Harrold 1 Inhibitors of Bacterial Wall Synthesis

Why are antibiotics that inhibit cell wall synthesis selectively toxic to bacteria?

Humans do not have a cell wall

Stage I of Bacterial Cell Wall Synthesis

Involves the formation of NAG and NAM

Stage II of Bacterial Cell Wall Synthesis

Involves the formation of a linear peptidoglycan that consists of alternating NAM and NAG units

Peptidoglycan synthetase

Key enzyme for linking NAG and NAM together

• Involved in Stage II of Bacterial Cell Wall Synthesis

Bactoprenol

Lipid soluble carrier that transports NAM-NAG units from the cytoplasm to the periplasm

• Monophosphate reacts with NAM

• Diphosphate carries NAM-NAG units for incorporation into linear peptidoglycan

At the end of Stages I and II of Bacterial Cell Wall Synthesis, you have numerous _____

Peptidoglycan strands

Which sugar component of peptidoglycan is attached to a pentapeptide containing D-Ala-D-Ala?

NAM

Stage III of Bacterial Cell Wall Synthesis

Involves the cross-linking of the linear peptidoglycan

Which amino acids are connected to cross-link peptidoglycan strands?

Gly and D-Ala

Inhibition of any stage of bacterial cell wall synthesis results in a _____ effect

Bacteriocidal

Mechanism of Cycloserine

Antimetabolite of D-Ala (mimics D-Ala)

Inhibits the synthesis of the D-Ala-D-Ala portion of the linear peptidoglycan

Antimetabolite

A substance competes with, replaces, or antagonizes a specific endogenous metabolite

• Normally inhibits an enzyme

The synthesis of the D-Ala-D-Ala dipeptide is controlled by which 2 enzymes?

Alanine racemase

• L-Ala → D-Ala

D-alanyl-D-alanine synthetase

• Join 2 D-Ala together

Both enzymes are inhibited by cycloserine

Why is the use of cycloserine limited to second-line tuberculosis therapy?

It has weak potency and high incidence of toxic reactions

Mechanism of Vancomycin

Binds tightly to the D-Ala-D-Ala portion of NAM, sterically hindering peptidoglycan synthetase

Spectrum of Action of Vancomycin

Gram positive only

• Likely because gram negative porins do not allow transport

Vancomycin has _____ oral absorption

Very poor

• Due to large size, many water soluble FGs, and peptide nature

How is vancomycin administered?

IV

Oral

• Very poor oral absorption, but can be used to treat local infections in the GI tract (Ex. Pseudomonas colitis)

Common adverse effect associated with vancomycin

Hypersensitivity reaction, red man syndrome

• Involves extreme flushing due to histamine release

What are lipoglycoproteins?

Semi-synthetic derivatives of vancomycin

Key difference is the inclusion of a lipid soluble tail attached to a sugar molecule

Common lipoglycoprotein antibiotics

Telavancin

Dalbavancin

Oritavancin

Indications for lipoglycoproteins

Complicated skin and skin structure infections by susceptible gram positive bacteria

Ineffective against gram negative bacteria

Effective against MRSA

Mechanism of lipoglycoproteins

Binds to D-Ala-D-Ala to inhibit peptidoglycan synthetase

Incorporation of the lipophilic side chain into the bacteria cell membrane

• Disrupts cell membrane integrity by causing depolarization and increasing the permeability of the membrane

• Different than vancomycin

Secondary effect of vancomycin and lipoglycoproteins binding to D-Ala-D-Ala

Inhibition of peptidoglycan polymerization by inhibiting:

• Transglycosylase

  • Required for NAM + NAG synthesis

• Transpeptidase

  • Crosslinks

Bacitracin is a mixture of polypeptide antibiotics, primarily _____

Bacitracin A

Parenteral bacitracin is rarely used due to the risk of _____

Serious nephrotoxicity

Mechanism of bacitracin

Inhibits the dephosphorylation of bactoprenol-diphosphate and traps it in the wrong phosphorylation state

Indications of bacitracin

Normally used topically to prevent superficial skin and eye infections following minor injuries

Transpeptidase

Enzyme that catalyzes the two step reaction of cross-linking peptidoglycan strands

Two steps involved in cross-linking peptidoglycan strands

1. A nucleophilic serine residue on the enzyme breaks the bond between D-Ala-D-Ala and becomes acylated

2. The terminal Gly on another strand reacts with the acylated enzyme

Mechanism of β-lactams

Mimic the D-Ala-D-Ala portion of the bacterial cell wall and one to transpeptidase in place of D-Ala-D-Ala

The β-lactam bond sits in the same location as the labile D-Ala-D-Ala bond

• β-lactam ring is hydrolyzed instead of D-Ala-D-Ala bond, which forms a covalent bond due to increased steric size

  • Irreversibly inhibits transpeptidase

Penicillin Binding Proteins (PBPs)

Targets for the actions of penicillins and cephalosporins

• All bacteria have these, but numbers can vary

• Most important one is transpeptidase

PBPs vary in affinity for β-lactams, but interactions eventually become _____

Covalent

PBPs 1a and 1b are of higher molecular weight, but most importantly include _____

Transpeptidase

Mechanisms of resistance to β-lactams

Enzymatic inactivation by β-lactamases

• Most common

Decreased affinity of transpeptidase for β-lactams

• Second most common

Abnormalities of autolysin activity by the bacteria

Inability of the compound to penetrate to the site of action

• More of an issue of spectrum than resistance

How do β-lactamases inactivate β-lactams?

Cleave the β-lactam ring so they no longer mimic D-Ala-D-Ala

Are β-lactamases PBPs?

No, the reaction between a β-lactam and a β-lactamase is NOT covalent

• β-lactamase quickly regenerates, allowing it to continually destroy β-lactam antibiotics

How does decreased affinity of transpeptidase for β-lactams lead to resistance?

Transpeptidase can still crosslink but cannot bind to β-lactams

• Can lead to resistance even if the compound is β-lactamase resistant

• Responsible for MRSA

  • Methacillin is β-lactamase resistant, but it cannot bind to transpeptidase

Structure of Daptomycin

Cyclic lipoprotein

Spectrum of Action of Daptomycin

Broad spectrum of gram postive organisms

Advantage of cyclical nature of daptomycin

Prevents exopeptidase from degrading it

Advantage of the D-amino acids in daptomycin

Decreased ability of endopeptidases to destroy the peptide

Mechanism of Daptomycin

Interferes with the integrity of cell wall structure

Binds to bacterial membranes and causes a rapid depolarization of membrane potential

The loss of membrane potential inhibits DNA, RNA, and protein synthesis and eventually leads to cell death

Does NOT penetrate the bacterial cytoplasm

Potential drug interaction with daptomycin

Increases creatinine phosphokinase (CPK) levels and could potentially interact with statins (increased myopathy)

• Potential solution is to temporarily discontinue the statin during therapy

Colistimethate is a sulfomethylated prodrug of the active drug _____

Colistin

Conversion of colistimethate to colistin requires _____

Hydrolysis of the five sulfomethyl groups

How are colistimethate and colistin administered?

IV or IM

• Not orally active

What is the advantage of colistimethate as a prodrug?

Increased H₂O solubility for IV and IM use

How does the acidity of colistimethate differ from that of colistin?

The polyionized prodrug is acidic, while the active drug is basic

Indications of colistimethate

Treatment of infections caused by gram-negative bacteria, with a focus on multidrug resistant organisms such as Pseudomonas aeruginosa, Acinetobacter baumannii, and Klebsiella pneumoniae

Mechanism of colistin

Acts as a surfactant and disrupts the structure of the bacterial cell membrane

Binds to gram-negative bacterial cell phospholipids by displacing Ca²⁺ and Mg²⁺

• Positive charges cause the displacement

The loss of membrane integrity and increased permeability of the cell envelope results in leaking of cell components and subsequent cell death

Actively binds to the lipid A portion of endotoxin in the outer membrane, which inactivates the endotoxin

Colistin has shown synergistic activity against gram negative microorganisms with _____

Tetracyclines, chloramphenicol, and the beta-lactams

Resistance mechanisms against colistin

Alterations of the outer membrane of the bacterial cell (major)

Decrease concentrations due to an efflux pump

Indication for fosfomycin

Single dose (usually 3 grams) oral treatment of uncomplicated UTIs in women

Mechanism of fosfomycin

Mimics phosphoenolpyruvate (PEP) and blocks synthesis of NAM