Bacterial structure Flashcards
What are the differences between bacteria and mammalian cells?
- Mammalian cells are typically 10000x bigger than bacterial cells
- Bacteria posses only a singular circular chromosome that lacks a nuclear membrane
- Prokaryotic rather than eurkayotic
- Rapid growth process (binary fission)
- Antbiotics take advantage of these differences by providing selective toxicity (act as ‘magic bullets’ that only harm bacterial cells and won’t hurt host cells’)
Describe the general structure of bacteria
- Uncompartmentalised
- Cytoplasmic membrane and cELL wall are only visible in transmission electron microscope
Describe the cytoplasm of bacteria
- Possess no structures comparable to ER or Golgi bodies in eukaryotic cells. Region between nucleus and cytoplasmic membrane packed with ribosomes (protein synthesis)
Describe ribosomes in bacteria
- Less complex than eukaryotic, smaller and involved in protein synthesis. Possible point of action for antibiotics
Describe the cytoplasmic membrane bacteria
- Surrounds cytoplasm
- Many complex function and provides osmotic bacteria and transport system
- Other than lack of sterols, membrane has similar composition to mammalian membranes (lipids and proteins)
- Not most effective target for antibiotics
What is gram staining?
- Gram positive and negative differ in surface structure
Give examples of gram positive bacteria
- E.g. staphylococci, streptococci and pneumococci
Describe gram positive bacteria
- Relatively homogenous cell wall under EM and limited number of components
- Cytoplasmic membrane surrounded by giant net-like molecule made up of long chains of amino-sugars cross-linked peptidoglycan
What is the purpose of peptidoglycan?
- This thick layer protects cytoplasmic membrane
from rupture
What antibiotic prevents the growth of peptidoglycan?
Penicillin
What phosphorylated polysaccharides are usually present in the outer layer of gram pos. cell wall?
- Polysaccharides or teichoic acids
- Exposed so main antigens for immune response
Give examples of gram negative bacteria
- E.coli
- Salmonella
Compare the cell wall of gram negative to gram positive
- more complex
- Peptidoglycan layer is only one molecule thick and outside there is another membrane containing lipid, protein and lipopolysaccharide
What is the importance of the outer membrane of gram negative bacteria?
- Presence of outer membrane responsible for gram staining properties
- Also prevents passage of penicillin to peptidoglycan
- More susceptible to antibody and component lytic action
Describe lipopolysaccharides
- Composed of repeating sugar unit (O-antigens) linked to terminal lipid region (lipid A)
- Sugar chains exposed on surface and suscetible to antigen binding
- However, usually contains unusual sugars new to host
- LPS largely involved in disease, responsible for many symptoms of gram neg. septicaemia including shock, fever and clotting abnormalities.
What are the exceptions gram staining?
- Mycobacteria, e.g. TB
- Has mycolic acids around peptidoglycan cell wall which stain cannot attach to (Acid-fast)
- Requires different stain- Ziehl-Neelsen
What are the non-essential bacterial components?
- Bacterial capsules
- Flagella
- Pilli
- Spores
What is a bacterial capsule?
- Contain hydrophilic jelly (outermost layer)
- Protect from phagocytosis
What are flagella?
- Long filamentous structures that aid movement in wet environments
- Also allows bacteria to penetrate mucus and reach gut epithelial cells
What are pilli?
- Hair-like projections for adhesion to mucosal surfaces
- e.g. gonococcus that needs to grow in urethra epithelial host cells
What are spores?
- Produced in adverse structure
- highly resistant to dehydrated forms with no metabolic activity
- Intracellular to extracellular
- Wait until conditions are more favourable to rejuvenate bacteria
