Exam 1 lecture 3

Question 1

What colors do gram positive and gram negative bacteria stain under microscope

Answer 1

Gram positive strain dark purple and gram negative stain light pink

Question 2

State the structural differences in gram positive and gram negative bacteria

Answer 2

Gram positive- drug molecules can get through gram positive cell wall. Drug molecules do not get through gram negative wall readily. Can get through porins if drug molecule is polar.

Beta lactamases are excreted out of peotidoglycan in gram positive. For gram negative, beta lactamases are in periplasmic space, so they do not have to be made in large amounts while gram positive needs to.

Peptidoglycan layer is thicker in positive than negative (accounts for staining difference)

Gram positive bacteria have one membrane while gram negative have 2 membranes. Gram negative have more complex cell wall.

In gram negative bacteria, the peptidoglycan contains a DAP residue, but in gram + bacteria, that residue is replaced by lysine

Question 3

Peptidoglycan backbone consists of what compounds

Answer 3

N acetylglucosamine
N acetylmuranic acid

Question 4

How can we structurally tell the difference between N acetylglucosamine and N acetylmuramic acid?

Answer 4

N acetylglucosamine does not have a large structure attached to it (it is smaller)

Question 5

How can we tell if a peptidoglycan wall is gram positive or gram negative

Answer 5

IF a mesodiaminoimetic acid reside is included it is in a gram negative residue. (it has a cooh near the bottom see structure)

Question 6

What is peptidogycan cross linked by?

Answer 6

Transpeptidase

Question 7

Where do cross links happen for gram + and gram - organisms

Answer 7

in gram -, the peptidoglycan is cross linked by a bridge between the DAP residue of one strand and the terminal D ala of another. ALternatively in Gram +, the bridge exists between the L-LYS strand and the terminal D-Ala of the second molecule.

Question 8

Look at transpeptidase mechanism on Jan 17 lecture

Question 9

Look at B lactam antibiotic mechanism of action on Jan 17 lecture

Question 10

How do B lactams inactivate transpeptidase? Why are B lactams systems reactive?

Answer 10

B lactam antibiotics acylate the transpeptidase Ser resudue in the enzyme active site to form a stable product. This inactivates the enzyme, inhibiting peptidoglycan cross linking, which results in a defectrive bacterial cell wall. The resulting cell wall will be led to cell lysis and death.

The reactivity of the B lactams is due to a hughly strained 4 member ring

Question 11

How do B lactams fool transpeptidases

Answer 11

The structures of B lactams resemble the D-ala-D-ala fragment that is acted on by transpeptidases, so the B lactams “fool” the transpeptidase into thinking that they are D-Ala-D-Ala fragments

Question 12

What could resistance to B lactam antibiotics stem from?

Answer 12

Resistance may result from decreased cellular uptake of the drug, mutation of the penicillin binding protein to decrease their affinity for penicillins or the presence of an efflux pump.

A more common mechanism is induction or elaboration of bacterial B lactamases. The B lactamases catalyze the hydrolysis of the B lactam moiety

Question 13

What is a common B lactam drug?

Answer 13

Penicillin

Question 14

What percent of people are allergic to penicillin?

Answer 14

6-8 percent

Question 15

What does the allergenicity of B lactam antibiotics result from? Is cross sensitivity common? What tests are recommended to test for B lactam allergenicity?

Answer 15

The allergenicity results from B lactams acting as a hapten and acylating the host cell proteins, which then raise antibodies that result in an allergic reaction.

Cross sensitivity is common and a person who has demonstrated an allergy to one beta lactam is likely to be allergic to another B lactam antibiotic

Topical flare and wheal tests are recommended for B lactam allergy

Question 16

Under acidic conditions, the main degradation products of Pen G are

Answer 16

Benzylpenicillenic acid
Benzylpenicillic acid
Benzylpenicilloic acid

Question 17

Memorize penicillin degradation mechanism on notes from Jan 17

Question 18

MEchanism of penicillin degradation under basic conditions

Answer 18

Penicillin is turned into Penicilloic acid

Question 19

Do penicillin hydrolysis products have antibiotic activity?
Is hydrilysis of B lactam reversible?

Answer 19

No antibiotic activity

Hydrolysis of B lactam is irreversible. Once the ring is opened it will not close.

Question 20

What are the use of electronegative substituents on the side chain carbonyl in B lactams

Answer 20

Electronegative substituents on the side chain carbonyl reduce the nucleophilicity of the side chain amide carbonly oxygen atom. This stabilizes the penicillin against hydrolysis under acidic conditions, since the first step in hydrolysis rxn is decelerated

Question 21

What is the difference between the penicillin G and Penicillin V structurally and functionally?

Answer 21

Penicillin V has an added oxygen before the ring that makes it more stable to hydrolysis in the stomach than penicilin G due to the electronegativity of the oxygen decreasing the nucleophilicity of the amide carbonyl.

PenG is 80% hydrolyzed in the stomach

Pen V is 35% hydrolyzed in the stomach

Question 22

Why should heavy metals be kept away from penicillin solutions

Answer 22

Heavy metal ions catalyze penicillin degradation reactions and should therefore be kept away from penicillin solutions

Question 23

What is special about penicillin with more lipophilic side chains? What happens when a drug is protein bound?

Answer 23

Penicillin with more lipophilic side chains are more highly protein bound.

Protein binding reduces bioavailability by reducing the effective concentration of the drug

Question 24

What value indicates lipid side chains

Answer 24

CLogP.

The higher the CLogP, the higher the lipophilicity

Question 25

How is penicillin usually excreted?

Answer 25

Renal or biliary routes. For those that are excreted by the kidneys, about 10% of renal excretion is by glomerular filtration and 90% by tubular secretion.

Question 26

What happens in the case of kidney disease/failure in penicillin excretion? What are the two mechanisms for tubular secretion? How to prolong half life?

Answer 26

In the case of kidney failure/disease, half life of penicillin is prolonged. Two mechanisms exist for tubular secretion, one for anions and one for cations. The penicillins are anionic and competition with the anion probenecid for the secretion echanism causes an increase in half life when probenecid is administered along with penicilin

Question 27

WHat are the 5 nomenclatures in B lactam ring system?

Answer 27

Penam Penem Carbapenem Cephem Monobactam

Question 28

Memorize the structures of the 5 nomenclatures of B lactam ring system in Jan 17 notes

Question 29

For benzylpenicillin (penicillin G), what is the antimicrobial spectrum? B lactamase sensitivity? Administration? Toxicity? Precautions?

Answer 29

Antimicrobial spectrum- Gram + cocci. Since Pen G resistant microogranisms are common, therapy should be guided by susceptibility testing. B lactamase sensitivity: Yes Administration: Parenteral Toxicity: Acute allergic rxn Precaution: Pen G should be used with caution in individuals with histories of significant allergies and/or asthma

Question 30

What is the main difference between penicillin G and V?

Answer 30

The main difference between pen G and V is that penicillin V is more stable in acid. That means Pen V is more stable in the stomach and produces higher and more sustained blood levels than pen G after oral administration

Question 31

Name a B lactamase resistant drug

Answer 31

Methicillin

Question 32

Why is methicillin not sensitive to B lactamases

Answer 32

This is due to steric hinderance of nucleophilic attack by the enzyme on the B lactam carbonyl . Methycillin is unstable in stomach so it must be administered via injection

Question 33

Why was Methicillin discontinued? Why is MRSA resistant to methicillin? What is the gene called that codes for MRSA resistance?

Answer 33

Dx bc too many bacteria were resistant to it. MRSA is resistant to methicillin bc of a mutation in a penicillin binding protein (transpeptidase). The gene coding for this protein is called methicillin resistant gene (mecA)

Question 34

The penicillin binding protein that mecA codes for is called

Answer 34

PBP2A

Question 35

Is Nafcillin B lactamase sensitive? COmpare with methicillin?

Answer 35

It is not B lactamase sensitive. It is B lactamase resistant. It is slightly more stable than methicillin in acid, but is clinically identical to methicillin

Question 36

What are B lactamase resistant oral penicillins

Answer 36

Oxacillin Cloxacillin Diclocxacillin

Question 37

Out of Oxacillin, cloxacillin, dicloxacillin, which ones are B lactamase sensitive? Which have been dx? What are they all called

Answer 37

All called isoxazoles Not B lactamase sensitive Oxacillin and cloxacillin have been dx for oral use, but dicloxacillin is still available for oral use. Oxacillin for inj is available

Question 38

Name B lactamase sensitive, broad spectryum, oral penicillins

Answer 38

Ampicillin AMoxacillin

Question 39

ANtimicrobial spectrum of ampicillin? Why?

Answer 39

Attacks gram negative bacteria. The inner surface of porins is hydrophilic and porins all transport ionic cpd. The charged amino group of ampicilin at physiological PH allows ampicilin to be transported into gram - bacteria through porins.

Question 40

Why is ampicilin an oral drug

Answer 40

The amino group is protonated in the stomach so the positively charged ntrogen is more electron attracting. This decreases the nucleophilicity of the amide carbonyl oxygen so that it does not participate in ring opening of the lactam

Question 41

Which drug has better oral absorption, ampicillin or amoxicillin

Answer 41

Amoxicillin has better oral absorption

Question 42

What are B lactamase inhibitors usually used with? How do they work?

Answer 42

Used with B lactamase sensitive penicillins. They work by accylating the serine hydroxyl group in the active site of B lactamase

Question 43

Give examples of how B lactamase inhibitors enhance the activities of B lactamase sensitive B lactams vs B lactamase producing resistant strains

Answer 43

Potassium clauvanate + Amoxicillin -> augmentin Sulbactam + Ampicillin-> Unasyn Tazobactam + Pipericillin -> Zosyn Avibactam + Ceftazidime -> Avycaz

Question 44

Name a B lactamase sensitive, broad spectrum Parenteral penicillin

Answer 44

Piperacillin

Question 45

Compare the potency of pipericillin and ampicillin. Why?

Answer 45

Pipericillin preserves activity against both gram positive and negative bacteria. This results because of the added side chain fragment that resembles a longer section of the peptidoglycan chain than ampicillin

Question 46

Describe the MOA of cephalosporins

Answer 46

The basic mechanism of action of cephalosporins is the same as with penicillins. Reaction with transpeptidasees (penicillin binding proteins) results in inhibition of peptidoglycan crosslinking. many cephalosporins contain leaving groups that facilitate B lactam ring opening

Question 47

Are cephalosporins hydrolyzed by B lactmases

Answer 47

Yes

Question 48

How many different B lactamases are known?

Answer 48

340. Some are penicillinases some are cephalosporinases. They can also be specific for certain antibiotics

Question 49

compare prevalence of allergenicity of cephalosporins and penicillins? Croo allergenicity? symptoms?

Answer 49

Allergic rxn to cephalosporins are generally less common and less severe than with penicillins. There is a small risk of cross allergenicity, cephalosporins should be used with caution in pts allergic to penicillins. fever and rash are the most common manifestations and are exhibited in 1-3% of population

Question 50

What are the classifications of cephalosporins and the trend seen between generations

Answer 50

Cephalosporins are classified as 1st, 2nd, 3rd, 4th or 5th gen. Going from 1st to 3rd, cephalosporins have enhanced gram negative activity and loss of efficacy toward gram positive bacteria

Question 51

What bacteria are 1st gen cephalosporins active towards?

Answer 51

Primarily active towards gram + cocci ( Staph aureus and staph pyogenes, ), group B streptococci (streptococcus agalactiae) and streptococcus pneumoniae

Question 52

How can we identify orally active cephalosporins

Answer 52

They have unreactive side chains at C 3. 1st gen orally active ceopalosporins have methyl group at C3

Question 53

how do 2nd gen cephalosporins differ from 1st gen

Answer 53

They retain anti gram + activity of the 1at group, but have better activity than gram - than 1st gen

Question 54

Name a 1st gen cephalosporin? Second gen cephalosporin?

Answer 54

1st gen Parenteral- Cefazolin 1st gen oral- Cephalexin 2nd- Cefuroxime (oral and parenteral)

Question 55

Structural difference between parenteral cephalosporin and orally active cephalosporin

Answer 55

Orally active cephalosporins have substituents at C3 that are not chemically reactive. Cephalexin is an example of an orally active 1st gen cephalosporin

Question 56

Using stereochemistry, how can we tell if a given antibiotic structure is less or more resistant to enzymatic hydrolysis

Answer 56

If it is syn to the carbonyl, it is more resistant to enzymatic hydrolysis If anti methoximino, it is less resistant to enzymatic hydrolysis

Question 57

Compare 3rd gen cephalosporin to 1st and 2nd get

Answer 57

They are less active against staphylococci than 1st gen, but they are much more active vs Gram - bacteria than both 1st and 2nd gen cephalosporin

Question 58

What do almost all 3rd gen cephalosporins have in their structure

Answer 58

aminothiazole substituent and oxime ether at 7 position

Question 59

What is the use of the large oxime ether moiety at C-7 and the charged pyridinium at C3

Answer 59

The large oxime ether moiety converys enhanced stability vs B lactamases The charged pyridinium ring at C 3 is a very good leaving group and strongly activates the B lactam ring. It also imparts enhanced aqueous solubility and makes it parenterally active

Question 60

Why do we get more activity against gram negative bacteria as we go from 1stgen to 3rd gen

Answer 60

More polar = it goes through gram negative wall

Question 61

Nam a 3rd gen cephalosporin

Answer 61

Ceftazidime (parenterally active) Cefixime (orally)

Question 62

Elaborate on reactive and unreactive substituents on the C3 and oral bioavailability

Answer 62

The presence of an unreactive substituent at C-3 increases oral bioavailability.

Question 63

What confers activity vs gram negative microorganism

Answer 63

The carboxylic acid in the side chains confer activity vs gram negative microorganisms

Question 64

4th gen cephalosporin example

Answer 64

Cefepime (parenterally active)

Question 65

No leaving group at C3 means what for cephalosporin

Answer 65

Oral activity and bioavailability

Question 66

compare 4th gen cephalosporins

Answer 66

They retain antibacterial spectrum of 3rd gen cephalosporins and add pseudomonas aeruginosa and some enterobacteria that are resistant to 3rd gen cephalosporins. They are also more active against gram + organisms than 3rd gen cephalosporins

Question 67

Why is cefepime stable against B lactamases

Answer 67

The syn methoximino group at C7 stabilizes cefepime against B lactamases

Question 68

Name a 5th gen cephalosporin

Answer 68

Ceftaroline

Question 69

What is unique about how ceftaroline acts in body

Answer 69

It is a prodrug that is hydrolyzed metabolically after IV infusion to ceftaroline.

Question 70

Why is Ceftaroline so special?

Answer 70

It is able to inhibit MRSA PBP2a

Question 71

What type of cephalosporins are more likely to be active against gram - bacteria

Answer 71

Cephalosporins with positively charged nitrogen on the side chain

Question 72

Is ceftaroline B lactamase sensitive or resistant?

Answer 72

Resistant