Gartenberg - Cell Wall Actibiotics

Question 1

If someone comes in with a serious bacterial infection what should you do?

Answer 1

1) GIve a broad spectrum antibiotic or utilize combination therapy
2) Grow a culture and figure out which organism is responsible for the infection.
3) Once identified, treat with a narrow-spectrum drug.

• keep in mind, Bacteriostatic drugs may inhibit the action of Bacteriostatic drugs that rely on active cell growth.

Question 2

why is it that cell wall antibiotics often don’t work as well on Gram - bacteria?

Answer 2

Because the gram - bacteria have a outer cell wall layer, they have difficulty getting access to the cell wall.

Question 3

Broad spectrum

Answer 3

Drugs that work against both gram+ and gram-

Question 4

Extended spectrum

Answer 4

A drug whose selectivity is broadened by chemical modification

Question 5

What types of molecules are best for treatment of gram - bacteria?

Answer 5

Small hydrophilic molecules that can fit through their transmembrane pores.

Question 6

What other factors dictate specificity once inside of the cell?

Answer 6

Affinity of the drug for their target

Ability of drugs to evade systems that eliminate them (efflux pumps)

Question 7

General structure of cell wall

Answer 7

NAG and NAM come together as one monomeric unit. They combine with other moomeric units by transglycosylases outside of the cell. Then in order to cross link a second polysaccharide we use transpeptidases.
- The peptides contain two D-Alas that are connected by Gylcines in the middle only in G+ (5 glycines).

Question 8

Transglycosylase

Transpeptidase

Answer 8

Transglycosylase - join monomeric sugars of NAG/NAM to other monomeric sugars outside of the cell in order to create a polysaccharide.

Transpeptidase - joins the polysaccharides together to cross link them.

Question 9

What are some common beta-lactam drugs?

Answer 9

Penicillin

Cephalosporin

Question 10

WHat is the biochemical mechanism of action of transpeptidases?

Answer 10

They have a serine group in their active site. The serine attacks the peptide bond between the two D-Alas. This causes one D-Ala to leave as a leaving group. Then, a glycine from a second polysaccharide comes in and attacks the D-Ala and covalently bonds to it while the transpeptidase is a leaving group.

Question 11

Mechanism of action of Penicllin And the beta-lactam class of drugs?

Answer 11

Penicllin looks a whole lot like the D-Ala D-Ala moiety. Therefore, the serine residue of the transpeptidase attacks Penicllin instead of the actively growing cell wall. Penicllin is actually larger than the D-Ala D-Ala within the active site, so there is no room for Glycine or H2O to come in and release Penicllin.

Question 12

What was the bacterias response to beta-lactam drugs?

Answer 12

Beta-lactamases

Question 13

What did microbes do in order to protect themselves from beta-lactams?

Answer 13

They began secreting beta-lactamases. These enzymes look like transpeptidases and they bind to the Penicllin, but these beta-lactamases have much larger active sites than does transpeptidases, so H2O is able to come in and hydrolyze the bond and release the penicillin but not in the original structure.

Question 14

What did the pharmaceutical industry do to combat the activity of beta-lactamases?

Answer 14

Beta-lactamase inhibitors

Question 15

What is an example of Beta-lactamase inhibitors?

Answer 15

Clavulanic acid

Avibactum

Question 16

How do beta-lactamase inhibitors work?

Answer 16

They are similar to penicillin but much larger. So they bind to the beta-lactamases and irreversibly inactivate them.
- they have no intrinsic antibacterial activity, they are just given with beta-lactam antibiotics like penicillin in order to increase their half-life and ability to do their job.

Question 17

Why is Avibactam different than the other beta-lactamase inhibitors?

Answer 17

Because usually beta lactamases have three classes - A, B, and D. Most beta-lactamases inhibitors work on class A enzymes. Avibactam works on class A, C, and D - it is broader spectrum.

Question 18

What was the progression of penicillins and what added benefit did they each have?

Answer 18

Penicllin G - this was the original. It is acid Labile so a lot of it would get destroyed in the stomach. It would be inactivated by beta-lactamases though.

Oxacillin, cloxacillin, dicloxacillin - acid stable so it would make it though the harsh environment of the stomach. It is resistant to beta-lactamases.

Amoxicillin - acid stable. It has enhances specificity against gram - because it could penetrate the outer membranes.
- often couples with a beta-lactamase inhibitor (Augmentin)

Question 19

What is Augmentin?

Answer 19

Combination of amoxicillin and clavulanate

- extended-spectrum Penicllin and a beta-lactamase inhibitor.

Question 20

Probenecid

Answer 20

Competitively inhibits secretion of Penicllin to increase its half life. Otherwise it is often excreted in 3-4 hours.

Question 21

Cephalosporins

Answer 21

Have increased resistance to beta-lactamases. 
- this class of drug starts with "cef"

Question 22

Progression of cephalosporin with each generation

Answer 22

1 - broad spectrum. Only really used for prophylactic surgery. Doesn’t penetrate CNS

2 - broad spectrum but increased coverage of G- bacteria. No allergic cross-reactivity with penicillin

3 - extend G- activity even further at the expense of G+. Somewhat crosses the blood-brain barrier.
- When combined with a beta-lactamase inhibitor it is called Avycaz

4 - increased activity. Against G+ and G-. Can cross CNS.

Question 23

What do you have to worry about when giving a cephalosporin?

Answer 23

Renal insufficiency because it is excreted through the kidney. It can cause seizures in these patients.

Question 24

What is the name of the 5th generation cephalosporin and what’s its deal?

Answer 24

ceftolozane
- often coupled with tazobactam (beta-lactamases inhibitor) to make a drug called Zerbaxa
- often used in case where microbe became resistant to other drugs - good against pseudomonas
-

Question 25

Ceftaroline

Answer 25

One of the cephalosporins that actively acts on transpeptidases unique to MRSA

Question 26

Monobactams

Answer 26

Looks like a beta-lactam but only has one ring.

• Because of this it is very active against G- microbes, specifically rods.
• aztreonam

Question 27

Carbapenems

- imbipenem

Answer 27

Broad spectrum antibiotic used to treat mixed infections.

• Resistant against beta-lactamases
• can be cleared by dehydropeptidases in renal tubules, which is why we give cilastatin, which is an inhibitor or dehydropeptidases

Question 28

Vancomycin

Answer 28

Very large so it can’t act on Gram -, it is used exclusively against G+, specifically staphylococci.

• it binds to the growing polysaccharide chain and doesn’t allow for the transpeptidase or transglycosylase to get to it.
• This causes lysis of the growing cell.
• resistant to strains of enterococci that switch from D-Ala and D-Ala to D-Ala and D-Lac

Question 29

what are some of the new types of vancomycin and why are they better?

Answer 29

Dalbavancin and oritavancin (“vancin”

- less frequent dosing - vanco needs to be given via IV for 7-10 days.

Question 30

Daptomycin

Answer 30

Causes pores in the cell, which are large enough for K+ to leak out. Cell does not rupture though so you don’t have to worry about toxins.
- Used for G+.

Question 31

Polymixins

Answer 31

a very large lipopeptide that works on G- cells.

• they split open the G- cell walls to allow it to be active to other drugs.
• commonly used in neosporin

Question 32

Fosfomycin

Answer 32

Drug inhibits the first committed in cell wall synthesis, the conversion of NAG-UDP to NAM-UDP.

• it does this by binding to the active site of the MurA enzyme
• TB is naturally resistant.

Question 33

Bacitracin

Answer 33

inhibit lipid phosphatase that dephosphorylates lipid carrier of peptidoglycan subunits.

• Used against G+
• nephrotoxic so only used topically

Question 34

D-cycloserine

Answer 34

Produced by soil bacteria, it is structurally related to D-Ala. It competitively inhibits alanine racemase and D-Ala ligase.
- second line treatment for TB, but only used for that indication due to severe CNS toxicity.