Exam 1 lecture 3 Flashcards
What colors do gram positive and gram negative bacteria stain under microscope
Gram positive strain dark purple and gram negative stain light pink
State the structural differences in gram positive and gram negative bacteria
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
Peptidoglycan backbone consists of what compounds
N acetylglucosamine
N acetylmuranic acid
How can we structurally tell the difference between N acetylglucosamine and N acetylmuramic acid?
N acetylglucosamine does not have a large structure attached to it (it is smaller)
How can we tell if a peptidoglycan wall is gram positive or gram negative
IF a mesodiaminoimetic acid reside is included it is in a gram negative residue. (it has a cooh near the bottom see structure)
What is peptidogycan cross linked by?
Transpeptidase
Where do cross links happen for gram + and gram - organisms
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.
Look at transpeptidase mechanism on Jan 17 lecture
Look at B lactam antibiotic mechanism of action on Jan 17 lecture
How do B lactams inactivate transpeptidase? Why are B lactams systems reactive?
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
How do B lactams fool transpeptidases
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
What could resistance to B lactam antibiotics stem from?
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
What is a common B lactam drug?
Penicillin
What percent of people are allergic to penicillin?
6-8 percent
What does the allergenicity of B lactam antibiotics result from? Is cross sensitivity common? What tests are recommended to test for B lactam allergenicity?
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
Under acidic conditions, the main degradation products of Pen G are
Benzylpenicillenic acid
Benzylpenicillic acid
Benzylpenicilloic acid
Memorize penicillin degradation mechanism on notes from Jan 17
MEchanism of penicillin degradation under basic conditions
Penicillin is turned into Penicilloic acid
Do penicillin hydrolysis products have antibiotic activity?
Is hydrilysis of B lactam reversible?
No antibiotic activity
Hydrolysis of B lactam is irreversible. Once the ring is opened it will not close.
What are the use of electronegative substituents on the side chain carbonyl in B lactams
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
What is the difference between the penicillin G and Penicillin V structurally and functionally?
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
Why should heavy metals be kept away from penicillin solutions
Heavy metal ions catalyze penicillin degradation reactions and should therefore be kept away from penicillin solutions
What is special about penicillin with more lipophilic side chains? What happens when a drug is protein bound?
Penicillin with more lipophilic side chains are more highly protein bound.
Protein binding reduces bioavailability by reducing the effective concentration of the drug
What value indicates lipid side chains
CLogP.
The higher the CLogP, the higher the lipophilicity
How is penicillin usually excreted?
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.
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?
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
WHat are the 5 nomenclatures in B lactam ring system?
Penam
Penem
Carbapenem
Cephem
Monobactam
Memorize the structures of the 5 nomenclatures of B lactam ring system in Jan 17 notes
For benzylpenicillin (penicillin G), what is the antimicrobial spectrum? B lactamase sensitivity? Administration? Toxicity? Precautions?
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
What is the main difference between penicillin G and V?
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
Name a B lactamase resistant drug
Methicillin
Why is methicillin not sensitive to B lactamases
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
Why was Methicillin discontinued? Why is MRSA resistant to methicillin? What is the gene called that codes for MRSA resistance?
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)
The penicillin binding protein that mecA codes for is called
PBP2A
Is Nafcillin B lactamase sensitive? COmpare with methicillin?
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
What are B lactamase resistant oral penicillins
Oxacillin
Cloxacillin
Diclocxacillin
Out of Oxacillin, cloxacillin, dicloxacillin, which ones are B lactamase sensitive? Which have been dx? What are they all called
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
Name B lactamase sensitive, broad spectryum, oral penicillins
Ampicillin
AMoxacillin
ANtimicrobial spectrum of ampicillin? Why?
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.
Why is ampicilin an oral drug
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
Which drug has better oral absorption, ampicillin or amoxicillin
Amoxicillin has better oral absorption
What are B lactamase inhibitors usually used with? How do they work?
Used with B lactamase sensitive penicillins.
They work by accylating the serine hydroxyl group in the active site of B lactamase
Give examples of how B lactamase inhibitors enhance the activities of B lactamase sensitive B lactams vs B lactamase producing resistant strains
Potassium clauvanate + Amoxicillin -> augmentin
Sulbactam + Ampicillin-> Unasyn
Tazobactam + Pipericillin -> Zosyn
Avibactam + Ceftazidime -> Avycaz
Name a B lactamase sensitive, broad spectrum Parenteral penicillin
Piperacillin
Compare the potency of pipericillin and ampicillin. Why?
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
Describe the MOA of cephalosporins
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
Are cephalosporins hydrolyzed by B lactmases
Yes
How many different B lactamases are known?
- Some are penicillinases some are cephalosporinases. They can also be specific for certain antibiotics
compare prevalence of allergenicity of cephalosporins and penicillins? Croo allergenicity? symptoms?
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
What are the classifications of cephalosporins and the trend seen between generations
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
What bacteria are 1st gen cephalosporins active towards?
Primarily active towards gram + cocci ( Staph aureus and staph pyogenes, ), group B streptococci (streptococcus agalactiae) and streptococcus pneumoniae
How can we identify orally active cephalosporins
They have unreactive side chains at C 3. 1st gen orally active ceopalosporins have methyl group at C3
how do 2nd gen cephalosporins differ from 1st gen
They retain anti gram + activity of the 1at group, but have better activity than gram - than 1st gen
Name a 1st gen cephalosporin? Second gen cephalosporin?
1st gen Parenteral- Cefazolin
1st gen oral- Cephalexin
2nd- Cefuroxime (oral and parenteral)
Structural difference between parenteral cephalosporin and orally active cephalosporin
Orally active cephalosporins have substituents at C3 that are not chemically reactive. Cephalexin is an example of an orally active 1st gen cephalosporin
Using stereochemistry, how can we tell if a given antibiotic structure is less or more resistant to enzymatic hydrolysis
If it is syn to the carbonyl, it is more resistant to enzymatic hydrolysis
If anti methoximino, it is less resistant to enzymatic hydrolysis
Compare 3rd gen cephalosporin to 1st and 2nd get
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
What do almost all 3rd gen cephalosporins have in their structure
aminothiazole substituent and oxime ether at 7 position
What is the use of the large oxime ether moiety at C-7 and the charged pyridinium at C3
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
Why do we get more activity against gram negative bacteria as we go from 1stgen to 3rd gen
More polar = it goes through gram negative wall
Nam a 3rd gen cephalosporin
Ceftazidime (parenterally active)
Cefixime (orally)
Elaborate on reactive and unreactive substituents on the C3 and oral bioavailability
The presence of an unreactive substituent at C-3 increases oral bioavailability.
What confers activity vs gram negative microorganism
The carboxylic acid in the side chains confer activity vs gram negative microorganisms
4th gen cephalosporin example
Cefepime (parenterally active)
No leaving group at C3 means what for cephalosporin
Oral activity and bioavailability
compare 4th gen cephalosporins
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
Why is cefepime stable against B lactamases
The syn methoximino group at C7 stabilizes cefepime against B lactamases
Name a 5th gen cephalosporin
Ceftaroline
What is unique about how ceftaroline acts in body
It is a prodrug that is hydrolyzed metabolically after IV infusion to ceftaroline.
Why is Ceftaroline so special?
It is able to inhibit MRSA PBP2a
What type of cephalosporins are more likely to be active against gram - bacteria
Cephalosporins with positively charged nitrogen on the side chain
Is ceftaroline B lactamase sensitive or resistant?
Resistant
