Antibiotics and Antibiotic Resistance

Question 1

What are antibiotics?

Answer 1

Molecules that kill bacteria or inhibit their growth - can be of biological origin or synthetic origin

Question 2

Selective toxicity

Answer 2

Antibiotics exert their activity by inhibiting gene products found in bacterial cells, but not in humans - minimize side effects

Question 3

Disinfectants

Answer 3

Toxic to humans and bacteria (bleach)

Nonspecific effects

Used to eliminate organisms on inanimate objects, surfaces

Question 4

Antiseptics

Answer 4

Generally toxic to bacteria

Nonspecific effects

Too toxic for systemic use in humans

OK for topical use

Question 5

Bacteriostatic:

Bactericidal:

Answer 5

Bacteriostatic: inhibit growth of bacteria - rely on immune system to eradicate

Bactericical: Kill bacteria directly - important for immunocompromised patients

Question 6

Pharmacology/Bioavailability

Answer 6

Not all antibiotics penetrate all tissues equally - to be effective, an antibiotic needs to get to site of infection at therapeutic levels

Question 7

Spectrum of activity

Narrow spectrum:

Broad Spectrum:

Answer 7

The collection of bacterial species that is susceptible to a given antibiotic

Narrow spectrum: Effective against a small group of bacteria

Broad Spectrum: Effective against a wide range of bacteria

Question 8

Broad spectrum

Advantage:

Disadvantage:

Answer 8

Advantage: Can be used when infectious agent is unknown or in emergency

Disadvantage: Affects many members of natural microflora, leading to undesirable secondary effects

Question 9

What does susceptibility mean?

What is a susceptibility test?

Answer 9

• Susceptible - Growth of bacteria can be inhibited by concentrations of antibiotic readily achieved in a patient
• Susceptibility test - in vitro test that requires pure cultures of infectious organism

Question 10

Why do we need to measure susceptibility

Answer 10

All isolates of bacterial species are NOT susceptible to the same antibiotics

Bacteria must be susceptible for antibiotics to work

Question 11

Minimum Inhibitory concentration:

Minimum bactericidal concentration:

Answer 11

Minimum Inhibitory concentration: Lowest concentration of antibiotic that inhibits growth

Minimum bactericidal concentration: Lowest concentration that kills a defined proportion of bacteria after specified time

Question 12

Disk diffusion test

Answer 12

• Innoculate surface of agar plate with test bacteria
• Disk containing antibiotic placed on “lawn” of bacteria and antibiotic diffuses away from growth medium
• Measure zone of inhibition to determine susceptibility

Question 13

Adverse effects of antibiotics

Answer 13

Toxic side effects of drug

Hypersensitivity

Alteration of normal microflora

Selection for antibiotic resistance

Question 14

C. Difficile antibiotic associated disease

Answer 14

1. Non spore forming anaerobes predominate
2. Antibiotics administered
3. Reduction in genera of anaerobes - C. difficile grows to high numbers
4. Production of exotoxins A and B (basis of diagnosis) → Diarrhea/ulceration of colon
5. Vancomycin administered
6. Symptoms abate
7. Cessation of therapy
8. 10-20% relapse

Question 15

Antibiotic Resistance

Horizontal Gene transfer:

Spontaneous mutations:

Answer 15

Genotypic changes that confer enhanced growth in the presence of an antibiotic

Horizontal gene transfer: Acquisition of foreign DNA encoding resistance genes; can enable rapid emergence of milti-drug resistant strains

Spontaneous mutations: selection for growth leads to resistant mutants

Question 16

3 basic mechanisms of resistance

Answer 16

1. Modification (inactivation) of antibiotic molecule itself
2. Modification (reprogramming) of antibiotic target
3. Reduction of antibiotic concentration/ prevent access to target

Question 17

Targets of antibiotics

Answer 17

• Peptidoglycan synthesis
• Ribosomes and protein synthesis
• DNA replication and repair
• Cell membrane
• Key metabolic reactions

Question 18

Bacterial growth and antibiotic sensitivity

Answer 18

Observe 4 phases of growth of bacteria in nutrient rich media

Antibiotic sensitivity depends on growth phase of the cell

Question 19

Peptidoglycan

Answer 19

Polymer of alternating sugars crosslinked via peptide side chains

Question 20

Why do we target peptidoglycan with antibiotics

Answer 20

• Peptidoglycan provides structure to bacterial cells and protects them from lysis due to osmotic pressure
• Peptidoglycan is unique to bacteria
• High specificity and low toxicity for host

Question 21

3 stages of peptidoglycan biosynthesis

Answer 21

1. Synthesis of MurNAc pentapeptide precursors - cytoplasm
2. Lipid linkage and transport of disaccharide precursors across membrane
3. Polymerization and crosslinking of precursors into peptidoglycan - extracellular

Question 22

What enzymes catalyze polymerization and crosslinking of peptidoglycan

Answer 22

Penicillin-binding proteins (PBPs)

Question 23

ß-Lactam antibiotics

Answer 23

Block peptidoglycan crosslinking by binding PBPs and inactivating them

Question 24

4 types of ß-Lactams

Answer 24

Penicillin

Cephalosporin

Carbapenem

Monobactam

Question 25

How to β-lactams inhibit PBP

Answer 25

Structure of β-Lactams mimic natural substrate of PBPs

Question 26

What is the purpose of “R” group modification for β-Lactam

Answer 26

Modification with different chemical substituents at the “R” positions alter spectrum of activity or pharmacology but not mechanism of action

Question 27

Primary mechanism of resistance to β-Lactam antibiotics

Answer 27

Production of β-lactamase enzyme catalyzes inactivation of β lactam antibotics - Cleavage of β-lactam ring

• Many different types of β-lactamses, each with specificity for certain subset of β lactams

Question 28

Primary mechanism of resistance to β-Lactam antibiotics (most common in Streptococcus (gram positive))

Answer 28

Mutations in PBPs that prevent binding of β-Lactam antibiotics (modification of target)

Question 29

β-lactamase inhibitors

Answer 29

Share structural featurses with β-lactam antibiotics

Possess little intrinsic antibacterial activity

Bind to β-lactamase and released very slowly, inhibiting β-lactamase activity

Administered in combination with β-lactam antibiotic

Question 30

β-lactamase inhibitors work mostly in ___ ______ bacteria

Answer 30

Gram negative