Antimicrobials targetting cell wall structure

Give examples of β-lactams and glycopeptides

• β-lactams - penicillins, cephalosporins, carbapenems, monobactams, clavulanic acid, sulbactam, tazobactam

• Glycopeptides - vancomycin, teicoplanin

• Cycloserine - targets peptidoglycan

• Isoniazid - targets mycolic acids

• Ethambutol - targets arabinogalactan

What are the differences in structure between gram- and gram+ bacteria?

Gram Negative

• 1-inner membrane

• 2-periplasmic space

• 3-impermeable outer membrane

• 4-phospolipid

• 5-peptidoglycan

• 6-lipoprotein

• 7-protein

• 8-Lipopolysaccharide

• 9 porins

Gram Positive

• 1-cytoplasmic membrane

• 2-peptidoglycan

• 3-phospholipid

• 4-protein

• 5-lipoteichoic acid

Why are gram- bacteria harder to treat and more resistant?

Impermeable outer membrane acting as a barrier, specialised efflux pumps that expel drugs, and high genetic plasticity.

• Limits entry of many antibiotics, making them difficult to treat

What is the benefits and downsides to bacteria having peptidoglycan?

• Benefits - strength, survive in hostile environment as unicellular organisms

• Downsides - easier for us to treat as we don’t have it to get damaged by antibiotics

What do β-lactams and glycopeptides do?

Inhibit the enzymes for the synthesis of peptidoglycan

What is gram- peptidoglycan made out of?

What is gram+ peptidoglycan made out of?

How are cross links formed in peptidoglycan?

• Transpeptidation (4–3 crosslinks) - DD-transpeptidases (PBP) form a bond between the 4th residue (D-alanine) of one chain and the 3rd residue (meso-diaminopimelic acid or L-lysine) of another.

• Alternative pathway (3–3 crosslinks) - LD-transpeptidases (LDTs) create 3-3 crosslinks, connecting the 3rd residue to the 3rd residue of neighboring chains.

• Energy generation - reaction involves the cleavage of a terminal D-Ala-D-Ala linkage, which provides the energy needed to form a new peptide bond.

• Bridge structures - In some bacteria, such as Staphylococcus aureus, the cross-link is not direct but involves a short interpeptide bridge, such as a pentaglycine chain, connected to the D-Ala of one stem and the L-lysine of another.

Amide → ester → amide

What is the structure of β-lactams and what is penicillins’ mechanism of action to inhibit petidoglycan synthesis of gram+ bacteria?

• Its β-lactam ring irreversibly binds to PBPS as it has a similar structure to D-Ala-D-Ala

• Inactivates penicillin-binding proteins, specifically DD-transpeptidase

• Blocking transpeptidation prevents the cross-linking of peptidoglycan chains.

• This weakens and unstabilises the cell wall, → to osmotic lysis and cell death

• May also activate bacterial autolysins, enzymes that degrade the cell wall, accelerating breakdown of cell wall, contributing to bactericidal activity

How are semi-synthetic penicillins made?

• When starved of phenylacetic acid, Penicillium chrysogenum produces the penicillin nucleus, 6-aminopenicillanic acid (6-APA).

• 6-APA has little intrinsic activity

• 6-APA can be converted to an active penicillin by reaction with an activated acid (e.g. acyl chloride)

• Can add side chains to make new antibiotics, that have better properties (Amp is stable in GI, large doses ok, slow metabolism, absorbs well across fat)

Structures of semi-synthetic penicillins

• Carbenicillin - good for gram-

• Methicillin - good for resistant (mrsa) and gram+

• Ampicillin - good for gram+, some gram- and β‑lactamases inhibitor

• Ticarcillin - good for gram-

• Flucloxacillin - good for gram+

• Amoxicillin - broad spec and β‑lactamases inhibitor

• Piperacillin - 2nd line as its broad spec antibiotic, good for resistant bacteria and β‑lactamases inhibitor

How do bacteria have resistance to penicillin?

• May produce β‑lactamases that hydrolyse the β‑lactam ring.

• Mutations in PBPs can reduce penicillin binding.

• Overuse accelerates the emergence of resistant strains

What is the mechanism of action of beta lactamases?

• β‑lactamases catalyse hydrolysis of the amide bond in the β‑lactam ring.

• This opens the ring, destroying the structural mimicry of D‑Ala‑D‑Ala needed for binding to PBPs.

• Once the ring is opened, the antibiotic cannot inhibit transpeptidases, so cell‑wall synthesis continues normally

What are the 2 types of β‑lactamases?

• Serine β‑lactamases (Classes A, C, D) - active‑site serine to attack the β‑lactam ring.

• Metallo β‑lactamases (Class B) - Zn²⁺ ions to activate water for hydrolysis.

What are the β‑lactamases inhibitors 2 mechanisms of action?

• They resemble β‑lactam antibiotics closely enough that β‑lactamases bind them over the actual antibiotic.

• Suicide - clavulanic acid, tazobactam, and sulbactam

  • Inhibitor enters the β‑lactamase active site.

  • The enzyme hydrolyses it → a covalent intermediate.

  • Inhibitor irreversibly inactivates the enzyme, destroying its catalytic function, no antibiotic hydrolysis

How does avibactam inhibit β‑lactamase?

• Binds reversibly to the β‑lactamase active site.

• Forms a stable, reversible covalent bond that prevents hydrolysis of the antibiotic.

• After dissociation, avibactam can inhibit additional enzyme molecules, giving it broad and sustained activity

What are cephalosporins?

β-lactam antibiotic

• For penicillin resistant bacteria

• Treats pneumonia, meningitis, and skin infections

• Limits clinical stability so semisynthetics (5 generations) are made

What are carbapenems?

β-lactam antibiotic - Thienamycin

• Broad spec (G+ve and G-ve) and resistant to β-lactamase

• Semi-synthetic carbapenems - Imipenem (sensitive to renal peptidase) Meropenem (resistant to renal peptidase)

• Prescribed for multi-drug resistant in hospitals

What are monobactams?

β-lactam antibiotic - Nocardicin A

• Aztreonam is a synthetic monobactam, resistant to β-lactamases and is the only clinically used monobactam

  • Tolerated by patients who are hypersensitive to penicillins

What are the ADRs for β-lactam antibiotics?

Type I (IgE) response believed to be due to “haptenization” of proteins

• Diarrhoea, nausea, urticarial rash

• Occurs in 1% of patients and severe anaphylaxis occurs in 0.01% of patients

• Cross-sensitivity is low between classes

What are glycopeptide antibioitcs?

Vancomycin

• Effective against gram+ bacteria as they’re big and too lipophilic

• Natural but no produced by fermentation

• Biosynthesis by unusual non-ribosomal peptide synthesis

What are the problems with teicoplanin as an example of glycopeptide antibiotic?

Mixture of 5 compounds, various side chains

• Different antibacterial activity

• Potential for variable toxicity

• Complex quality control

• Differences in pharmacokinetics

• Relative proportions of each component can vary between manufacturing batches

How do glycopeptides work on gram+ bacteria?

• Preventing the formation of the linear glycan (NAG-NAM) strands by transglycosylase. Inhibiting peptide cross linking by transpeptidase

• They do this by binding tightly to the terminal D-alanyl-D-alanine of the peptidoglycan by hydrogen bonding

• Transglycosylases → cannot polymerise glycan chains

• Transpeptidases (PBPs) → cannot cross‑link the chains

• Cell wall cannot be extended or strengthened so cell lysis happens

1. Which of these structures is a carbapenem?

2. Which of these antibiotics is not a β-lactam antibiotic?

3. Which of these structures is a cephalosporin?

4. Which of these structures is a β-lactamase inhibitor?

5. Which of these structures might be resistant β-lactamase activity?

6. Which of these structures could be a product of β-lactamase activity on a penicillin?

7. Which of these might be tolerated by a patient with penicillin sensitivity?

1. C → carbon and not sulfur on 4,5 ring

2. B, F and E, G -. they don’t have b-lactam

3. A, H → 4 membered ring fused to 6

4. B, F → 4,5 ring system but no side chain, has oxygen or modified sulfur

5. D,E,G and A,C → they’re late gen penicillins

6. E → b-lactam ring is open

7. D,G