Gram negative and positive

Question 1

GRAM-POSITIVE CELL WALLS

Answer 1

May also contain large amounts of teichoic acids
(negatively charged):
– Help maintain structure of cell envelope.
– Protect from environmental substances (such as
antibiotics and host defense molecules).
– May help pathogens bind to host tissues in initiating
infection.

Question 2

Structure of teichoic acids

Answer 2

Polymers of glycerol or ribitol linked by phosphate groups.
Only present in Gram-positive bacteria.

Question 3

GRAM-NEGATIVE CELL WALLS

Answer 3

• Consist of a thin layer of peptidoglycan surrounded by an outer membrane.
• Outer membrane composed of lipids,
  lipoproteins, and lipopolysaccharide (LPS).
• Outer membrane lies outside the thin
  peptidoglycan layer.
• Braun’s lipoproteins connect outer membrane to peptidoglycan.

Question 4

• Periplasmic space differs from that in Grampositive cells:

Answer 4

May constitute 20-40% of cell volume.
– Many enzymes present iperiplasm (hydrolytic enzymes, transport proteins and other proteins).

Question 5

Lipopolysaccharides (LPSs)

Answer 5

• Consists of three parts:
  – lipid A
  – core polysaccharide
  – O side chain (O antigen; varies between
  bacterial strains)

Question 6

Importance of lipopolysaccharides

Answer 6

Contributes to negative charge on cell surface.
* Helps stabilise outer membrane structure.
* May contribute to attachment to surfaces and biofilm formation.
* Creates a permeability barrier.
* Protection from host defenses (O antigen – elicits immune
response from host).
* Lipid A part of LPS can act as an endotoxin (i.e. is toxic to host).
LPS or lipid A in human bloodstr

Question 7

Gram-Negative Outer Membrane Permeability

Answer 7

• More permeable than plasma membrane due to presence of porin proteins and transporter
  proteins.
• Porin proteins form channels through which small molecules (600-700 daltons) can pass

Question 8

S Layers

Answer 8

Regularly structured layers of protein or
glycoprotein that self-assemble:
– in Gram-negative bacteria the S layer
adheres to the outer membrane.
– in Gram-positive bacteria it is associated
with the peptidoglycan surface.

Question 9

S Layer Functions

Answer 9

Protect from ion and pH fluctuations, osmotic stress, enzymes, and predation.
* Maintains shape and rigidity.
* Promotes adhesion to surfaces.
* Protects from host defenses.
* Potential use in nanotechnology (because S layer spontaneously associates).