MIIM - Antibiotics II - Week 9

Question 1

Name 3 nucleic acid pathways that can be targetted.

Answer 1

Inhibiting synthesis of precursors
Inhibiting DNA replication
Inhibiting RNA polymerase

Question 2

What nucleic acid pathway do quinolones inhibit? What enzyme specifically?

Answer 2

Inhibits DNA replication

Affects DNA gyrase

Question 3

What is the action of DNA gyrase?

Answer 3

Required to supercoil bacterial DNA

Question 4

Are quinolones bactericidal or bacteriostatic?

Answer 4

Bactericidal

Question 5

Name three reasons to use antibiotic combinations.

Answer 5

To cover a range of organisms which might reasonably be expected to be present
To achieve synergy
To cover the possibility of the presence of antibiotic resistant mutants

Question 6

What is the therapeutic guideline for staph blepharitis?

Answer 6

Antibiotic efficacy uncertain - chloramphenicol 1% ointment 1-2x daily for 1 week

Question 7

What is the therapeutic guideline for chronic blepharitis (rosacea)?

Answer 7

Doxycycline one daily for 8 weeks

Question 8

What is the therapeutic guideline for conjunctivitis (empirical treatment) (2)?

Answer 8

Delayed prescription approach - chloramphenicol or framycetin

Question 9

What is the therapeutic guideline for conjunctivitis (non-empirical treatment) (3)?

Answer 9

Gentamycin, tobramycin, or quinolone drops when indicated

Question 10

What is the therapeutic guideline for conjunctivitis (chlamydia)?

Answer 10

Systemic therapy with azithromycin

Question 11

What is the therapeutic guideline for HSV keratitis?

Answer 11

Acyclovir 3% 5x daily for 14 days

Question 12

What should be done with bacterial keratitis?

Answer 12

Referral

Question 13

Define minimal inhibitory concentration.

Answer 13

The lowest concentration of an antimicrobial agent that will inhibit the growth of an organism under test over a suitable defined time interval

Question 14

Describe the tube method for determining minimal inhibitory concentration (3).

Answer 14

Serial dilutions of the antibiotic are made in a suitable broth and a standard number of bacteria from the patient is added.
After overnight incubation, the growth is scored.
The last dilution which inhibits growth is the minimal inhibitory concentration.

Question 15

Define the term sensitive or susceptible.

Answer 15

An infection with that organism will respond to the usually recommended dose on an antimicrobial

Question 16

Define intermediate in terms of minimal inhibitory concentration (2).

Answer 16

The microorganism may be inhibited by attainable concentrations of certain drugs if higher doses can be used safely, or if the infection involves a body site in which the drug is physiologically concentrated.

Question 17

Define resistant in terms of minimal inhibitory concentration.

Answer 17

Antimicrobials that do not inhibit the test organism within the range of achievable levels.

Question 18

Describe the disc diffusion method for determining minimal inhibitory concentration (3).

Answer 18

A standard number of bacteria from the patient is inoculated onto an agar plate.
Discs impregnated with appropriate concentrations of different antibiotics are added.
After overnight inculation, zone diameters are measured and correlated with minimal inhibitory concentration.

Question 19

Name 4 mechanisms of resistance to antibiotics. Give two examples for each (6).

Answer 19

Drug inactivation
-by hydrolysis
-by covalent modification
Altering the target of drug action
-modify target to a less sensitive form
-overproduce the target
Reduce access of drug to target
-reduced entry into the cell
-increase efflex from the cell
Failure to activate inactive precursor of drug

Question 20

Consider resistance to B-lactam antibiotics involving reduced affinity for penicillin binding proteins. Does this cause problems of resistance at other sites?

Answer 20

Yes, but not in the eye

Question 21

Name three ways resistance to B-lactams can occur.

Answer 21

Production of B-lactamases
Alteration of penicillin-binding proteins
Reduced entry into the cell

Question 22

Briefly explain how B-lactamase acts.

Answer 22

It breaks the B-lactam ring, disabling the molecule

Question 23

List and describe the two ways resistance to glycopeptides can occur.

Answer 23

Alteration of the target - mutation of the terminal amino acids into lactate
Production of excess target - peptidoglycan

Question 24

Name 4 ways resistance to aminoglycosides can occur.

Answer 24

Efflux from the cell
Modified outer membrane leading to reduced entry
Enzymatic inactivation
Ribosomal mutation leading to reduced binding

Question 25

Name two ways resistance to macrolides can occur.

Answer 25

Increased efflux from the cell

Modification of the target - methylation of the 23S renders 50S resistant

Question 26

Describe the main way resistance to tetracyclines occurs.

Answer 26

Predominantly efflux from the cell

Question 27

Name two ways resistance to quinolones can occur.

Answer 27

Increased efflux from the cell

Modification of the target - mutation in DNA gyrase reducing antibiotic binding

Question 28

Name 4 ways genetic material for antibiotic resistance can be acquired by a bacteria.

Answer 28

Transformation
Transduction
Conjugation
Transposition

Question 29

Name three intrinsic resistances to antibiotics.

Answer 29

Cell wall impermeability
Lack of a target
Chromosomal resistance gene

Question 30

Name two acquired resistances to antibiotics.

Answer 30

Mutation

Horizontal gene transfer

Question 31

Briefly describe transformation.

Answer 31

A dead bacteria with the genes for resistance releases free DNA when lysed.
The recipient bacteria take up the free DNA and incorporate it into their genome.

Question 32

Briefly describe transduction.

Answer 32

A bacteriophage infects a bacteria with resistance.
Once it finishes killing the bacteria, the bacteriophage has a copy of the resistance gene.
It infects another bacteria, which receives the resistance gene from the bacteriophage.

Question 33

Briefly describe conjugation.

Answer 33

The gene for resistance is found on a plasmid.

The bacteria connects to a recipient bacteria via a pilus, and sends it a copy of the plasmid.

Question 34

Briefly describe transposition.

Answer 34

The gene for resistance is found on a plasmid with a transposon.
The plasmid is transferred to a recipient via conjugation.
The transposon transposes the plasmid into the genome.

Question 35

List the 5 drug resistance mechanisms.

Answer 35

Prevent drug entry
Drug efflux
Inactivate drug
Modify drug target
Use an alternate pathway

Question 36

How can the threat of resistance be reduced.

Answer 36

Controlled use of antimicrobials to reduce the natural selection of resistant strains.