Study guide: Mechanisms of antibacterial & microbial resistance

Question 1

What are 3 features unique to gram-negative bacteria?

Answer 1

1. thin peptidoglycan (PG)
2. Safranin (red)
3. outer lipid rich membrane + lipoproteins repel many drug: polar drugs enter through porins to access PG

Question 2

Which is easier for a drug to penetrate- gram positive or negative bacteria, and why?

Answer 2

Gram positive, low MW enter easily across exposed PG layer, itself a key target

Question 3

How does the murein (peptidoglycan) layer differ between gram positive and negative bacteria?

Answer 3

Gram positive: thick PG

Gram negative: thin PG

Question 4

How does gram staining distinguish between gram positive and negative bacteria?

Answer 4

Gram _p_ositive: purple dye

Gram negative: safranin (red)

Question 5

Ribosomes and cell walls are common targets for antibiotics – which is unique to bacteria, and which is slightly different from its human counterpart?

Answer 5

bacteria vs humans:

• cell wall (humans have no cell walls)
• 70S ribosome (humans: 80S)
• outer membrane (gram negative only)
• Different needs for substrates

Question 6

What are the differences between bactericidal and bacteriostatic antibiotics, and which one requires a competent immune system to resolve the infection?

Answer 6

Bactericidal: kills bacteria; irreversible (penicillin)

Bacteriostatic: prevent replication; reversible (tetracycline) - the patient’s own immune system must deal with getting rid of rest of the infection

Question 7

Describe 3 (there are 4 in the lecture) categories of adverse effects of antibiotics – which is specific to antibiotics?

Answer 7

1. Direct toxicity: aminoglycosides generate free radicals that damage neurons in inner ear
2. Allergic reactions: rapid, immune-mediated development of rash, hives
3. Idiosyncratic reactions: hemolysis in G-6-PD-deficient patients treated with sulfonamides
4. Changes in normal body flora: killing some bacteria allows other to proliferate; vaginal yeast infections (this is specific to antibiotics)

Question 8

Contrast between prophylactic, pre-emptive, empiric and definitive / directed therapy.

Answer 8

Prophylactic: antibiotic used before an infection

Pre-emptive therapy: antibiotic used during symptoms occur to prevent anticipated infection or symptoms

Empiric therapy: selection of an antibiotic based on most likely cause of infection

Definitive/directed therapy: selection of an antibiotic based on positive identification of the causative organisms

Question 9

Define the terms “sensitivity”, “MIC”, and “clinical breakpoint”

Answer 9

Sensitivity: the ability of a bacteria to be inhibited/killed by a particular antibiotic; drug affects the bacteria at lowest concentration is the one to which it’s most sensitive

Minimum inhibitory concentration (MIC): lowest concentration of drug that inhibits growth of organism

Clinical breakpoint: highest plasma concentration that can safely be achieved in a patient

Question 10

What is the purpose of broth dilution and disk diffusion testing?

Answer 10

Broth dilution: bacteria in liquid are exposed to increasing concentrations of a drug - the lowest concentration that eliminates growth is MIC

Disk diffusion: bacteria are plated onto agar studded with small disks containing different antibiotics - if bacteria surrounding a disk are dead, they are susceptible to that drug

Question 11

How are MIC and breakpoint concentrations used together to determine bacterial sensitivity?

Answer 11

Sensitive: MIC < breakpoint

Intermediate: MIC near breakpoint

Resistant: MIC > breakpoint

Question 12

Contrast between the terms “broad spectrum” and “narrow spectrum” – why would a narrow spectrum drug sometimes be more appropriate?

Answer 12

Broad spectrum: active against many types of bacteria

Narrow spectrum: active against only one or a few types of bacteria (might be more appropriate: to kill/inhibit only unwanted bacteria; lessen resistance)

Question 13

Contrast between time- and concentration-dependent antibiotic effects, and the dosing strategy required for each – give an example drug for each.

Answer 13

Time dependent: constant rate of killing, provided that drug concentration exceeds MIC (beta lactams)

Concentration-dependent: rate of killing increase with drug concentration above MIC (aminoglycosides)

Question 14

State 3 advantages of combining antibiotic drugs - are all combinations of antibiotics equally beneficial, or can some be detrimental?

Answer 14

1. both MOA to produce a synergistic effect (greater than sum of individual drugs), in some cases combined antibiotics can antagonize each other’s effects
2. Combining antibiotics can also minimize risk of resistance development
3. Lower doses of each agent can be used, minimizing side effect severity

Question 15

What is the difference between intrinsic and acquired resistance?

Answer 15

Intrinsic: trait that confers protection against antibiotic action, shared by all members of a bacterial species, not related to antibiotic exposure

Acquired: certain gene changed become favored as a result of pressure from antibiotic exposure

Question 16

What are 4 general mechanisms by which bacteria can become resistant to antibiotics, and which one of these is specific to gram negative bacteria?

Answer 16

1. Altered receptors or targets so drugs cannot bind (vancomycin)
2. Drug destruction or inactivation (beta lactams)
3. New resistant pathway (sulfonamides)
4. Decreased drug exposure: less uptake/more efflux (regulating outer membrane pore function, or efflux transporter expression) - this mechanism is relevant to gram negative bacteria

Question 17

Can more than one resistance mechanism appear in the same strain of bacteria?

Answer 17

With in same species of bacteria, there may be multiple strains with varying types of resistance

A single species may have multiple acquired resistance mechanisms

Question 18

What are the 3 main modes of horizontal transfer of acquired resistance, and how does each work?

Answer 18

1. Transformation: uptake of naked DNA containing resistance genes
2. Transduction: DNA transferred by infection with viruses
3. Conjugation: DNA transfer between bacterial cells via plasmid exchange

Question 19

Are resistance genes always acquired horizontally (from another organism)?

Answer 19

Bacterial proliferation and transfer between hosts (poor hygiene, hospital acquired infections)

Increased exposure of bacteria to antibiotics -> selection pressure (over prescription, wrong prescription, use in agriculture)

Question 20

What is “antibiotic stewardship”?

Answer 20

coordinated data-driven programs that focus on reducing inappropriate antibiotic use in clinical settings and agriculture

Question 21

What are two general categories of initiatives that are being supported to combat drug resistance development?

Answer 21

1. Government
2. WHO, World Bank