Antibacterials & Resistance

Question 1

What are the definitions for the following terms; antibacterial, antibiotic, antimicrobial?

Answer 1

antibacterial - a substance that kills bacteria, or inhibits their growth

antibiotic - substances produced by living organisms capable of destroying or inhibiting the growth of micro-organisms

antimicrobial - a substance that kills microorganisms or inhibits their growth. Includes antivirals, antifungals, and antiparasitics, as well as antibacterials

Question 2

What is the difference between bactericidal and bacteriostatic antibiotics?

Answer 2

bactericidal: kill bacteria but low concentrations can be bacteriostatic

bacteriostatic: slow bacterial growth or reproduction but high concentrations or combinations can be bactericidal

Question 3

What is the difference between gram -ve vs gram +ve bacteria in terms of antibiotics resistance?

Answer 3

gram -ve bacteria have two membranes, making them less susceptible to antibiotics

gram +ve bacteria have 1 membrane making it more permeable to antibiotics

Question 4

Why are cell wall inhibitors effective antibiotics?

Answer 4

• highly selective (no cell wall in mammals)
• disrupt bacterial maintenance of osmotic pressure
• generally more effective against gram positives
• first antibiotics
• generally bactericidal

Question 5

How do beta lactam cell wall inhibitors work?

Answer 5

NAM and NAG peptides crosslink peptidoglycan in cell wall making a v stable complex

beta lactams prevent this crosslinking, causing significant loss of integrity

crosslinking require DD-transpeptidases, beta-lactams mimic this

Question 6

How does fosfomycin inhibit the cell wall?

Answer 6

blocks action of phosphoenolpyruvate which converts NAG to NAM –> less crosslinking opportunities

Question 7

How does cycloserine inhibit the cell wall?

Answer 7

inhibits alanine racemase preventing conversion of L-ala to D-ala –> NAM side chain unable to form

Question 8

How does bacitracin inhibit the cell wall?

Answer 8

inhibits isoprenylpyrophosphatase which dephosphorylates the lipid carrier molecule Und-PP, preventing its conversion to the active form, Und-P

Und-P serves as a carrier molecule that transports peptidoglycan precursors across the cell membrane during cell wall synthesis.

Question 9

How do vancomycin and teicoplanin inhibit the cell wall?

Answer 9

vancomycin and teicoplanin are last line of defense antibiotics, saved for serious gram positive infection

vancomycin mimics D-ala and takes its place, preventing NAG-NAM crosslinking

Question 10

How is synthetic penicillin made and what are the advantages?

Answer 10

• strip side chains from natural penicillin and replace w new ones
• stop synthesis of penicillin early by the fungus, and purify the central beta-lactam ring core
• susceptibility to penicillinases overcome by methicillin, oxacillin
• narrow specificity - overcome by amoxicillin, ampicillin

eg
- cephalosporins
- carbopenems
last line os defense

Question 11

What is the benefit of beta-lactamase inhibitors?

Answer 11

can be used in conjunction w previous beta lactams to make them effective for sure
prolongs the use of antibiotics that bacteria has become resistant to

Question 12

What is the action of rifampicin?

Answer 12

inhibition of bacterial DNA-dependent RNA synthesis

high affinity for prokaryotic RNA polymerase

usually bacteriostatic

synthesis blocked by steric clashes w the growing oligonucleotide
inhibits elongation of the transcript

antibiotic ineffective once RNA strand is being read

Question 13

What is the action of fluoroquinolones?

Answer 13

• DNA synthesis inhibitors
• contain a fluorine atom
• broad spectrum
• potential for human toxicity
• rapidly bactericidal

can be modified to gram +ve and gram -ve or just gram +ve

gyrase: introduces transient break in DNA, to allow uncoiling

• stabilises gyrase-DNA complex so DNA cannot be released and replication is blocked
  –> strand broken, leading to cell death

Question 14

What is the action of metronidazole?

Answer 14

• selectively absorbed by anaerobic bacteria
• reduced by reacting w reduced ferredoxin
• reduced intermediates damage enzymes and from unstable molecules in DNA

can be used to treat bacteria in the gut

Question 15

What is the action of folic acid synthesis inhibitors?

Answer 15

folic acid enzymes needed for synthesis of amino acids, therefore necessary for bacterial protein synthesis
–> singularly = bacteriostatic
–> in combination = bactericidal

eg sulphonamides, diaminopyrimidines

Question 16

Why is it important to use antibiotics in combination?

Answer 16

• delays emergence of resistance
• synergy between the two drugs
• individually bacteriostatic, bactericidal in combination
• act in sequential steps in pathway

Question 17

What are monobactams?

Answer 17

• synthetic
• similar to beta-lactam
• single ring

Question 18

What is isoniazid?

Answer 18

• effective against mycobacteria
• inhibits mycolic acid synthesis

Question 19

What is the action of polymyxin B (colistin)?

Answer 19

• disrupts plasma membrane
• real last line of defence
• deemed to toxic in the 70s, brought back in the 2000s
• acts on lipopolysaccharides of gram -ves
• bacteria resistant to this would have changed its membrane structure = no LPS detection by the immune system

Question 20

What is meant by bypass in the context of antimicrobial resistance?

Answer 20

microbes with mutated genes for antimicrobial resistance can bypass the deleterious effects if they possess a plasmid w the corresponding genes

Question 21

What ways can bacteria be resistant to beta-lactams?

Answer 21

• production of beta-lactamases, located in the periplasmic space
  –> usually plasmid mediated, but can be chromosomal encoded on mobile genetic elements
• perturbation of the penicillin binding proteins eg enzyme inhibition or competitive binding
• permeability (influx and efflux)

Question 22

How can alterations in beta-lactamase expression confer resistance?

Answer 22

1. induction
   - interference with repressor protein
2. de-repression
   - mutation in repressor gene

Question 23

What are extended spectrum beta lactamases?

Answer 23

enzymes that can degrade cephalosporins

4 classes: A-D

enzymes that can degrade newer generation beta-lactams can degrade older ones but not vice versa

Question 24

What is the action of beta lactamases?

Answer 24

binds to certain substrate and cleaves the amide ring making the antibiotic completely ineffective

Question 25

How do bacteria resist cephalosporins and carbapenem?

Answer 25

ESBLs
These are enzymes that can hydrolyse extended-spectrum cephalosporins and monobactams but are inhibited by beta-lactamase inhibitors such as clavulanic acid

AmpC beta-lactamases
cephalosporinases that can hydrolyse cephalosporins, penicillins, and monobactams but are not inhibited by most beta-lactamase inhibitors

Porin loss
Loss or alteration of porins can reduce the permeability of the outer membrane, limiting the entry of beta-lactam antibiotics into the bacterial cell.

Question 26

What is conservative transposition?

Answer 26

only 1 transposon that moves around

facilitated by transposase to target replicon

Question 27

What is replicative transposition?

Answer 27

• fusion mediated by the action of transposase
• cointegrate
• resolution of cointegration by site-specific recombination between the two res sites mediated by resolvase