Mechanisms of Antimicrobial Resistance

Question 1

Describe the difference between intrinsic and acquired antibiotic resistance and list ways a bacterium may acquire antibiotic resistance.

Answer 1

Intrinsic- Properties of bacteria don’t allow for antibiotic targeting.
Ex Gram - organisms and Vancomycin. Vanc cant penetrate Gram - outer membrane.

Acquired- Genetic mutation or acquiring new genes.
Ex: plasmid, transposon, or bacteriophages.

Question 2

Which bacteria use porins for resistance? How do porins work?

Answer 2

1. Found in the outer membrane of gram-negative bacteria

2. Porins selectively take in nutrients/drugs. Change in number or configuration of porins can effect drug activity.

Question 3

Which bacteria use efflux pumps for resistance? How do they work?

Answer 3

1. Both gram-positive and gram-negative organisms.

2. They pump drug out from cytoplasm.

Question 4

Which protein cross links peptidoglycans? What motif is cleaved during cross linking?

Answer 4

Penicillin Binding Proteins (PBPs)

D-Ala D-Ala motif is cleaved.

Question 5

How do β - lactam antibiotics work?

Answer 5

β-lactam antibiotics act by irreversibly inhibiting PBPs thereby inhibiting peptide cross-linking and peptidoglycan synthesis.

Question 6

How do bacteria become resistant to β - lactams?

Answer 6

1. Break down the drug via B lactamase

2. Modify PBP to have lower binding affinity for b-lactam.

Question 7

In which bacteria are β-lactamases found?

Answer 7

BOTH gram-positive and gram-negative organisms.

In general, the BROAD spectrum β-lactamases are found in gram-negative bacteria.

Question 8

Which bacteria are Narrow spectrum β-lactamases? Which antibotics are they resistant to?

Answer 8

1. Staphylococci
   * 2. E. Coli/ H. Influenza
   * 3. Klebsiella pneumonia
   * newer resistance- prompted 2nd and 3rd gen celphalosporin devel.

Resistant to penicillin and ampicillin

Question 9

What is a difference between Narrow spectrum β-lactamases and ESBLs?

Answer 9

ESBL= Extended spectrum

ESBLs can attack cephalosporins

Question 10

Where is the ampC gene found in ampC-encoded β-lactamases?

Answer 10

AmpC found in the CHROMOSOME of certain Gram (-) rods: Enterobacter, Pseudomonas, and a few others.

Question 11

Why is ampC special?

Answer 11

Encodes for cephalosporinase. Can hydrolize penicillin and cephalosporins.

Also, it can be induced or constitutively expressed.

Question 12

How is ampC expression determined?

Answer 12

Basically ampC isn’t expressed all the time, but if it develops a mutation, it can be expressed all the time and be permanently resistant to a ton of antibiotics.

Question 13

What drug is used to treat a bug (most commonly Enterobacter strains) with constitutive expression of ampC?

Answer 13

Carbapenems

Question 14

What is KBC and what does it do?

Answer 14

Klebsiella pneumoniae carbapenemase:

Hydrolyzes ALL B-lactams and Carbapenems.

Found in PLASMIDS

Question 15

What are the 2 mechanisms for developing Altered penicillin-binding proteins (PBPs)?

Answer 15

1. acquiring mecA gene (in staphy bacteria)–> alters binding affinity (when present in staph–>MRSA)
2. Mosaic PBPs (strep and gono)- DNA transformation alters protein structure

Question 16

Which type of altered PBP resistance is generally a slow process?

Answer 16

Mosaic PBPs

Question 17

How does Vancomycin work?

Answer 17

It binds to the D-Ala motif and prevents cleaving and thus, cell wall synthesis.

Question 18

How do bacteria become resistant to Vancomycin?

Answer 18

1. Modifying the target in Enterococcus (VRE). Changes the D-Ala Motif to D-ala D lac
2. Preventing drug-target interaction by binding free Vanc (Staph A.)

Question 19

How do bacteria become resistant to macrolides?

Answer 19

1. Di-methylating target ribosome (erm)

2. Efflux pumps

Question 20

How do bacteria become resistant to Flouroquinolones?

Answer 20

Modifying target via point mutations in gyrase/topo

Question 21

What is erm resistance?

Answer 21

Erm gene encodes demethylase which confers resistance to CLINDAMYCIN and MACROLIDES.

Question 22

Why is the D-test used?

Answer 22

To test resistance to macrolide and sensitivity to clindamycin. Also can tell if it is via efflux pump or constitutive erm expression.

Question 23

How can you tell if resistance is via efflux pump or constitutive erm expressed (in d-test)?

Answer 23

If bacteria has inducible erm system, then the zone of clearing around clindamycin disk (on agar plate) is blunted and appears like a D instead of a circle.

Efflux= Regular resistance pattern (circle around antibiotic)

*Remember this plate from lab?