The Bacterial Cell Wall and Beta-Lactam Antibiotics Flashcards
Gram positive bacteria are ____ under a microscope (after staining)
Dark Purple
Gram negative bacteria are ____ under a microscope (after staining)
Light Pink
Describe a Gram positive cell wall
Peptidoglycan – lots of it Bacterial membrane underneath
Describe a Gram negative cell wall
Outer envelope w/Porins Periplasmic space Less peptidoglycan Bacterial membrane
Other name for Penicillin-binding proteins
Transpeptidases
Difference in drug penetrance between G+ and G- bacteria
In G+ –> Drugs can penetrate outer layers of the wall effectively, membrane is main protective layer In G- –> Outer membrane excludes drugs, but some can still get in through porins in the outer membrane
Difference in beta-lactamase secretions between G+ and G-
G+ –> excreted through the cell well to the external environment. Larger quantities produced G- –> Confined to periplasmic space
Which type of bacteria has a thicker peptidoglycan layer
G+
Which type of bacterial membrane is more lipoidal?
G-
In G- bacteria, the peptidoglycan contains a ______ residue. In G+ this residue is replaced by a ______ residue.
Meso-diaminopimelic acid residue (DAP) L-lysine residue (COOH of DAP replaced by an H)
Main components of peptidoglycan
N-acetylglucosamine and N-acetylmuramic acid
In G- cells, peptidoglycan is cross linked by…
Bridge between DAP residue of one stand and the terminal D-Ala of the other
In G+ cells, peptidoglycan is cross linked by…
L-Lys strand and the terminal D-Ala of the second molecule
The cross link between peptidoglycan molecules typically consists of ___ amino acids
5 Ex. S. aureus has 5 glycines
What happens in the transpeptidase reaction to connect S. aureus peptidoglycan
Since S. aureus is G+, The terminal glycine of one residue attaches to the penultimate Alanine residue, releasing the terminal D-Ala on the second molecule.
beta-Lactam mechanism of action?
Inhibition of transpeptidases that glue the peptidoglycan strands together by cross linking
beta-lactam antibiotics work by ______ the transpeptidase on the _____ residue
Acetylating Serine
Following beta-lactam exposure, the bacterial cell wall…
is subject to cell lysis and cell death
The beta-lactam system is very reactive due to steric strain. The NCC bond angle is a ____ angle, compared to the normal _____ angle.
90 degree 120 degree
Aside from steric strain, the beta-lactam carbonyl is also more reactive due to….
its poor stereoelectronics. These make it more like a ketone carbonyl than an aminde carbonyl due to the overlap on non-bonded electrons. In Bryan words – to make the Nitrogen’s electrons fit with the C=O, they have to fold up in a weird shape that inhibits their ability to have resonance. Makes it more ketone-like
Penicillins assume what structure
Folded ring with sp3 hybridized Nitrogen
Why don’t bacterial transpeptidases catalyze reactions with host cell proeins?
The bacterial substrate contain D-Ala amino acid residue not found in host cell proteins
Causes of beta-lactam resistance
Decreased cellular uptake of drug Mutation of Pen-binding proteins Presence of an Efflux pump Induction/Elaboration of bacterial beta-lactamases
What do beta-lactamases do?
Catalyze the hydrolysis of the beta-lactam moiety
Difference between the chemical reaction of a beta-lactam+transpeptidase and the reaction of a beta-lactam and beta-lactamase
Basically nothing except the beta-lactamases are recyclable
T or F. Resistance to beta-lactams is uncommon.
False. It is increasingly common
Describe the speed of the hydrolysis of the acetylated intermediate
Its fast, so the enzyme can hydrolyze many drug molecules rapidly
About ___% of the US population is allergic to beta-lactams
6-8
Cause of the allergenicity of beta-lactams
The drug acts as a hapten It acetylates host cell proteins, which then raise Abs
Why can’t the drug be modified to cause no allergic reaction
The site that causes the allergy is also the one that causes the antimicrobial fxn
Can a person who has demonstrated a penicillin allergy of one kind be given a different type of penicillin to avoid allergic response.
No. No they cannot.
Recommendations for patients that are unsure about allergy status?
Topical flare and wheal test for beta-lactam allergy
Under acidic conditions (the stomach), the main degradation products of Pen G are…
BenzylpenicILLENIC Acid BenzylpenILLIC Acid BenzylpenicILLOIC Acid
How is penicillenic acid made in acidic degradation.
Beta-elimination of sulfhydryl group in the second penicillin break down intermediate
How much antibiotic activity is retained by penicillin hydrolysis products
No antibiotic activity
T or F. Hydrolysis of the beta-lactam is irreversible.
True. Once the ring has been opened, it can’t be pushed back shut
Significance of electronegative substituents on the side chain carbonyl. How does that work.
Electronegative substituents on the side chain carbonyl reduce the nucleophilicity of the side chain amide carbonyl oxygen atom. This stabilizes the penicillin against hydrolsis under acidic conditions and increased bioavailability.
Give an example of a Penicillin made more stable by addition of an electronegative group
Penicillin G is 80% hydrolyzed in te stomach An extra Oxygen makes Penicillin V 35% hydrolyzed in the stomach.
Storage and longevity of penicillin solutions.
They should be used promptly (if being used parenterally) Store in fridge btw pH 6-6.8
_____ catalyze penicillin degradation reactions and should be kept away from solutions
Heavy metal ions
What makes certain penicillins more able to bind serum proteins more effectively.
More lipophilic side chains
Pro of serum protein binding
In general, it protects drugs from degradation
Serum protein binding _____ the bioavailability. Why?
Reduces Reduces effective concentration of free drug.
Why aren’t penicillin half lives altered by protein binding?
Their dissociation rates from the protein are fast and the renal excretion rates are rate limiting
___% of penicillin renal excretion is performed by glomerular filtration. _____% by tubular secretion.
10% 90%
Relationship of Probenecid and Penicillin
Porbenecid is an anion administered with penicillin that will compete with penicillin for secretion, increasing the half life of penicillin.
Antimicrobial spectrum for Benzylpenicillin (Pen G)
G+ N. gonorrhoeae + H. influenza
Is Benzylpenicillin (Pen G) sensitive to beta-lactamase?
yes
How is Benzylpenicillin (Pen G) administered?
Orally in large doses (due to stomach acid) Most effective parenteral
Toxicity associated with Benzylpenicillin (Pen G)?
Acute Allergic Reactions
Precautions for Benzylpenicillin (Pen G)
Patients with significant allergy/asthma history
Methicillin. Beta-lactamase sensitive?
No Steric hindrance of the nucleophilic attack by the enzyme on the beta-lactam carbonyl
Removal of one Methicillin methoxyl group (or a change to para position) will cause…
Abolished beta-lactamase sensitivity
How is Methicillin administered?
By injection Unstable to acid in the stomach due to electron donation toward the added o-methoxy groups
Antimicrobial spectrum of Methicillin?
beta-lactamase producing S. aureus
Why should Methicillin only be used when required
It induces beta-lactamase production, making other penicillins less effective
How does MRSA avoid Methicillin?
Mutation in a transpeptidase Methicillin resistance gene (mecA) and Penicillin binding protein (PBP2)
Cephapirin is what type of antibiotic
Cephalosporin
What is fused to the beta lactam in Cephapirin
A six-membered sulfur containing ring (instead of the boring 5 member one)
Cephalosporins with have an acetate in the 3-position are…
metabolically inactivated by hydrolysis by host esterases Makes hydroxymethyl acid that lactonizes
What are carbapenems?
Carbon analogs of penicillins Sulfur group in thiazolide ring of penicillins becomes methylene
Imipenem. beta lactamase sensitivity?
Reacts with an inhibits beta-lactamases
Imipenem is hydrolyzed by…. How to get around it?
Renal dihydropeptidase-1 Cilastatin (a dihydropeptidase inhibitor)
Perks of Imipenem + Cilastatin
Broader spectrum of antibiotic activity than anything else in US Gram + and Gram - “Magic Bullets”
Why is Imipenem + Cilastatin restricted?
To avoid bacterial resistance
Imipenem + Cilastatin is typically used to treat…
Serious gut infections, GU, Bone, Skin, and Endocardium
Imipenem + Cilastatin route of administration
Parenterally
Aztreonam is completely synthetic, but inspired by…
monocyclic beta-lactams (monobactams
In Aztreonam, the C2 carboxyl group is replaced with
Sulfamic Acid
Aztreonam antibiotic spectrum.
Almost entirely G-
What makes Aztreonam so good anyway?
The Sulfamic Acid activates the beta-lactam ring toward hydrolysis and to rxn with penicillin binding proteins
Indication for Aztreonam?
Severe G- Infections, especially with penicillin-resistant hospital organisms.
Cross allergenicity with penicillins and cephalosporins…is it a thing?
No (except for Ceftazidime)
Identify
Benzylpenicillin
Identify.
Methicillin
Identify.
Cephaprin
Identify.
Imipenem
Identify.
Aztreonam
