Lecture 5: LPS, periplasm, flagella

Question 1

What does LPS stand for?

Answer 1

Lipopolysaccharide

Question 2

Describe a Lipopolysaccharide

Answer 2

• Large complex molecule containing lipid and carbohydrate
• Contains: lipid A, core polysaccharide, and O side chain

Question 3

What are 2 examples of bacteria that the LPS is found in?

Answer 3

Salmonella and E.coli

Question 4

What is the LPS called when it is free in its host cell?

Answer 4

An endotoxin

Question 5

Is it true that LPS can induce an immune response (i.e septic shock) ?

Answer 5

Yes, and whilst this isn’t a specific function of the LPS, it does induce an immune response when it comes into contact with an innate immune system

Question 6

do gram negative bacteria have an LPS?

Answer 6

No, only gram positive

Question 7

What are the 3 main components of the LPS?

Answer 7

Lipid A

Core polysaccharide

O side chain

Question 8

Describe the Lipid A component of the LPS

Answer 8

• Made up of 2 glucosamine residues linked to fatty acids and phosphate (occasionally pyrophosphate)
• Integrated into the outer membrane and so can act as an endotoxin and trigger an immune response
• Straight
• Very variable composition
• Stabilises the outer membrane structure

Question 9

Describe the core polysaccharide component of the LPS

Answer 9

• Also referred to as R-antigen or R-polysaccharide
• Has side chains of NAG, phosphate and ethanolamine

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Question 10

Describe the O side chain component of the LPS

Answer 10

• Has a variable region that extends outwards, which is responsible for the antigenic makeup of bacteria. Different O side chains are linked to different diseases.
• O side chain is flexible and bent
• Highly variable composition (at least 20 different sugars)
• Rough/smooth variants depend on the side chain length

Question 11

Why do bacteria move between rough and smooth variants?

Answer 11

To protect them from the immune system

Question 12

What are the functions of the LPS

Answer 12

• Stability of the outer membrane (provided by Lipid A)
• Contributes to negative charge on the surface of the bacteria (due to the core polysaccharide being charged)
• Charged, hydrophilic external layer reduces permeability of hydrophobic substances (e.g. preventing entry of bile salts, antibiotics
  ).
• Protects against host defences

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Question 13

What’s more susceptible to phagoctytosis, rough variants or smooth variants?

Answer 13

Rough variants

Question 14

What does the loss of the O antigen in salmonella lead to ?

Answer 14

Reduced virulence

Question 15

Describe endotoxins

Answer 15

• Produced by many pathogens and are released during cell division or by lysis of bacterial cells, and trigger an immune response
• Can act to prime immune system against a pathogen
• If LPS is in the blood, can cause septic shock syndrome, of which there is no direct treatment
• Immunogenic even in absence of living cells
• Can be tested for using the Rabbit pyrogen test and the Limulus amoebocyte lysate (LAL) assay

Question 16

What are some important properties of endotoxins?

Answer 16

• Heat stable
• Toxic in small amounts
• Interacts with innate immune system cells
• Has serious infects, including: inflammation, fever, shock, and even death

Question 17

How can endotoxins interact with innate immune system cells?

Answer 17

Can trigger the release of cytokines in a cascade

Activates transcription factors such as interferon-β and tumour necrosis factor (TNF)

Question 18

Describe porins

Answer 18

• Make the outer membrane more permeable than the inner membrane
• Links the inter membrane space with the outside
• Protein channels that facilitate the diffusion of small molecules
• Made up of transmembrane proteins
• Highly conserved structure
• Form water-filled channels in the outer membrane
• Most porins are non-specific, but some are selective
• High thermal stability
• Resistant to protease and detergent degradation
• Have an unusual, stable, and closed structure

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Question 19

Is it true that compounds diffuse through porins and into the periplasm?

Answer 19

Yes

Question 20

What is the periplasm?

Answer 20

the space between outer membrane and cytoplasmic membrane