Lecture 19 - Antibiotics A

Question 1

What is differential toxicity?

Answer 1

Antibiotics target components that are unique to the bacterium - cell wall, ribosomes, DNA pol
etc., but often also cause some damage to the host (“differential toxicity” – more toxic to
bacteria)

Question 2

How are antibiotics classified?

Answer 2

Based on bacterial spectrum (broad vs narrow), route of administration (injectable vs
oral versus topical), type of activity (bactericidal vs bacteriostatic), chemical structure (blactams,
aminoglycosides, etc)

Question 3

How is resistance acquired by bacteria?

Answer 3

Resistance is most often acquired by horizontal gene transfer, usually on plasmids (conjugation) but also via natural transformation and phage transduction. Point mutations.

Question 4

Four general strategies bacteria

use to resist antibiotics?

Answer 4

Restrict acces/efflux, enzymatic modification of antibiotic, enzymatic modification of antibiotic target, immunity and bypass

Question 5

What are the biggest reasons behind antibiotic resistance?

Answer 5

Misuse and overuse of antibiotics by doctors and
patients; spread of resistant bacteria by poor hand hygiene in hospitals; release of antibiotic resistant bacteria from hospitals into waterways in regions of poor sanitation; regulation; wide-spread use of antibiotics in agriculture to promote growth, prevent infection; release of antibiotics into wastewater by drug manufacturers, agriculture

Question 6

How NAM-NAG cross-linked in gram-neg and gram-pos bacteria?

Answer 6

Gram-neg: direct isopeptide linkage

Gram-pos: pentaglycine bridge

Question 7

What do Beta-lactams bind to?

Answer 7

The enzymes that
catalyze the
transpeptidation step:
transpeptidases/
penicillin binding
proteins (PBPs)

Question 8

What do vancomycins bind to?

Answer 8

D-Ala-D-Ala substrate,
which prevents
transpeptidases from
binding

Question 9

How do bacteria acquire resistance to Beta-lactams and glycopeptides (vancomycin)?

Answer 9

(1) b-lactamases or (2) altered penicillin
binding proteins, making them resistant to b-lactam antibiotics (3) mutations in porin genes – porin channels in OM no longer take up the b-lactams; or by upregulation of expression ofefflux pumps to expel the antibiotics

Question 10

How are PG monomers attached to the PM before integration into growing PG cell wall?

Answer 10

In the form of Lipid II - NAM-NAG-prenylchain anchor

Question 11

How are Beta-lactams so strongly bound to transpeptidases/PBPs?

Answer 11

Covalent linkage

Question 12

What is the structure of beta-lactams?

Answer 12

Fused beta-lactam structure, free carboxylic acid and one or more R groups

Question 13

What are the different structural groups of beta-lactams?

Answer 13

Penicillins, cephalosporins, monobactams, penems/carbapenems

Question 14

How do Beta-lactams kill pathogens?

Answer 14

Upon treatment with b-lactam antibiotics, cell wall synthesis is disrupted and existing
peptidoglycan is degraded by bacterial hydrolytic enzymes; cells lyse from osmotic pressure

Question 15

What are carbapenems?

Answer 15

Carbapenems are powerful broad spectrum b-lactam antibiotics considered the drugs of last
resort as they are the only effective drugs for some multidrug resistant pathogens – however
resistance to these is increasing

Question 16

How do Beta-lactamases modify Beta-lactams?

Answer 16

Hydrolyze C-N bond in b-lactam ring

Question 17

How are beta-lactamase-resistant penicillins made?

Answer 17

Add large bulky group
at R (eg. methicillin, oxacillin) - resist degradation by some b-lactamases.

Question 18

What are CRE’s? How do they acheive antibiotic resistance against carbapenems?

Answer 18

Carbapenem-resistant Enterobacteriaceae (CRE) are an emerging class of superbugs -
multidrug-resistant Gram negative enteric bacteria that are also resistant or nearly resistant to
carbapenems. CRE are common commensals or pathogens such as Klebsiella and E. coli.that
have acquired a two-plasmid-encoded b-lactamase carbapenemase – hydrolyzes carbapenem
and many other b-lactam antibiotics.

Question 19

What are two examples of CRE carbapenemases?

Answer 19

• KPC for Klebsiella pneumoniae carbapenemase
• NDM-1 for New Delhi metallo-b-lactamase, named after the city where the initial patient
  was thought to have contracted the infection (name is controversial)

Question 20

Name two drugs used to treat CRE infections.

Answer 20

CRE infections are treated with drug combinations that are not always effective, can be toxic:
tigecycline, a new class of glycylcyclines related to the tetracyclines; and colistin (polymyxin
E), a cyclic peptide – old antibiotic that disrupts bacterial outer membranes but is toxic to
kidneys.

Question 21

Why is NDM-1 classified as a metallprotease?

Answer 21

Binds to conserved regions of the Beta-lactam antibiotics using 2 zinc atoms, beta-lactam ring is hydrolyzed inactivating the molecule.

Question 22

Which type of bacteria is vancomycin usually used and why?

Answer 22

Used for Gram-positive bacteria, can’t penetrate the outer membrane of Gram-negative bacteria
(too big for the pass through the OM porin channels)

Question 23

What does vancomycin bind to ?

Answer 23

C terminal D-Ala

Question 24

Which species of pathogens are van genes (vancomycin resistance genes) found?

Answer 24

Enterococci species (VRE) and other gram positive cocci including Staph aureus.

Question 25

What is the function of PBPs?

Answer 25

Transpeptidation of cell wall pentapeptides (cross-linking) and transglycosylation of NAM and NAG.