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MedEd + PathSoc + Laz's Qs + Romesh Path > Micro 20: Antimicrobial agents 1 > Flashcards

Micro 20: Antimicrobial agents 1 Flashcards

1
Q

What are the 3 main targets for antibiotics?

A

Peptidoglycan layer of cell wall

Inhibition of bacterial protein synthesis

DNA gyrase and other prokaryote-specific enzymes

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2
Q

Difference in structure of gram +ve v gram -ve bacteria

A

Gram +ve:
- Thick peptidoglycan cell wall on outside

Gram -ve:

  • Thinner peptidoglycan cell wall
  • Has an outer membrane which prevents some abx from working on the cell wall
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3
Q

Name two groups of antibiotics that inhibit cell wall synthesis (peptidoglycan layer of cell wall) + group of abx in each / examples

A

Beta-lactams:

  • Penicillins
  • Cephalosporins
  • Carbapenems

Glycopeptides:
- Vancomycin
- Tiecoplanin
(these are abx not groups like in b-lactams)

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4
Q

MoA of b-lactams?

A

Beta-lactam is a structural analogue of the enzyme substrate

Inhibits transpeptidases (aka penicillin-binding protein)

Inactivate enzymes that are involved in the terminal stages of cell wall synthesis

This means that there are no peptide crosslinks between peptidoglycan chains so the cell wall is weak

This causes bacteria to burst and die due to osmotic lysis (from osmotic pressure)

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5
Q

Limitations of b-lactams?

A

They work on the stationary phase of the cell cycle - only effective against rapidly dividing bacteria

They are also ineffective against bacteria w/ no cell wall - intracellular pathogens (eg. Mycoplasma & chlamydia)

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6
Q

4 types of penicilins?

A

Penicillin
Amoxicillin
Flucloxacillin
Piperacillin

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7
Q

Describe the coverage and mechanism of resistance of penicillin?

A

Active against Gram-positives (e.g. Streptococci, Clostridia)

Broken down by beta-lactamases (mainly produced by S. aureus)

NOTE: penicillin is the MOST ACTIVE beta-lactam antibiotic

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8
Q

Describe the coverage, mechanism of resistance and limitations of Amoxicillin

A

Broad-spectrum penicillin

Extends coverage to Enterococci and Gram-negative organisms

Broken down by beta-lactamase produced by S. aureus and many Gram-negatives

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9
Q

Describe the coverage, mechanism of resistance and limitations of Piperacillin

A

Similar to amoxicillin

Extends coverage to Pseudomonas and other non-enteric Gram-negative organisms

Broken down by beta-lactamase produced by S. aureus and many Gram-negatives

Note: P for Piperacillin, P for Pseudomonas

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10
Q

Describe the coverage and limitations of Flucloxacillin

A

Similar to penicillin but less active

Does NOT get broken down by beta-lactamase produced by S. aureus

  • This is why S. aureus is treated w/ flucloxacillin
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11
Q

Name two beta-lactamase inhibitors.

What is the benefit of giving beta-lactamase inhibitors with beta-lactams?

A

Clavulanic acid
Tazobactam

Protect penicillins from breakdown by beta-lactamases thereby increasing the coverage to include S. aureus, Gram-negatives and anaerobes

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12
Q

What is co-amoxiclav?

A

Amoxicillin + Clavulanic Acid

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13
Q

Examples of 1st, 2nd and 3rd gen cephalosporins?

What is the difference between different generations?

A

1st = Cephalexin

2nd = Cefuroxime

3rd = Cefotaxime, Ceftriaxone, Ceftazidime

NOTE: as you go up the generations you get increasing activity against Gram-negatives and less activity against Gram-positives

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14
Q

Benefit of cephalosporins v penicillins?

A

Stable to many b-lactams produced by gram -ve bacterias

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15
Q

Coverage of cefuroxime (2nd gen cephalosporin)

A

Stable to many b-lactamases made by gram -ves

Similar cover to co-amoxiclav

But less active against anaerobes -> combine w/ metronidazole to cover anaerobes

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16
Q

What cephalosporin is the mainstay of bacterial meningitis mx? Why?

What adverse reaction is this associated w/?

A

Ceftriaxone (3rd gen cephalosporin) - This is excreted less than other b-lactams hence maintains conc in CNS

Associated w/ C.Diff infection

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17
Q

What cephalosporin provides good cover against Pseudomonas?

A

Ceftazidime (3rd gen cephalosporin)

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18
Q

What are ESBLs?

A

Type of beta-lactamase that also breaks down cephalosporins as well as penicillins

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19
Q

List three examples of carbapenems

A

Meropenem (Broadest + anti-pseudomonal activity)

Imipenem (Similar to meropenem, can cause renal failure + seizures - give cilastatin)

Ertapenem (not as broad as others, doesnt cover pseudo or enterococci)

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20
Q

What is the main benefit of carbapenems?

A

Stable to EBSL enzymes

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21
Q

Outline the key features of beta-lactam antibiotics (5)

A

Relatively non-toxic

Renally excreted (reduced dose in renal impairment)

Short half-life

Will not cross an intact blood-brain barrier (may cross inflamed meninges in meningitis)

Cross allergenic (penicillins have 5-10% cross-reactivity with cephalosporins and carbapenems)

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22
Q

List examples of bacteria that have shown carbapenem resistance and how?

A

Acinetobacter and Klebsiella

Presence of carbapenemase enzyme

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23
Q

Examples of glycopeptide ABX?

A

Vancomycin

Teicoplanin

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24
Q

MoA of Glycopeptides?

A

Glycopeptides bind to amino acid chains at the end of peptidoglycan precursors

This prevents glycosidic bonds being formed (via transglycosidase) and prevents peptide crosslinks being formed (via transpeptidase)

NOTE: they are similar to beta-lactams but instead of binding to the enzymes, they bind to substrates (cell wall component precursors)

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25
What are glycopeptides often used to treat?
Serious MRSA infections C. difficile infections (oral vancomycin)
26
Which type of bacteria are glycopeptides effective against and why?
Gram-positives They are large molecules so they cannot cross the outer membrane of Gram-negative cell walls
27
Major side effect of glycopeptides? prevention?
Nephrotoxic - monitor blood levels to prevent accumulation
28
How are Bacterial ribosome different to human ribosomes?
Both have 2 subunits but bacterial is smaller: - Human = 60S + 40S - Bacterial = 50S + 30S
29
What are some antibiotics that work by inhibiting protein synthesis?
ATMCO ``` Aminoglycosides - 30S subunit Tetracyclines - 30S subunit MLS Group (Macrolides, Lincosamides, Streptogramins) - 50S subunit Chloamphenicol - 50S subunit Oxazolidinones - 23S subunit of 50S ```
30
MoA of aminoglycosides? Which class of antibiotics can be used in combination to produce a synergistic effect?
Binds to amino-acyl site of the 30S ribosomal subunit and prevents elongation of the polypeptide chain It also causes misreading of the codons along the mRNA -------------------------------------- B-lactams (eg. in endocarditis)
31
Major side effect of aminoglycosides? prevetion?
Ototoxic and nephrotoxic - monitor levels
32
Limitations of aminoglycosides?
No activity against anaerobes Inhibited by low pH - won't work in abscesses
33
Examples of Aminoglycosides?
Gentamicin Amikacin Tobramycin
34
What are tetracyclines? activity against what? Examples of tetracyclines?
Broad-spectrum agents with activity against intracellular pathogens - no cell wall (e.g. Chlamydiae, Rickettsiae and Mycoplasmas) as well as most conventional bacteria Doxycycline Lymecycline
35
MoA of tetracyclines?
Binds to the ribosomal 30S subunit and prevents the binding of aminoacyl-tRNA to the ribosomal acceptor site, thereby inhibiting protein synthesis
36
Major issue w/ tetracycline use in terms of effectiveness? How has this been overcome?
Most gram -ves show widespread resistance Tigacycline - broadened spectrum of tetracyclines
37
Characteristic side effect of tetracyclines? Who should not recieve tetracycline treatment and why?
Light sensitive rash Children and pregnant women - Because it can deposit in bone and cause discoloration of growing teeth
38
Examples of macrolides?
Erythromycin Clarithromycin Azithromycin
39
MoA of macrolides? Limitation
Binds to the 50S ribosomal subunit and interferes with translation Also stimulates the dissociation of peptidyl-tRNA Limited activity against gram -ves due to outer membrane (generally gram -ve are macrolide resistant)
40
What does the MLS group of abx consist of + examples of each?
Macrolides - Azithromycin Lincosamides - Clindamycin Streptogramins - Synercid
41
MoA of Oxazolidinones + example?
Binds to the 23S components of the 50S subunit to prevent the formation of a functional 70S initiation complex (needed for translation) Linezolid
42
What are Oxazolidinones active against? Drawbacks of their use?
Highly active against gram-positives (including MRSA and VRE) Not active against Gram-negatives Expensive + may cause optic neuritis, thrombocytopaenia and anaemia
43
What types of abx inhibit DNA synthesis + examples of each?
Quinolones: Ciprofloxacin Moxifloxacin Levofloxacin Nitroimidazoles: Metronidazole Tinidazole
44
MoA of Quinolones?
Acts on the alpha-subunit of DNA gyrase predominantly with other actions
45
What are quinolones active against?
Broad antibacterial activity, especially against Gram-negatives, including Pseudomonas aeruginosa NOTE: newer agents (levofloxacin, moxifloxacin) increased activity against Gram-negatives and intracellular organisms (eg chlamydia)
46
Uses of quinolones? - think conditions (not organisms) Limitations? (reasons why use has decreased)
UTI Pneumonia Atypical pneumonia Bacterial gastroenteritis Use has decreased due to resistance against E.Coli and side effects (tinnitus) Note - well absorbed after oral administration
47
MoA of nitroimidazoles? Coverage?
Under anaerobic conditions, an active intermediate is formed, which causes DNA strand breakage Only active against anaerobic bacteria and protozoa (e.g. Giardia) - low resistance against anaerobes
48
What is a related compound to nitroimidazoles + what are their advantages? R
Nitrofurans (eg. Nitrofurantoin) Benefits: - Concentrates in bladder - hence doesnt cause C. Diff - Active against lots of EBSL organisms - not inhibited by mechanisms of resistance
49
What drugs are inhibitors of bacterial RNA synthesis? | Examples?
Rifamycins such as: Rifampicin Rifabutin
50
Main activity of rifampicin? MoA?
Mainly Mycobacteria and Chlamydiae Inhibits protein synthesis by binding to DNA-dependent RNA polymerase thereby inhibiting initiation
51
Side effects of Rifampicin?
Inducer of CYP450 - important to monitor interactions (OCP, warfarin) + LFTs May turn urine + contact lenses orange
52
Can rifampicin be used as single agent? Why?
No, rifampicin should never be used alone Resistance develops rapidly due to chromosomal mutation (single amino acid change in beta-subunit of RNA polymerase)
53
Name two cell membrane toxins
Daptomycin Colistin
54
Describe the activity of: Daptomycin Colistin +specific organisms?
Daptomycin - Gram-positives - Likely to be used in treating MRSA and VRE NOTE: it is a cyclic lipopeptide Colistin - Active against Gram-negatives including Pseudomonas aeruginosa, Acinetobacter baumanii and Klebsiella pneumoniae NOTE: it is a polymyxin
55
Name two families of antibiotics that work by inhibiting folate metabolism + examples? MoA?
Sulphonamides - sulphamethoxazole Diaminopyrimidines - trimethoprim Act indirectly on DNA through interference with folic acid metabolism NOTE: they act on sequential stages in same pathway so have synergistic action
56
What is co-trimoxazole? why is this used and when?
Sulphamethoxazole + trimethoprim Sulphonamide RESISTANCE is common - hence extends coverage Useful for PCP (immunocompromised), HAP (less likely to cause C. Diff)
57
What is used to treat community-accquried UTIs?
Trimethoprim
58
List some mechanisms of antibiotic resistance.
INACTIVATION - Chemical modification or inactivation of the drug ALTERED TARGET - Modification or replacement of the target REDUCED ACCUMULATION - Reduced antibiotic accumulation (impaired uptake or enhanced efflux) BYPASS - Bypass antibiotic-sensitive step in cell division
59
Which bacteria produce beta-lactamases?
S. aureus and Gram-negative bacilli (coliforms) NOTE: this is not the mechanism of resistance in pneumococcus and MRSA
60
In which groups of bacteria is penicillin resistance not reported in?
Group A, B, C and G beta-haemolytic streptococci
61
Describe how MRSA uses ‘altered targets’ as a mechanism of resistance.
MRSA has a mecA gene which encodes novel PBP2a (peptidoglycan transpeptidase) This has a low affinity for binding beta-lactams therefore is not inactivated by beta-lactams
62
Describe the mechanism of resistance in Streptococcus pyogenes.
Results from acquisition of a series of stepwise mutations in PBP genes Lower level resistance can be overcome by increasing the dose
63
What are extended spectrum beta-lactamases? Where is this more common? Advice?
Able to breakdown cephalosporins as well as penicillins Becoming more common in E. coli and Klebsiella NOTE: if there is > 10% resistance then empirical therapy is not advised
64
What are AmpC beta-lactamases?
Breakdown penicillins and cephalosporins but are not inhibited by clavulanic acid
65
MoA of resistance to Macrolides?
Adenine-N6 methyltransferase modifies the 23S RNA This reduces the binding of macrolides thereby resulting in resistance Encoded by erm (erythromycin ribosome methylation) genes NOTE: caution when using clindamycin in Staphylococcus and Streptococcus which is resistant to macrolides because lincosamides can induce this mechanism of resistance
66
Which abx have resistance due to chemical modification or inactivation of the drug ? - INACTIVATION
B-Lactams Aminoglycosides Chlroamphenical
67
Which abx have resistance due to modification or replacement of the target ? ALTERED TARGET
``` B-Lactams Glycopeptides Macrolides Chloramphenicol Linezolid Quinolones Rifampicin ```
68
Which abx have resistance due to reduced antibiotic accumulation (impaired uptake or enhanced efflux) ?- REDUCED ACCUMULATION
``` B-lactams Aminoglycosides Tetracyclines Chloramphenicol Quinolones ```
69
Which abx have resistance due to bypass antibiotic-sensitive step in cell division ?
Trimethoprim | Sulphonamides
70
What does bactericidal and bacterostatic mean?
Bactericidal - Kills bacteria Bacteriostatic - Stops bacterial growth
71
Which classes of Abx are bactericidal and bacteriostatic?
Bactericidal: - Anything that doesn't work via inhibition of protein synthesis - Protein synthesis abx = Aminoglycosides + Oxazolidinones (against strep) Bacteriostatic: - Only protein synthesis abx - Except aminoglycosides + oxazolidinones (work for staph + enterococci)
72
What abx work against Gram +ve? What work against gram -ve?
Gram +ve: - B-lactams (penicillins, cephalosporins, carbapenems) - Glycopeptides - Macrolides - useful in penicillin allergy - Oxazolidinones Gram -ves: - Aminoglycosides - Fluroquinolones
73
MoA of Chloramphenicol?
- Chloramphenicol binds to the **peptidyl transferase of the 50S ribosomal subunit** - And inhibits the formation of peptide bonds during translation

MedEd + PathSoc + Laz's Qs + Romesh Path (99 decks)

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