acids ). Play an important role in virulence for all Gram positive bacteria, are antigenic leading to formation of antiteichoic antibodies.

4.  Gram negative cell wall

  1. Thinner than gram positive bacteria
  2. Structurally more complex
  3. Peptidoglycan layer single unit thick. Chlamydia lack or have very low amounts of peptidoglycan in CW
  4. Outside to peptidoglycan : outer membrane; phospholipid bilayer in which other large molecules embedded (outer membrane protein, porins )
  5. Relatively high antibiotic resistance of gram negative bacteria
  6. Outer membrane anchored to peptidoglycan by lipophilic lipoprotein
  7. Lipopolysaccharide (LPS) molecule unique to gram negative outer membrane
  8. Complex lipid: lipid ‘A’, to which is attached core polysaccharide and a terminal series of repeat unit (O or the somatic antigen)
  9. Firmly bound to the cell surface, released only when cells are lysed
  10. Endotoxic acitivity: lipid ‘A’
  11. Polysaccharide : major surface antigen, O antigen
  12. Periplasmic space : b/w inner and outer membrane; contains peptidoglycan layer and penicillin binding proteins(transpeptidase enzymes)

5.  Acid-fast cell wall

Certain genera : Mycobacteria, Nocardia,Corynebacteria (CMN group)

  1. Gram positive cell wall structure + waxy layer of glycolipids and fatty acids (mycolic acids) bound to the exterior of cell wall
  2. Difficult to stain with gram stain
  3. Cell wall cannot be seen by light microscopy, does not stain by simple dye
  4. Can be demonstrated by plasmolysis, microdissection, reaction with specific antibody, mechanical rupture, differential staining & electron microscopy
  5. Protoplast : cytoplasmic membrane and its contents left following action of lysozymes on gram positive cell
Extra edge

CMN group- groups of bacteria with mycolic acids            

  1. Corynebacteria contain 20-40 C mycolic acids              
  2. Nocardia contain 40-60C mycolic acids   
  3. Mycobacteria contain 60-80 C mycolic acids

f.    Spheroplast : protoplast with some cell wall attached after action of lysozyme on gram negative cell


6.  Difference between Protoplast & Spheroplast


Protoplast Spheroplast
Gram positive gram negative
No cell wall remnant cell wall remnant +
Grow in size but do not multiply multiply by budding or fission
Osmotically more sensitive osmotically less sensitive
Cannot revert to parent bacterial form Can revert to parent bacterial form                                        
Also called unstable 'L' forms
B.  L-forms
  1. Cell wall deficient bacteria
  2. Derived from normal bacteria in laboratory by growing in the presence of cell wall inhibitors/ spontaneously
  3. Stable : special conditions are not required for preventing their reversion
  4. Capable of growing and multiplying on a suitable medium
  5. 0.1 - 20 μm in diameter
  6. Solid media: fried egg colony. Microscopy required for visualization
  7. Could account for bacterial persistence during therapy with certain antibiotics 

C.  Cytoplasmic membrane/ cell membrane

  1. 5-10 nm, limits bacterial protoplast externally
  2. Elastic, phospholipid bilayer, lacks sterols
  3. Semipermeable
  4. Passive diffusion: water, small molecules
  5. Selective transport: specific enzymes (permeases)

D. Cytoplasm

  1. Viscous watery solution of soft gel
  2. Organic and inorganic solutes, ribosomes
  3. Organelles (-), cytoplamic steraming (-), amoeboid movement (-)

E.  Ribosomes

  1. Ribosomal RNA + ribosomal proteins
  2. 70s divided into 50s & 30s
  3. Polysomes : thousands of ribosomes strung together on strands of mRNA
  4. Site of protein synthesis
  5. Different from ribosomes of eukaryotes : basis of selective action of aminoglycosides and tetracyclines (act on 30s); chloramphemicol, macrolides and lincosamides (act on 50s)  

F.  Mesosomes

  1. Convoluted body, develops by invagination of cytoplasmic membrane
  2. membrane surface, respiratory enzymes (analogous to mitochondria)
  3. Formation of cross wall during cell division
  4. More prominent in gram positive bacteria 

G.  Intra cytoplasmis inclusions

  1. Volutin granules : Babes-Ernst, metachromatic, polymetaphosphate, polar bodies
    1. Stained by albert's, neisser's, ponder's stain
    2. Act as energy stores.
    3. diphtheriae, diphtheroides
  2. Lipid granules : polymerised β-hydroxy butyric acid
a. Stained by sudan black
3. Poly saccharide granules : stained with iodine
  1. Glycogen (red brown), starch (blue)
Extra edge-
  1. Other bacteria with volutin granules                 
  2. Gardnerella vaginalis
  3. Bordetella pertussis                 
  4. Yersinia pestis
  5. Mycobacteria


H. Bacterial nucleus

  1. Single, circular, double stranded DNA molecule
  2. Absence of nuclear membrane, nucleolus & basic proteins
  3. Plasmid: extra chomosomal genetic material 

I.   Bacterial capsule

  1. Gelatinous layer
  2. Diffuses into the medium : slime layer
  3. Too thin to be seen with light microscope : microcapsule, eg.Staphylococcus
  4. Chemically:
    a.  usually polysaccharide : Klebsiella, Hemophilus, S. pneumoniae, Cryptococcus neoformans
    b.  Polypedtide/protein : Bacillus anthracis (poly D-glutamate)
  5. Not demonstrated by gram stain but by india ink, nigrosin staining , quellung reaction (swelling of capsule when specific antiserum is added)
  6. Protects from antibacterial agents, inhibits phagocytosis, lost on repeated subculture 

J.   Flagella

  1. Organs of locomotion
  2. 3-20 micron long, 10-20 µm thick
  3. Chemically : protein called flagellin
  4. Three parts :
  5. (i). filament,
  6. (ii). hook,
  7. (iii). basal body: Rod, Rings ≥ 2 (M [cytoplasmic membrane],S [periplasmic space above cytoplasmic membrane], P[ peptidoglycan], L [outer membrane]). Rings P & L are absent in gram positive bacteria 

Arrangement :

  1. Monotrichous   :  Vibrio
  2. Lophotrichous  :  Spirilla, Helicobacter, Campylobacter
  3. Amphitrichous :  single/tuft at both ends,Campylobacter
  4. Peritrichous    :  all Enterobacteriacea, Bacillus, Clostridia( with non-motile exceptions)

1)  Demonstrated by : dark ground microscopy, impregnation stains, hanging drop, u tube, craigie's tube, electron microscopy

2)  Antigenic, serodiagnois


K. Fimbriae / pili

  1. Hair like, 1-1.5 micron length, 4-8 nm thick
  2. Straigher, thinner, shorter than flagella
  3. Present on many gram-negative bacteria: adherence
  4. More numerous than flagella
  5. Originate from cytoplasmic membrane
  6. Composed of protein pilin
  7. Function : adhesion, agglutinate rbc's of various species, sex pili (F pili) involved in conjugation
  8. Can be demonstrated by Haemagglutination test/EM/detection of antigen 

L.  Endosproes

  1. Found in Bacillus, clostridium, sporosarcina
  2. Each bacterium forms one endospore which germinates into single vegetative cell
  3. Method of preservation not replication
  4. Highly resistant resting stage formed during nutrient depletion
  5. Layers : (from inside out) spore cytoplasm, spore wall (inner membrane), spore cortex, spore coat (outer membrane), exosporium
  6. Round, oval, elongated ; terminal , subterminal, central ; bulging, non bulging
  7. Dormant for many years
  8. Extremely resistant to chemical, physical agents may be due to dipicolinic acid, a calcium ion chelator found only in bacterial spores
  9. Killed by moist heat 121°c, 15min, 15 psi
  10. Demonstrated by: modified ZN staining, unstained in gram stain, unstained preparation : refractile  

M.        Involution forms

  1. Swollen aberrant forms seen in aging cultures, penicillin, glycine, lithium chloride, salt concentration, organic acids
  2. Many non-viable, others revert to normal form (Y. pestis)

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