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Introduction to Microbiology > Bacterial cell walls > Flashcards

Bacterial cell walls Flashcards

1
Q

What is the function of the cell wall?

A

Strength, mechanical protection, osmotic protection, shape and rigidity

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2
Q

What are bacterial cell walls made of?

A

Peptidoglycan

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3
Q

What is the monomer of peptidoglycan?

A

Glycan tetrapeptide

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4
Q

What are the components of a glycan tetrapeptide monomer?

A

2 sugars: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)
Short chain of AA attached to NAM: L-ala, D-glu, DAP, D-ala

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5
Q

What is interesting about the amino acid chain attached to NAM in peptidoglycan?

A

It contains D amino acids and DAP, which are unusual amino acids that can’t be used by ribosomes to build proteins

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6
Q

Which two components of glycan tetrapeptide monomers are only found in bacteria and nowhere else?

A

DAG (diaminopimelic acid) and NAM

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7
Q

What is the polymer structure of peptidoglycan?

A

Repeating units of NAG then NAM linked by glycosidic bonds. The tetrapeptides are covalently cross linked to each other between the 3rd position on one chain and the 4th position on the other

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8
Q

How are peptidoglycan chains usually cross-linked together?

A

3rd position on the first chain, the DAP, crossed linked to the 4th position on the second chain, the D-ala

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9
Q

What modifications to the peptidoglycan cross-linking are found in gram-negative bacteria?

A

None. Its always position 3 to position 4

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10
Q

What 3 modifications to the peptidoglycan cross-linking are found in gram-positive bacteria?

A
  1. -NH2 group attached to D-Glu
  2. DAP replaced with L-Lys
  3. Peptide crossbridge made of a bunch of Gly between the 3rd position and 4th position
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11
Q

Which species have all 3 modifications to the cross linking of peptidoglycan in their cell walls?

A

Staphylococcus

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12
Q

Why is targeting peptidoglycan a good way to kill bacteria?

A

A weakened cell wall makes the cells really vulnerable to osmotic stress. Drugs that target PG will also not hurt our cells because it’s only found in bacteria

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13
Q

What does lysozyme do to peptidoglycan?

A

Cleaves the glycosidic bond between NAG and NAM

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14
Q

Where is lysozyme found?

A

Tears and saliva

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15
Q

How do beta-lactam antibiotics work?

A

They prevent formation of new PG crosslinks, which makes growing cells weaker and vulnerable to osmotic stress

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16
Q

Do peptidoglycan cross-links occur in just one plane?

A

No, form a 3D lattice

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17
Q

What is the cell wall structure of gram-positive bacteria?

A

Thick layer of PG, about 20-40 strands. Highly crosslinked in all directions

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18
Q

What do gram-positive bacteria have embedded in their cell walls?

A

Teichoic acids

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19
Q

What are teichoic acids?

A

Ribitol phosphate polymers that are covalently attached to NAM

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20
Q

What are the 2 types of teichoic acids?

A

Wall teichoic acids and lipoteichoic acids

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21
Q

What are wall teichoic acids?

A

Teichoic acids that are only attached to the cell wall

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22
Q

What are lipoteichoic acids?

A

Teichoic acids with a lipid tail that is inserted into the membrane

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23
Q

What is the purpose of having the wall teichoic acids in the gram-positive cell wall?

A

Gives the cell surface a negative charge that will facilitate binding and transport of positively charged ions

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24
Q

What is the purpose of having the lipoteichoic acids in the gram-positive cell wall?

A

Helps anchor the cell wall to the membrane

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25
Q

What is the cell wall structure of gram-negative cell walls?

A

Thin layer of PG, only about 3 strands, not as highly cross-linked. PG cell wall sandwiched between the cytoplasmic membrane and the outer membrane

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26
Q

What is the periplasm?

A

Fluid between the inner and outer membranes of gram negative bacteria

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27
Q

What is in the periplasm?

A

Soluble proteins and PG

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28
Q

What is the structure of the outer membrane of gram negative bacteria?

A

Inner leaflet contains normal phospholipids, outer leaflet is mostly LPS and a few phospholipids

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29
Q

Where else is LPS found besides gram negative bacteria?

A

Nowhere

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30
Q

What are the 3 parts of LPS?

A

Lipid A, core polysaccharide, O-polysaccharide

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31
Q

Which two parts of LPS are highly conserved among gram-negative bacteria?

A

Lipid A and the core polysaccharide

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32
Q

What is Lipid A of LPS made of?

A

Tons of fatty acids and glucosamine sugars that are chemically similar to NAG. Contains no glycerol

33
Q

What is the core polysaccharide of LPS made of?

A

Tons of sugars, including some heptoses and an 8C sugar called KDO

34
Q

What is the O-polysaccharide of LPS made of?

A

Hydrophilic sugars

35
Q

Is the O-polysaccharide highly conserved between gram-negative bacteria?

A

No, varies widely between species and strains

36
Q

How can the O-polysaccharide be different between gram-negative species?

A

Sugar composition and length

37
Q

Which part of LPS is recognized by our immune system?

A

The O-polysaccharide. Antibodies will be made against that

38
Q

Why is it a problem if gram-negative bacteria get into our bloodstream?

A

They can lyse from immune attack and expose the Lipid A of LPS, which causes an immune system freak out and septic shock syndrome

39
Q

What happens when our immune systems see Lipid A?

A

Freaks out and elicits a really strong immune response, which causes massive inflammation, high fever, and organ failure

40
Q

What is another name for the Lipid A of pathogens?

A

Endotoxin

41
Q

What are the treatments for septic shock syndrome?

A

Dialysis to get rid of the endotoxin or endotoxin-binding drugs

42
Q

What are 3 functions of the outer membrane of gram-negative bacteria?

A

Permeability barrier, reduces drying stress, adherence

43
Q

How does the outer membrane act as a better permeability barrier than the cytoplasmic membrane?

A

The hydrophobic portion of LPS has 6 fatty acid tails compared to 2 for phospholipids, which allows them to pack tighter and exclude anything hydrophilic. The hydrophilic portions of LPS also bind a ton of water and exclude anything hydrophobic

44
Q

How do nutrients and things the cell needs get across the outer membrane if its so inpermeable?

A

Transport proteins. Lets the cell be really selective since that’s the only way across

45
Q

Why are gram-negative pathogens more difficult to kill than gram-positive pathogens?

A

Gram-positive species only have the thick PG layer, which provides physical protection but not much chemical protection. The outer membrane of gram-negative species also provides chemical protection, since it excludes both hydrophobic and hydrophilic antibiotics and lysozyme

46
Q

How does LPS reduce drying stress?

A

The polysaccharides will associate with a lot of water, which keeps the surface of the cell hydrated

47
Q

Considering that the polysaccharides of LPS have a bunch of negatively charged phosphates on them, how do they not repel each other?

A

They associate with divalent cations that bridge the negative charges so they stop repelling each other

48
Q

What happens if you treat a gram-negative cell with a chelator?

A

The cations between the LPS get removed and they start repelling each other, which makes the outer membrane looser and antibiotics can get in

49
Q

Why can’t we use chelators to treat gram-negative infections?

A

They would also bind the ions we need

50
Q

What are two types of outer membrane proteins?

A

Lipoprotein and porins

51
Q

What does lipoprotein do?

A

Anchors the outer membrane to the PG by crosslinks

52
Q

Where is lipoprotein found?

A

The inner leaflet of the outer membrane and extends into the cell wall

53
Q

What is the structure of porins?

A

Small trimeric protein channels that pass through the outer membrane

54
Q

What do porins let through the outer membrane?

A

Small hydrophilic molecules

55
Q

What are the two types of porins in the outer membrane?

A

Non-specific and specific

56
Q

How do non-specific porins avoid letting toxins into the cell?

A

They’re extremely small, only water and ions can get through. Anything larger than that can’t fit

57
Q

What do non-specific porins let into the cell?

A

Water and ions

58
Q

What do specific porins let into the cell?

A

Amino acids and sugars

59
Q

How do specific porins ensure substrate specificity?

A

They have a binding site in the middle of the channel. Only substrates that can make all the correct interactions can get all the way through

60
Q

Are specific porins an active transport mechanism since they have binding sites?

A

No, the binding site is only for specificity. No conformation change happens

61
Q

What is an adaptation in porins that is seen in pathogens? What’s the downside?

A

Their porins are often smaller than usual to exclude host toxins. But its harder to get nutrients in so they grow slower

62
Q

Did acid-fast bacteria evolve from gram-positive or gram-negative bacteria?

A

Gram-positive

63
Q

What is the structure of the cell wall of acid fast bacteria?

A

PG layer on the inside, then a layer of polysaccharides in the middle, then an outer layer of mycolic acids

64
Q

What do mycolic acids do?

A

Are extremely hydrophobic and exclude anything hydrophilic. They form a sort of waxy coating on the cell surface

65
Q

Why are acid fast cell walls even more impermeable than gram-negative outer membranes?

A

The mycolic acids exclude anything remotely hydrophilic, and the polysaccharide layer underneath excludes anything remotely hydrophobic

66
Q

How do acid fast bacteria get nutrients in if their cell walls are so impermeable?

A

Have evolved porins, which is unusual for species that evolved from gram-positive bacteria. But still hard to get nutrients in so they grow really slow

67
Q

Why don’t acid fast bacteria gram stain?

A

The mycolic acid layer excludes the dyes, so they end up colourless

68
Q

How do you stain acid fast bacteria?

A

Need to use phenols to disrupt the mycolic acids enough for the stain to enter

69
Q

What type of cell walls do Mycoplasma species have?

A

No cell walls, lost them over the course of evolution

70
Q

Did Mycoplasma species evolve from gram-positive or gram-negative bacteria?

A

Gram positive

71
Q

How do Mycoplasma species survive osmotic stress if they have no cell walls?

A

They’re obligate intracellular pathogens, and a eukaryotic cell is an isotonic environment

72
Q

Do archaeal cell walls have peptidoglycan?

A

No, they’re much more chemically diverse

73
Q

What is pseudomurein?

A

A cell wall polymer found in archaea that is structurally similar to PG, but isn’t PG

74
Q

How is pseudomurein similar to peptidoglycan?

A

Uses NAG sugars and the overall structure is very similar

75
Q

How is pseudomurein different than peptidoglycan?

A

Uses NAT sugars instead of NAM, and the tetrapeptide attached to it contains only L amino acids. The crosslink is different

76
Q

Why can’t pseudomurin be destroyed by lysozyme?

A

Different glycosidic linkage

77
Q

Why can’t pseudomurin be destroyed by antibiotics that target the cross links?

A

Different cross links than the ones found in bacteria

78
Q

What is the surface layer (S-layer)?

A

A glycoprotein layer that forms a crystal lattice on the archaeal cell surface

Introduction to Microbiology (18 decks)

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