Antimicrobial Agents 1

Question 1

What are the inhibitors of cell wall synthesis?

Answer 1

Beta-lactam antibiotics - Penicillins, cephalosporins and carbapenems

Glycopeptides - Vancomycin and Teicoplanin

Question 2

What is the difference between Gram positive and Gram negative bacteria?

Answer 2

Gram positive - peptidoglycan cell wall, cytoplasmic membrane

Gram negative - outer membrane, peptidoglycan cell wall, cytoplasmic membrane

Question 3

What are the features of beta-lactam antibiotics?

Answer 3

Inactivate the enzymes that are involved in the terminal stages of cell wall synthesis (transpeptidases also known as penicillin binding proteins) – β-lactam is a structural analogue of the enzyme substrate

Bactericidal

Active against rapidly-dividing bacteria

Ineffective against bacteria that lack peptidoglycan cell walls (e.g. Mycoplasma or Chlamydia)

Question 4

What are the common penicillins?

Answer 4

Penicillin - Gram positive organisms, Streptococci, Clostridia
Broken down by an enzyme (β-lactamase) produced by S. aureus

Amoxicillin – Broad spectrum penicillin, extends coverage to Enterococci and Gram negative organisms
Broken down by β-lactamase produced by S. aureus and many Gram negative organisms

Flucloxacillin- Similar to penicillin although less active.
Stable to β-lactamase produced by S. aureus.

Piperacillin – similar to amoxicillin, extends coverage to Pseudomonas and other non-enteric Gram negatives
Broken down by β-lactamase produced by S. aureus and many Gram negative organisms.

Question 5

What are some adjuncts for penicillins?

Answer 5

Clavulanic acid and tazobactam

β-lactamase inhibitors.

Protect penicillins from enzymatic breakdown and increase coverage to include S. aureus, Gram negatives and anaerobes.

Question 6

What are some examples of cephalosporins?

Answer 6

First generation: Cephalexin

Second generation: Cefuroxime
Stable to many β-lactamases produced by Gram negatives. Similar cover to co-amoxiclav but less active against anaerobes.

Third generation: Cefotaxime, Ceftriaxone (associated with C.diff), Ceftazidime (anti-Pseudomonas)

These increase in activity against Gram negative bacteria as you progress through the generations.

Question 7

What is resistant to cephalosporins regardless of in vitro results?

Answer 7

Extended Spectrum β-lactamase (ESBL) producing organisms

Question 8

What are the carbapenems?

Answer 8

Stable to Extended Spectrum β-lactamase (ESBL) enzymes

Meropenem, Imipenem, Ertapenem

Carbapenemase enzymes becoming more widespread. Multi drug resistant Acinetobacter and Klebsiella species.

Question 9

What are the pharmacodynamics and pharmacokinetics of beta-lactams?

Answer 9

Relatively non-toxic

Renally excreted (so ↓dose if renal impairment)

Short half life

Will not cross intact blood-brain barrier

Cross-allergenic (penicillins approx 10% cross-reactivity with cephalosporins or carbapenems)

Question 10

What are the pharmacodynamics and pharmacokinetics of glycopeptides?

Answer 10

Large molecules, unable to penetrate Gram –ve outer cell wall; active against Gram +ve organisms

Inhibit cell wall synthesis

Important for treating serious MRSA infections (iv only)

Oral vancomycin can be used to treat serious C. difficile infection

Vancomycin and Teicoplanin are examples of glycopeptides

Slowly bactericidal

Nephrotoxic – hence important to monitor drug levels to prevent accumulation

Question 11

What are inhibitors of protein synthesis?

Answer 11

Aminoglycosides (e.g. gentamicin, amikacin,tobramycin)

Tetracyclines

Macrolides (e.g. erythromycin) / Lincosamides (clindamycin) / Streptogramins (Synercid) – The MSL group

Chloramphenicol

Oxazolidinones (e.g. Linezolid)

Question 12

What are the features of aminoglycosides?

Answer 12

Bind to amino-acyl site of the 30S ribosomal subunit
Rapid, concentration-dependent bactericidal action
Require specific transport mechanisms to enter cells (accounts for some intrinsic R)

Ototoxic & nephrotoxic, therefore must monitor levels

Gentamicin & tobramycin particularly active vs. Ps. aeruginosa
Synergistic combination with beta-lactams

No activity vs. anaerobes

Question 13

In summary, how do aminoglycosides work?

Answer 13

Prevent elongation of the polypeptide chain

Cause misreading of the codons along the mRNA

Question 14

What are tetracyclines and when are they used?

Answer 14

Broad-spectrum agents with activity against intracellular pathogens (e.g. chlamydiae, rickettsiae & mycoplasmas) as well as most conventional bacteria

Bacteriostatic

Widespread resistance limits usefulness to certain defined situations

Do not give to children or pregnant women

Light-sensitive rash

Question 15

How do tetracyclines work?

Answer 15

Reversibly bind to the ribosomal 30S subunit

Prevent binding of aminoacyl-tRNA to the ribosomal acceptor site, so inhibiting protein synthesis.

Question 16

What are macrolides and how do they work?

Answer 16

Bacteriostatic

Minimal activity against Gram –ve bacteria

Useful agent for treating mild Staphylococcal or Streptococcal infections in penicillin-allergic patients

Also active against Campylobacter sp and Legionella. Pneumophila

Newer agents include clarithromycin & azithromycin with improved pharmacological properties

Question 17

How do macrolides work?

Answer 17

Bind to the 50s subunit of the ribosome, interfering with translocation, thus stimulating dissociation of peptidyl-tRNA

Question 18

What are the features of chloramphenicols?

Answer 18

Bacteriostatic

Very broad antibacterial activity

Rarely used (apart from eye preparations and special indications) because risk of aplastic anaemia (1/25,000 – 1/45,000 patients) and grey baby syndrome in neonates because of an inability to metabolise the drug

Question 19

How do chloramphenicols work?

Answer 19

Chloramphenicol binds to the peptidyl transferase of the 50S ribosomal subunit and inhibits the formation of peptide bonds during translation

Question 20

How do oxazolidinones (Linezolid) work?

Answer 20

Binds to the 23S component of the 50S subunit to prevent the formation of a functional 70S initiation complex (required for the translation process to occur).

Question 21

When is linezolid used?

Answer 21

Highly active against Gram positive organisms, including MRSA and VRE. Not active against most Gram negatives.

Is expensive, may cause thrombocytopoenia and should be used only with consultant Micro/ID approval

Question 22

What are some examples of DNA synthesis?

Answer 22

Quinolones e.g. Ciprofloxacin, Levofloxacin, Moxifloxacin

Nitroimidazoles e.g. Metronidazole & Tinidazole

Question 23

How do Fluoroquinolones work?

Answer 23

Act on beta-subunit of DNA gyrase predominantly, but, together with other antibacterial actions, are essentially bactericidal

Broad antibacterial activity, especially vs Gram –ve organisms, including Pseudomonas aeruginosa

Newer agents (e.g. levofloxacin, moxifloxacin) have increased activity vs G +ves and intracellular bacteria, e.g. Chlamydia spp

Question 24

When are fluoroquinolones used?

Answer 24

UTIs

Pneumonia

Atypical pneumonia

Bacterial gastroenteritis

Question 25

How do Nitroimidazoles work?

Answer 25

Include the antimicrobial agents metronidazole & tinidazole

Under anaerobic conditions, an active intermediate is produced which causes DNA strand breakage

Rapidly bactericidal

Active against anaerobic bacteria and protozoa (e.g. Giardia)

Nitrofurans are related compounds: nitrofurantoin is useful for treating simple UTIs

Question 26

What are inhibitors of RNA synthesis?

Answer 26

Rifamycins, e.g. rifampicin & rifabutin

Question 27

How do rifampicins work?

Answer 27

Inhibits protein synthesis by binding to DNA-dependent RNA polymerase thereby inhibiting initiation

Bactericidal

Active against certain bacteria, including Mycobacteria & Chlamydiae

Monitor LFTs

Beware of interactions with other drugs that are metabolised in the liver (e.g oral contraceptives)

May turn urine (& contact lenses) orange

Question 28

Why should you never use rifampicins as a single agent?

Answer 28

Except for short-term prophylaxis (vs. meningococcol infection) you should NEVER use as single agent because resistance develops rapidly

Resistance is due to chromosomal mutation.

This causes a single amino acid change in the ß subunit of RNA polymerase which then fails to bind Rifampicin.

Question 29

What is daptomycin and when is it used?

Answer 29

A cyclic lipopeptide with activity limited to G+ve pathogens. It is a recently-licenced antibiotic likely to be used for treating MRSA and VRE infections as an alternative to linezolid and Synercid

Question 30

What is colistin and when is it used?

Answer 30

A polymyxin antibiotic that is active against Gram negative organisms, including Pseudomonas aeruginosa, Acinetobacter baumannii and Klebsiella. pneumoniae.

It is not absorbed by mouth. It is nephrotoxic and should be reserved for use against multi-resistant organisms

Question 31

What are inhibitors of folate metabolites?

Answer 31

Sulfonamides

Diaminopyrimidines (e.g. trimethoprim)

Question 32

How do sulphonamides and diaminopyrimidines work?

Answer 32

Act indirectly on DNA through interference with folic acid metabolism

Synergistic action between the two drug classes because they act on sequential stages in the same pathway

Sulphonamide resistance is common, but the combination of sulphamethoxazole+trimethoprim (Co-trimoxazole) is a valuable antimicrobial in certain situations (e.g. Treating Pneumocystis. jiroveci pneumonia)

Trimethoprim is used for Rx community-acquired UTIs

Question 33

What are the common mechanisms of resistance?

Answer 33

Chemical modification or inactivation of the antibiotic

Modification or replacement of target

Reduced antibiotic accumulation (impaired uptake or enhanced efflux)

Bypass antibiotic sensitive step

Question 34

Which antibiotics are resisted through inactivation?

Answer 34

Penicillins

Aminoglycosides

Chloramphenicols

Question 35

Which antibiotics are resisted through altered target sites?

Answer 35

Beta lactams
Macrolides
Quinolones
Rifampicin
Chloramphenicol
Linezolid
Glycopeptides

Question 36

Which antibiotics are resisted through reduced accumulation?

Answer 36

Tetracylcines
Beta lactams
Aminoglycosides
Quinolones
Chloramphenicols

Question 37

Which antibiotics are resisted through bypass?

Answer 37

Trimethoprim
Sulphonamides

Question 38

Which pathogens inactive beta lactams?

Answer 38

ß Lactamases are a major mechanism of resistance to ß Lactam antibiotics in Staphylococcus aureus and Gram Negative Bacilli (Coliforms).

Question 39

Which pathogens alter target site for beta lactams?

Answer 39

Methicillin Resistant Staphylococcus aureus (MRSA)

mecA gene encodes a novel PBP (2a).
Low affinity for binding ß Lactams.
Substitutes for the essential functions of high affinity PBPs at otherwise lethal concentrations of antibiotic.

Streptococcus pneumoniae

Penicillin resistance is the result of the acquisition of a series of stepwise mutations in PBP genes. Lower level resistance can be overcome by increasing the dose of penicillin used.