Microbio - Antimicrobials 1

Question 1

What are the three broad targets for antibiotics

Answer 1

Peptidoglycan layer of the cell wall
Inhibition of bacterial protein synthesis
DNA gyrase and other prokaryote-specific enzymes

Question 2

Which antibiotic classes are inhibitors of cell wall synthesis

Answer 2

Beta-lactam antibiotics e.g. penicillins, cephalosporins, carbapenems
Glycopeptides e.g. vancomycin and teicoplanin

Question 3

What is the difference between gram positive and gram negative bacteria

Answer 3

Gram positive - thick peptidoglycan wall, stains purple
Gram negative - thin peptidoglycan wall and an extra outer membrane, stains pink

Question 4

What is the MOA for beta-lactams

Answer 4

Structural analogue that inactivates transpeptidase enzymes (involved in forming crosslinks between peptidoglycans in the cell wall) → weakened cell wall → osmotic lysis during division

Question 5

In what situation are beta-lactams ineffective

Answer 5

They are only bactericidal against rapidly-dividing bacteria, so are not effective when they are not dividing
e.g. Biofilms, abscesses

Ineffective against bacteria lacking a peptidoglycan cell wall e.g. mycoplasma, chlamydia

Question 6

What are the following features of beta lactams (toxicity, safety in pregnancy, excretion, half life, ability to cross the BBB, cross-reactivity)

Answer 6

Relatively non-toxic, safe in pregnancy
Renally excreted, so may need to reduce the dose if there is renal impairment
Short half life
Will not cross an intact blood-brain barrier, but will cross the meninges and therefore can be used in meningitis
Cross-allergenic (Penicillin approx. 10 % cross reactivity with cephalosporins or carbapenems)

Question 7

What is the coverage for penicillin and what is the mechanism of resistance against it

Answer 7

Gram positive organisms, Streptococci, Clostridia
Broken down by an enzyme (β-lactamase)
produced by S. aureus (although most S. aureus is resistant to penicillin)

Question 8

What is the coverage for amoxicillin and what is the mechanism of resistance against it

Answer 8

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

Question 9

What is the coverage for flucloxacillin

Answer 9

Similar to penicillin although less active.
Stable to β-lactamase produced by S. aureus. Mainstay treatment for S. aureus

Question 10

What is the coverage for piperacillin and what is the mechanism of resistance against it

Answer 10

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 11

What are clavulanic acid and tazobactam and what do they cover

Answer 11

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

Question 12

What do cephalosporins cover (separated by generation)

Answer 12

First
Cephalexin: E. Coli

Second
Cefuroxime: Klebsiella, GI infections

Third
Cefotaxime: sepsis in neonates
Ceftriaxone: Haemophilus and meningococcus meningitis
Ceftazidime: Pseudomonas

Question 13

Which cephalosporin is associated with C. difficile

Answer 13

Ceftriaxone

Question 14

What do carbapenems cover

Answer 14

Stable to ESBL (but becoming widespread → multi drug resistance)

Meropenem: broad-spectrum, S. Aureus, streptococcus, pseudomonas, ESBLs
Ertapenem: broad spectrum, ESBLs

Question 15

How do the properties of antibiotic change with each generation of cephalosporin

Answer 15

Activity against gram negative bacilli increases with each generation
Activity against E. Coli decreases with each

Question 16

What are the mechanisms of resistance against beta-lactam antibiotics

Answer 16

Beta lactamases
Extended spectrum beta-lactamases (ESBL)
Altered target

Question 17

Describe the beta lactamase mechanism of resistance

Answer 17

Beta lactamases are enzymes produced by bacteria such as S.aureus and gram -ve bacilli (Coliforms)
Separated in to 4 groups: broad-spectrum, extended-spectrum, AmpC, carbapenemase (not inhibited by beta-lactamase inhibitors)

It is NOT the mechanism of resistance in penicillin resistance pneumococci and MRSA

Question 18

Describe the altered target mechanism of resistance to beta-lactams

Answer 18

MRSA: mecA gene which encodes a novel transpeptidase (PBP) (2a), which has a low affinity for binding beta lactams

Streptococcus pneumoniae: acquisition of a series of stepwise mutations in PBP genes.

Question 19

What are ESBLs and what do they confer resistance to + what organisms are they seen in

Answer 19

Extended spectrum beta-lactamases
Resistance to cephalosporins
More common in E. coli and klebsiella

Question 20

Describe glycopeptides and what they can be used for

Answer 20

Active against Gram +ve organisms (NOT gram -ve - unable to penetrate outer wall)
Inhibit cell wall synthesis by binding to the peptide cross link that comes out of the peptide precursors → blocks transpeptidase from forming the links

Important for treating serious MRSA infections (IV only)
Vancomycin - C. difficile

Question 21

Give examples of antibiotics classes that inhibit protein synthesis

Answer 21

Aminoglycosides e.g. gentamicin, amikacin, tobramycin
Tetracyclines
Macrolides e.g. erythromycin/lincosamides (clindamycin)
Chloramphenicol
Oxazolidinones e.g. Linezolid

Question 22

What is the MOA for aminoglycosides and what side effect do clinicians worry about

Answer 22

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 - must monitor levels
No activity against anaerobes

Question 23

What is the MOA for Tetracyclines, what do they cover, and what is a common side effect

Answer 23

Bacteriostatic - Binds to the ribosome and prevents binding of tRNA
Broad-spectrum agents with activity against intracellular pathogens (e.g. chlamydiae, rickettsiae & mycoplasmas) as well as most conventional bacteria
Useful for MRSA

SE: light-sensitive rash, teratogenicity (do not give to children or pregnant women)

Question 24

What is the MOA for Macrolides, what do they cover

Answer 24

Bacteriostatic
Binds to the 50S subunit of the ribosome
Pencillin allergic0: Useful for staphylococcal or streptococcal infection
Campylobacter and legionella

Question 25

What is the mechanism of resistance against macrolides

Answer 25

Altered targets encoded by erythromycin ribosome methylation (erm) gene
Modification of the 23s rRNA that reduces binding of antibiotics → resistance

Same resistance fo clindamycin - be cautious using where there is known macrolide resistance

Question 26

What is the MOA for chloramphenicol, what do they cover, and what is a common side effect

Answer 26

Bacteriostatic
Binds to 50S ribosome and inhibits peptide bond formation

very broad activity, though rarely used
Used in eye preparations

SE: aplastic anaemia, grey baby syndrome

Question 27

What is the MOA for Oxazolidinone (linezolid), what do they cover, and what is a common side effect

Answer 27

Binds to the 23S component of the 50S subunit, prevents formation of a functional 70S initiation complex

Highly active against Gram +ve organisms, including MRSA and VRE

SE: thrombocytopenia

Question 28

Which antibiotic classes are inhibitors of DNA synthesis

Answer 28

Quinolones e.g. ciprofloxacin, levofloxacin
Nitroimidazoles e.g. metronidazole, tinidazole

Question 29

What is the MOA for Fluoroquinolones, what do they cover

Answer 29

Act on 𝛼-subunit of DNA gyrase predominantly
Bactericidal

Broad antibacterial activity, especially vs Gram –ve organisms, including Pseudomonas aeruginosa
Newer agents (e.g. levofloxacin, moxifloxacin) increased activity vs G +ves and intracellular bacteria, e.g. Chlamydia spp
Use for UTIs, pneumonia, atypical pneumonia & bacterial gastroenteritis

Question 30

What is the MOA for Nitroimidazoles, what do they cover

Answer 30

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 31

Which antibiotics are inhibitors of RNA synthesis

Answer 31

Rifamycins e.g. rifampicin, rifabutin
Sulfonamides - used for UTIs

Question 32

What is the MOA for Rifampicin, what do they cover, and what is a common side effect

Answer 32

Binds to DNA-dependent RNA polymerase thereby inhibiting initiation
Bactericidal

Active against Mycobacteria, chlamydia

SE: hepatotoxicity (monitor LFTs), turns urine and contact lenses orange, high risk of resistance in monotherapy (always use in combination)

Question 33

What is the most common mechanism of resistance against rifampicin

Answer 33

Chromosomal mutation - a single amino acid change in the beta-unit of RNA polymerase, which fails to bind rifampicin.

Question 34

What is daptomycin and what does it cover

Answer 34

Cell membrane toxin, a cyclic lipopeptide

MRSA, VRE, gram +

Question 35

What is colistin and what does it cover

Answer 35

Polymyxin cell membrane toxin

Gram negative, pseudomonas, acinetobacter, klebsiella

Question 36

How to inhibitors of Folate metabolism work and give examples

Answer 36

Acts directly on DNA through interference with folic acid metabolism
Sulphonamide, trimethoprim (UTI)

Question 37

When are sulphonamides used

Answer 37

As a combination - Co-trimoxazole
Used for PCP

Question 38

What are the four main mechanisms of resistance and give examples of 2 ABx that they work against

Answer 38

Chemical modification or inactivation of the antibiotic - beta-lactams, aminoglycosides
Modification or replacement of target - beta lactams, macrolides
Reduced antibiotic accumulation - tetracyclines, aminoglycosides
- Impaired uptake
- Enhanced efflux
Bypass antibiotic sensitive step - trimethoprim, sulphonamides