Bacterial cell structure and function Flashcards
Outline the key features of bacteria
- Prokaryotes (no nucleus)
- Typically unicellular (in organisation)
- Diverse metabolism (heterotrophs/photoautotrophs, aerobes/anaerobes)
- Typically have a cell size of around 1-2um
- No membrane-bound organelles (in eukaryotic cells, organelles often surrounded by a membrane similar to the cell membrane)
Outline features of spherical bacterial cells
- Cocci (plural), coccus (singular) = from Greek (‘berry’)
- May aggregate into: chains (streptococci), clumps (staphylococci), or pairs (diplococci)
- They can form chains
- An example would be streptococcus pneumoniae, responsible for pneumonia, common most to the elderly
Why do spherical cells arrange in different shapes and sizes?
- They may have different division points
- You can have more than one plane of division (can be three, equally divided)
Outline features of rod bacterial cells
- Bacilli (plural), bacillus (singular) = from Latin ‘(stick’)
- Hence, the bacillus genus (such as bacillus anthracis (causing anthrax)
- But many other rods exist, so it is termed rod (NOT bacillus)
What is the capsule/mucilage layer?
- A layer surrounding the cell wall and protects the bacterial cell
- Also known as Glycocalyx
- Not essential for cell viability
- Outside of the cell wall
- When well defined, it is a capsule
- When less defined, it is a slime layer
- The structure is usually a single polysaccharide (the type of which can help to identify the bacteria’s species/strain)
What are the functions of a capsule/mucilage layer?
- Adherence = (capsule is sticky) ‘biofilms’
- Protection = against water loss
- Protection = against phagocytes in pathogenic bacteria
- Protection = against chemicals (e.g., disinfectants)
Outline the key features of a bacterial cell wall
- Has a major component of peptidoglycan, or murein
- It is a complex polymer made of carbohydrates and amino acids
- Has long (s)glycan chains with repeating subunits: (N-acetylglucosamine, N-acetylmuramic acids)
- Is crosslinked by short peptide chains
- The peptidoglycan monomers are synthesised in the cytosol of the bacterial, where they attach to a membrane carrier molecule called bactoprenol
What is the function of bactoprenol?
- They transport the peptidoglycan monomers across the cytoplasmic membrane and work together with other enzymes to insert the monomers into existing peptidoglycan, enabling bacterial growth after the binary fission
Outline the properties of peptidoglycan
- A unique polymer
- Forms a single molecule-surrounding cell
- Very strong, but permeable
- Protects the cell from bursting due to turgor and maintains cell shape
- Two major types of cell wall = gram positive and gram negative (two dyes used, only one retained by bacteria - due to major cell wall differences, can be told apart)
How do you stain gram-negative bacteria?
- Crystal violet = primary stain
- Fixing of the dye - use of iodine (iodine complex prevents easy removal of dye)
- A decolourizer solvent (ethanol) used to remove the dye
- All bacteria will take up the crystal violet dye, but…
- In gram-positive microorganisms (with a higher peptidoglycan content), the alcohol will dehydrate the cell walls, with the closure of pores, preventing any diffusion of the violet-iodine complex - so cells will remain purple-blue
- In gram-negative microorganisms (with a higher lipid content), the solvent will dissolve the lipid bilayer and cells will loose the primary stains
The length of decolourisation is critical as prolonged exposure can remove ALL the stains from both cell types
- Use of counterstain (safranin) in order to give the decolourized gram-negative bacteria a pink colour, allowing for easier identification
Outline the bacterial envelope (wall plus membranes) in gram-positive bacteria
- A relatively thick cell wall (>20nm)
- High internal osmotic pressure (turgid)
- High peptidoglycan content (>50%)
- Wall contains other polymers (teichoic acids)
- Typically, sensitive to lysozyme and penicillin
- No further layers outside the cell wall
What roles do surface polymers, such as teichoic acids, provide?
- Cell shape
- Cell determination
- Regulation of cell division
- Pathogenesis
- Antibiotic resistance
Outline the bacterial envelope (wall plus membranes) in gram-negative bacteria
- Relatively thin cell wall (<10nm)
- Low internal osmotic pressure
- Low peptidoglycan content (<20%)
- No teichoic acids
- A further membrane, outside of the peptidoglycan wall (outer membrane)
- Mostly resistant to lysozyme and penicillin
Name and outline the main major component of the outer membrane in gram-negative bacteria
Lipopolysaccharide:
- Localized in the outer layer of the membrane
- In non-capsulated strains, it is exposed on the cell surface
- Intact bacterial lipopolysaccharides are macromolecules of molecular mass 10-20 kDa (made up of three structural components)
- O-antigen structure is variable and repeats
What is a porin?
- beta-barrel structure
- Sometimes arranged as trimers
- Central pore allows passage of small molecules
- Some are specific for particular molecules (e.g., maltoporin for maltodextrins)
