Cell wall Flashcards
Describe the functions of the cell wall.
The cell wall has many important functions such as: determining cell shape; protecting the cell from osmotic lysis; protection from toxic substances; and contributing to pathogenicity (Willey, et al., 2011, pp.54-61).
References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.
Describe peptidoglycan structure.
Peptidoglycan is a polymer made up of a backbone of N-acetylmuramic acid and N-acetylglucosamine, joined by 1,4 glycosidic bonds, with a short peptide of four D and L amino acids joined to the carboxyl group of N-acetylmuramic acid (Willey, et al., 2011, pp.54-61).
The peptidoglycan strand is helical with peptides extending at right angles, allowing cross-links between peptides (Willey, et al., 2011, pp.54-61).
Cross-links occur between the carboxyl group of the terminal D-alanine (position 4) and the amino group of diaminopimelic acid (position 3) (Willey, et al., 2011, pp.54-61).
References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.
Why does peptidoglycan contain the unusual D-isomers of alanine and glutamic acid rather than the L-isomers observed in proteins?
Peptidases only recognise L-amino acids so D-amino acids protect against degradation (Willey, et al., 2011, pp.54-61).
References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.
Describe the mechanism of gram staining.
First, the cell is stained with crystal violet (Wiley, et al., 2011, pp.54-61).
Next, it is treated with iodine, after which it is treated with ethanol (Willey, et al., 2011, pp.54-61).
Then it is counterstained with safranin (Willey, et al., 2011, pp.54-61).
Gram positive bacteria remain purple as the crystal violet is retained by peptidoglycan (Willey, et al., 2011, pp.54-61).
Gram negative bacteria turn red or pink as the crystal violet is lost from bacteria due to the ethanol (Willey, et al., 2011, pp.54-61).
Ethanol shrinks the pores of peptidoglycan in gram positive bacteria, causing it to act as a permeability barrier (Willey, et al., 2011, pp.54-61).
Ethanol in gram negative bacteria increases porosity of cell wall by extracting lipids (Willey, et al., 2011, pp.54-61).
Peptidoglycan is already thin
and isn’t as highly cross-linked and has larger pores in gram negative bacteria (Willey, et al., 2011, pp.54-61).
References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.
State what happens to cells in hypotonic solutions and in hypertonic solutions.
Cells swell or lyse in hypotonic solutions as water diffuses in (Willey, et al., 2011, pp.54-61).
The cytoplasm shrivels up in hypertonic solutions, called plasmolysis, as water flows out (Willey, et al., 2011, pp.54-61).
References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.
Describe the difference between protoplasts, spheroplasts and mycoplasmas.
Protoplasts are gram positive cells without a cell wall (Willey, et al., 2011, pp.54-61).
Spheroplasts are gram negative cells without the peptidoglycan layer and cell wall (Willey, et al., 2011, pp.54-61).
They are osmotically sensitive (Willey, et al., 2011, pp.54-61).
Mycoplasmas lack cell wall and are osmotically sensitive but can grow (Willey, et al., 2011, pp.54-61).
Reference: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.
Describe the structure of gram positive cell walls.
Gram positive bacteria have thick cell walls containing peptidoglycan and secondary cell wall polymers (teichoic acids) (Willey, et al., 2011, 54-61).
The periplasmic space is smaller than that in gram negative bacteria and contains few proteins, as the peptidoglycan sac is porous and so proteins may pass through (Willey, et al., 2011, pp.54-61).
The periplasmic space contain exoenzymes that degrade polymeric nutrients for transport across plasma membrane (Willey, et al., 2011, pp.54-61).
The peptidoglycan surface has a layer of proteins either covalently or non-covalently attached, which are involved in cell interaction with the environment (Willey, et al., 2011, pp.54-61).
Gram positive cell walls also have sortases attached to the plasma membrane which catalyse the attachment of surface proteins to peptidoglycan (Willey, et al., 2011, pp.54-61).
References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.
Explain the composition of teichoic acids in gram positive bacteria.
Teichoic acids are secondary cell wall polymers (Willey, et al., 2011, pp.54-61).
They are polymers of glycerol or ribitol joined by phosphate groups, with amino acids (D-alanine) or sugars (glucose) attached (Willey, et al., 2011, pp.54-61).
Lipoteichoic acids are covalently connected to peptidoglycan or plasma membrane lipids (Willey, et al., 2011, pp.54-61).
Teichoic acids are negatively charged and not present in gram negative bacteria (Willey, et al., 2011, pp.54-61).
References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.
Describe the functions of teichoic acids.
Teichoic acids create and maintain cell envelope structure (Willey, et al., 2011, pp.54-61).
They provide protection from harmful substances (Willey, et al., 2011, pp.54-61).
They also bind pathogenic species to host tissues (Willey, et al., 2011, pp.54-61).
References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.
Describe the structure and composition of gram negative cell walls.
Gram negative cell walls have a thin peptidoglycan layer surrounded by periplasmic space (Willey, et al., 2011, pp.54-61).
The outer membrane is joined to the cell by Braun’s lipoprotein or contact sites which join the outer and plasma membranes (Willey, et al., 2011, pp.54-61).
The lipoproteins hydrophobic end is embedded in outer membrane and is covalently joined to peptidoglycan (Willey, et al., 2011, pp.54-61).
The periplasmic space contains proteins and enzymes involved in nutrient acquisition (hydrolytic enzymes and transport proteins), energy conservation (electron transport proteins), peptidoglycan synthesis and toxic compounds modification (Willey, et al., 2011, pp.54-61).
Gram negative cell walls also contain lipopolysaccharides and porin proteins (Willey, et al., 2011, pp.54-61).
Porin proteins allow movement of small molecules across outer membrane (Willey, et al., 2011, pp.54-61).
They are tube-shaped, narrow, water-filled channel (Willey, et al., 2011, pp.54-61).
Cell membrane also contains specific carriers for transport of larger molecules (Willey, et al., 2011, pp.54-61).
References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.
Describe the structure and function of lipopolysaccharides.
Lipopolysaccharides are composed of lipid A, core polysaccharide and O side chain (Willey, et al., 2011, pp.54-61).
Lipid A is composed of two glucosamine sugar derivatives each attached to three fatty acids and phosphate or pyrophosphate (Willey, et al., 2011, pp.54-61).
Core polysaccharide is joined to lipid A and consists of ten sugars (Willey, et al., 2011, pp.54-61).
The O side chain is a polysaccharide chain extending outward from core (Willey, et al., 2011, pp.54-61).
Lipopolysaccharides have several important functions (Willey, et al., 2011, pp.54-61).
For example, they contribute to negative charge, stabilise outer membrane structure, attach bacteria to surfaces and form biofilms (Willey, et al., 2011, pp.54-61).
They also create a permeability barrier, protect cell from host defences and act as an endotoxin (Willey, et al., 2011, pp.54-61).
References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.
Describe the characteristic shapes bacteria can assume.
The two main shapes bacteria can assume are cocci and bacilli (Willey, et al., 2011, pp.48).
Cocci are spherical cells which can exist singly or can form diplococci, when they divide and form pairs (Willey, et al., 2011, pp.48).
Long chains of cocci are formed when ells divide in one plane while irregular clumps are formed when cells divide in random planes (Willey, et al., 2011, pp.48).
Bacilli are rod shaped cells (Willey, et al., 2011, pp.48).
The end of the rods can have different shapes in different species (Willey, et al., 2011, pp.48).
The end may be flat, rounded, cigar-shaped or bifurcated (Willey, et al., 2011, pp.48).
Rods can exist singly or form pairs or chains (Willey, et al., 2011, pp.48).
Bacterial cells can have other shapes too, such as: vibrios, which have a comma shape; spirilla, which have a rigid spiral shape; and spirochetes, which have a flexible spiral shape (Willey, et al., 2011, pp.48).
References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.
What advantages might a microbial species that forms multicellular arrangements (eg. clusters or chains) have that are not afforded unicellular microbes?
○Forming multicellular arrangements can be very beneficial as it has many advantages, such as: resistance to physical and chemical stresses; improved nutrient acquisition; and efficient colonisation of territories (Lyons et al., 2015).
○Another advantage is that larger or filamentous cells are less susceptible to predation (Lyons et al., 2015).
○This can be seen with the bacterium E. Fishersoni, which has a large size that protects it from being eaten by protists (Willey, et al., 2011, pp.50).
○Furthermore, a large size gives more chances of surviving intermicrobial conflicts (Lyons et al., 2015).
References: Lyons, N.A., and Kolter, R. (2015) ‘On the evolution of bacterial multicellularity’, Current opinion in microbiology, volume 24, pp. 21-28.
Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.
List the functions of bacterial plasma membranes.
Act as a selectively permeable barrier, allowing movement of some molecules and preventing other molecules (Willey, et al., 2011, pp.52-54).
Contain transport systems used for uptaking nutrients, excreting waste and secreting proteins (Willey, et al., 2011, pp.52-54).
Important metabolic processes occur at PM, such as photosynthesis, respiration, and lipids and cell wall constituents synthesis (Willey, et al., 2011, pp.52-54).
References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.
With a few exceptions, the cell walls of gram-positive bacteria lack porins. Why?
Because they only have one plasma membrane, which allows proteins to transport across it (Willey, et al., 2011, pp.54-61).
Gram-negative cells need porins because the LPS in the cell wall creates a permeability barrier, therefore porins are needed to allow transport of proteins across the membrane (Willey, et al., 2011, pp.54-61).
References: Willey, J.M., Sherwood, L.M., Woolverton, C.J. (2011) Prescott’s Microbiology. 8th edn. New York: McGraw-Hill.
