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Bacterial Pathogenicity > L5. Staph A Antibiotic Resistance > Flashcards

L5. Staph A Antibiotic Resistance Flashcards

1
Q

How soon did penicillin resistance appear after its introduction?

A

Clinical resistance for penicillin appeared a few years after its introduction.

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2
Q

What is the mechanism behind penicillin resistance?

A

Penicillin resistance is mediated by β-lactamase, an enzyme that bacteria use against each other.

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3
Q

What are semi-synthetic β-lactams and give an example?

A

Semi-synthetic β-lactams are chemically modified antibiotics derived from β-lactam structures; an example is Methicillin.

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4
Q

How long did it take for resistance to Methicillin (Semi-synthetic B-lactam) to appear?

A

Resistance to Methicillin appeared within a few years of its use in Staphylococcus aureus (MRSA).

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5
Q

Why was Vancomycin considered a last resort antibiotic?

A

> Vancomycin was thought to be resistant-proof because resistance would require a massive change in the bacterial cell wall.

> But it was very toxic

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6
Q

What is the state of antibiotic resistance as of 2024?

A

By 2024, penicillin is considered useless, semi-synthetic β-lactams can still be used, and Vancomycin resistance is spreading.

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7
Q

What is MRSA and why is it a major issue?

A

MRSA stands for Methicillin-resistant Staphylococcus aureus, and it is a major issue because it is resistant to all β-lactams.

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8
Q

What antibiotics are MRSA resistant to?

A

MRSA is resistant to penicillins, cephalosporins, carbapenems, penems, and all β-lactam derivatives.

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9
Q

What other resistances does MRSA have?

A

MRSA has the capacity to be resistant to other antibiotics such as erythromycin, tetracycline, streptomycin, and even disinfectants.

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10
Q

Why is it problematic to administer many antibiotics?

A

Many antibiotics can have significant side effects and are not pleasant for patients to take.

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11
Q

How do β-lactams kill Staphylococcus aureus?

A

β-lactams bind to penicillin-binding proteins (PBPs) involved in the final stage of peptidoglycan production where substrate precursors are transglycosylase and transpeptidase work to cross link, which is essential for maintaining cell viability under pressure.

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12
Q

Why are β-lactams effective at killing Staphylococcus aureus?

A

Peptidoglycan synthesis occurs outside the cell membrane, making PBPs accessible targets for β-lactams for gram positive bacteria (not for gram negative due to outer membrane).

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13
Q

What is PBP2a and its significance in methicillin resistance?

A

PBP2a, encoded by the mecA gene, has a low affinity for β-lactams, allowing Staphylococcus aureus to resist these antibiotics.

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14
Q

How does PBP2a function despite the presence of β-lactams?

A

PBP2a works by having a low affinity for β-lactams due to an altered binding site, allowing peptidoglycan cross-linking to continue.

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15
Q

Where did the mecA gene come from, and how did it contribute to resistance?

A

The mecA gene likely originated from an exogenous source, such as Staphylococcus sciuri, and was acquired by horizontal transfer.

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16
Q

What happens during the horizontal transfer of mecA in Staphylococcus aureus?

A

Staphylococcus aureus acquires a 30-50kb mec element flanked by insertion sequences, facilitating the transfer and integration of the mecA gene.

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17
Q

What is the benefit of mecA only being expressed in the presence of antibiotics?

A

As mecA is from a foreign bacteria, it has not evolved to work with Staph A machinery so continuous expression of mecA could be harmful to the bacterium, so it is regulated to be expressed only when antibiotics are present.

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18
Q

What are the two regulatory mechanisms controlling mecA expression?

A

The two regulatory mechanisms are mecI and mecR.

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19
Q

What is the role of mecI in mecA regulation?

A

mecI is a repressor that binds upstream of mecA and blocks its transcription in the absence of β-lactams.

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20
Q

How does mecR regulate mecA expression in response to β-lactams?

A

mecR senses β-lactams, undergoes a conformational change, and hydrolyses mecI, relieving its repression on mecA and allowing mecA expression.

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21
Q

What happens to mecI in the presence of β-lactams?

A

In the presence of β-lactams, mecR hydrolyzes mecI, leading to mecA expression.

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22
Q

Describe the environmental signaling process involving mecR and mecI.

A

mecR senses β-lactams and activates to hydrolyze mecI, allowing mecA expression only when antibiotics are present.

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23
Q

What happens when no β-lactam is present?

A

When no β-lactam is present, mecR is inactive, and mecI binds to mecA, repressing its transcription.

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24
Q

What type of protein is mecR and what does it do?

A

mecR is a metalloprotease activated by β-lactams, which hydrolyzes mecI, leading to mecA transcription.

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25
Q

When were fluoroquinolones introduced and why were they initially effective?

A

Fluoroquinolones were introduced in the mid-1980s. They were initially effective because they were synthetic and bacteria had not encountered them before, so no existing resistance mechanisms had evolved.

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26
Q

What class of antibiotics does Ciprofloxacin belong to and what is its MIC for MRSA?

A

Ciprofloxacin belongs to the fluoroquinolones class, with an MIC (Minimum Inhibitory Concentration) of less than 2 µg/ml against MRSA.

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27
Q

What was Ciprofloxacin (type of fluoroquinolone) used for in MRSA treatment?

A

Ciprofloxacin was used to cure the nasal carrier state of MRSA.

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28
Q

How quickly did MRSA strains develop resistance to Ciprofloxacin (type of fluoroquinolone)?

A

Resistance in MRSA strains to Ciprofloxacin increased from 5% to 80% within one year.

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29
Q

What causes resistance to Ciprofloxacin (type of fluoroquinolone) in MRSA?

A

Resistance is due to point mutations in the DNA gyrase target, specifically altered codon 84 or 85, and can spread via horizontal transfer and evolutionary pressure.

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30
Q

What is the current status of mupirocin in treating infections?

A

Mupirocin is now used, but resistance has appeared, as it does with every antibiotic.

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31
Q

What is the only currently effective drug against MRSA?

A

Vancomycin, a glycopeptide antibiotic, is currently the only effective drug against MRSA.

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32
Q

How does Vancomycin work and why is it effective against Gram-positive bacteria?

A

Vancomycin inhibits peptidoglycan biosynthesis by binding to D-Ala D-Ala at the end of peptide building blocks causing steric hinderance (as vancomycin is so large it stops incorporation into peptidoglycan), effective against Gram-positive bacteria because it targets the cell wall outside the membrane.

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33
Q

How does Vancomycin work in both mature and growing cells?

A

Vancomycin binds to a building block for cell wall growth, target is present in the mature wall too.

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34
Q

What is VISA and how did it arise?

A

VISA (Vancomycin-Intermediate Staphylococcus aureus), now usually called GISA (Glycopeptide-Insensitive Staphylococcus aureus), arose from the use of Vancomycin leading to strains with reduced sensitivity.

35
Q

How can the activity of Vancomycin be inhibited in VISA?

A

Vancomycin activity can be inhibited by changes in permeability (as vancomycin is large, will block it reaching targets) or a thicker cell wall (Vancomycin binds to other cell wall molecules instead of target), making it harder for the antibiotic to reach its target.

36
Q

What are the consequences of altering the cell wall to resist Vancomycin?

A

Small alterations in cell wall permeability or thickness can decrease the effectiveness of Vancomycin, making it less effective within its narrow therapeutic window.

37
Q

What is VRSA and when was it first reported?

A

VRSA (Vancomycin-Resistant Staphylococcus aureus) was first reported in the USA in 2002, caused by acquiring the VanA gene from Enterococcus (co-infected with both)

38
Q

How does the VanA gene confer resistance to Vancomycin?

A

The VanA gene encodes a D-Ala D-Lac ligase, which alters the peptidoglycan structure by replacing D-Ala D-Ala with D-Ala D-Lac, preventing Vancomycin binding.

39
Q

What is the overall difference between phenotypes of VISA and VRSA?

A

VISA is just insensitive to Vancomycin, so just has lesser effect (as target is altered), but VRSA is completely resistant to it (as has no target to even target)

40
Q

Why was there no successful Staph A vaccine despite 3-4 years of effort?

A

The vaccine development efforts failed, but a lot was learned about the disease and the challenges of making vaccines for it.

41
Q

What were the targets for vaccine/antibody therapy against Staph A?

A

The targets included the capsule (CP5, CP8), extracellular polysaccharides, and surface proteins (IsdB).

42
Q

What is a limitation of targeting the capsule for a Staph A vaccine?

A

Not all Staph A strains produce a capsule.

43
Q

Why did three Phase III clinical trials for a Staph A vaccine fail?

A
  1. Clinical trials cost 100 million pounds, and it is difficult to show vaccine efficacy because the most likely candidates (surgery patients) are treated with antibiotics, obscuring results.
  2. As Staph A is a human organism, people already have a high titer of antibodies against it, making additional antibodies from vaccination less effective.
44
Q

What is the challenge in proving the efficacy of a Staph A vaccine in clinical trials?

A

Proving efficacy is difficult because individuals most at risk are typically undergoing surgery and receiving antibiotics, making it hard to determine if the vaccine or antibiotics are preventing infection.

45
Q

Why is making more antibodies against Staph A not a good tactic against infection?

A

People already have high levels of antibodies against Staph A, so additional antibodies from vaccination are unlikely to provide significant additional protection.

46
Q

What are therapeutic antibodies and what is their limitation for Staph A?

A

Therapeutic antibodies can be opsonizing or neutralizing antibodies against Staph A, but they are very expensive and might not reach inside abscesses where Staph A bacteria can reside.

47
Q

How much does it cost to bring one drug to market?

A

It costs $1 billion to bring one drug to market.

48
Q

How long does it typically take to bring a new antibiotic to market?

A

It typically takes 10-12 years to bring a new antibiotic to market.

49
Q

Why does it take a long time to develop new antibiotics?

A

There are hundreds of compounds that kill Staphylococcus aureus in vitro, but it is difficult to ensure they are safe and effective in humans. Most antibiotics fail in clinical trials due to unpredictable factors such as instability in vivo or toxicity.

50
Q

What are the economic challenges in antibiotic development related to resistance?

A

Developing new antibiotics is challenging because resistance will eventually develop, reducing the drug’s effectiveness and market lifespan.

51
Q

Why is the market for broad-spectrum antibiotics preferred over Staph A specific antibiotics?

A

Broad-spectrum antibiotics are preferred because they target many types of bacteria, expanding the market, whereas Staph A specific antibiotics limit the market.

52
Q

What is the issue with antibiotic use in developing countries?

A

Low-income countries (LMICs) have the highest demand for antibiotics but lack the funds to buy them. Therefore, antibiotics need to be seen as a public good that government funds.

53
Q

What is the empirical method in drug discovery?

A

The empirical method involves screening every part of the environment, from mud to water, to find organisms that produce compounds capable of killing pathogens.

54
Q

What is the rational method in drug discovery?

A

The rational method involves targeting existing mechanisms and modifying drugs, such as creating semi-synthetic β-lactams or finding novel targets with new chemistry to avoid preexisting resistance.

55
Q

What are the existing targets for new antibiotics?

A
  1. Analogues of existing drugs.
  2. Understanding the molecular basis of resistance.
56
Q

What are examples of drug analogues stable to degradation?

A

Amikacin is an analogue of kanamycin, which is stable to degradation.

57
Q

How do some drug analogues inhibit inactivating enzymes?

A

Clavulanic acid is an example of a β-lactamase inhibitor that prevents enzyme inactivation of antibiotics.

58
Q

Why are drug analogues that are not effluxed important?

A

They are important because drug efflux is a major type of resistance. An example is glycylcyclines, which are analogues of tetracycline.

59
Q

How do some drug analogues bind to modified targets?

A

Carbapenem derivatives can bind to modified targets such as MecA, which is crucial for resistance in MRSA.

60
Q

What are the 2 new classes of Chemistry used by Novel Drugs against Staph A

A
  1. Daptomycin (Cubicin)
  2. Linezolid (Zyvox)
61
Q

What is the mechanism of action for Daptomycin (Cubicin)?

A

Daptomycin is a cyclic lipopeptide that acts at the membrane of Staph A. It oligomerizes (Ca2+ dependent) in the cytoplasmic membrane, forming a patch where K+ ions are lost from the cell, disrupting K+ homeostasis crucial for life, and causing loss of pH homeostasis and membrane potential adjustments.

62
Q

Why doesn’t Daptomycin work on Gram-negative bacteria?

A

> Gram-negative bacteria have an outer membrane that prevents Daptomycin from getting in.

> The peptidoglycan layer in Gram-positive bacteria is porous enough to allow daptomycin to reach the cytoplasmic membrane.

63
Q

What type of resistance has already appeared for Daptomycin?

A

Resistance in membrane fluidity and structure, despite the bacteria not being in contact with the drug yet.

64
Q

What class of drug is Linezolid (Zyvox)?

A

Linezolid is an Oxazolidinone.

65
Q

How does Linezolid (Zyvox) inhibit bacterial growth?

A

Linezolid binds to the ribosome 50S subunit (this is a novel drug as despite this being a common target, this drug effects it in a new way), preventing the initiation complex formation necessary for translation, thereby inhibiting protein synthesis.

66
Q

What type of resistance has already appeared for Linezolid?

A

Resistance due to mutation in the binding site, despite not being in contact with the drug yet.

67
Q

Why is MRSA resistant to many β-lactam antibiotics?

A

MRSA expresses PBP2a due to the mecA gene. PBP2a has an active site that doesn’t bind β-lactams well, but it has an allosteric site that β-lactams bind to, causing a conformational change and opening the active site.

68
Q

How is the expression of mecA regulated?

A

The mecA gene has transcriptional control by mecR and mecI, and posttranslational control that ensures it is only active in the presence of antibiotics.

69
Q

What are the two novel β-lactam derivative drugs that target PBP2A?

A

Ceftobiprole (Zevtera) and Ceftaroline (Teflaro).

70
Q

How does Ceftobiprole (Zevtera) work?

A

Ceftobiprole is a cephalosporin that binds to the active site of PBP2a.

71
Q

How does Ceftaroline (Teflaro) work?

A

Ceftaroline is a cephalosporin that binds to both the active site and allosteric site of PBP2a, causing a conformational change that opens the active site and allows it to bind, combating PBP2a.

72
Q

What has been observed about resistance to Ceftaroline and how has this helped lab techqniues improve?

A

Resistance to Ceftaroline helps us understand fundamental mechanisms of growth and division, as it shows how peptidoglycan biosynthesis occurs. Knocking out PBPs usually causes immediate death, but knocking out PBP2A causes less resistance, not death, in the presence of antibiotics- can use this as a screen as bacteria don’t die

73
Q

What makes Teixobactin difficult to mass produce?

A

Teixobactin is a complex molecule, which makes it harder to be mass produced.

74
Q

How was Teixobactin discovered?

A

Teixobactin was discovered using the iChip method, which involved taking mud samples from various locations and identifying microbes with antibiotics against Staph A.

75
Q

What is the significance of the iChip method in the discovery of Teixobactin?

A

The iChip method allowed researchers to study soil organisms in the presence of other microorganisms (as in soil all microbes are needed to survive- microorganisms are interdependent), but we could tell which microbe produced what. Helping them identify which microbe produced specific antibiotics.

76
Q

Which microorganism was found using the iChip method and what is its significance?

A

Elephtheria terrae, a Gram-negative proteobacterium, was found using this method. It was isolated and derivatives were plated until finding those that could grow alone.

77
Q

What is the molecular weight and composition of Teixobactin?

A

Teixobactin has a molecular weight of 1,242 Da and contains Depsipeptide (enduracididine, methylphenylalanine, and 4 different D-amino acids).

78
Q

what is a depsipeptide?

A

A depsipeptide is a type of peptide in which one or more of the peptide bonds (-CONH-) are replaced by ester bonds (-COO-). The presence of ester bonds instead of some of the usual amide bonds in the peptide backbone can affect the molecule’s stability, solubility, and interaction with biological targets, often enhancing its biological activity.

79
Q

How is Teixobactin synthesized? (extra reading Ling et al (2015))

A

Teixobactin is non-ribosomally encoded and synthesized by two peptide synthetase proteins encoded by txo1 and txo2, which assemble the molecule by linking specific amino acids together in sequence without the need for ribosomes.

80
Q

What is the activity of Teixobactin against bacteria?

A

Teixobactin effectively kills bacteria and showed no resistance in the specific methodology used during the study.

81
Q

Why does Teixobactin not induce resistance?

A

Teixobactin has two activities that inhibit cell wall synthesis, making it harder for bacteria to evolve resistance.

82
Q

What are the specific targets of Teixobactin in bacterial cell wall synthesis?

A

Teixobactin binds to Lipid II, preventing its incorporation into peptidoglycan, and binds to Lipid III, preventing the biosynthesis of teichoic acid.

> By binding to Lipid II, Teixobactin prevents it from being incorporated into peptidoglycan, thus inhibiting cell wall synthesis.

> By binding to Lipid III, Teixobactin prevents the biosynthesis of teichoic acid (important polymers in the cell wall, especially in host-pathogen interactions, and cell division/ cell morphology))

83
Q

Why might Teixobactin not reach clinical use despite its efficacy?

A

Teixobactin is likely too complex a molecule to be easily produced on a clinical scale, but being the first of its class to have two activities/targets more antibiotics can be modelled off of this.

Bacterial Pathogenicity (11 decks)

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