Antibiotic Targets and Mechanisms

• To understand the 6 targets of antibiotics and how these antibiotics affect normal bacterial cell function.
• To describe four emerging paradigms in how bacterial infectious diseases are being approached.

Antibiotic Targets

• Antibiotics have six primary targets within bacterial cells.

Cell Wall Synthesis Inhibitors

• Cycloserine, Vancomycin, Bacitracin, Penicillins, Cephalosporins, Monobactams, and Carbapenems.
• Penicillin, Cephalosporin, and Vancomycin are examples.
• They inhibit cell wall synthesis at various steps, including transpeptidation and peptidoglycan precursor synthesis.
• β-Lactam antibiotics have a β-lactam ring and inhibit peptidoglycan cross-links by targeting transpeptidases.
• Vancomycin binds to the ends of stem peptides, preventing cross-link formation by targeting the transpeptidase substrate, and is similar to penicillin but with different activity.
• Cycloserine blocks peptide formation for cross-links.
• Bacitracin blocks movement across the membrane, and disaccharide subunits don't reach the periplasm because the lipid carrier bactoprenol can't be recycled and is a component of Neosporin ointment for topical use only.

Cytoplasmic Membrane Structure

• Polymyxins and Daptomycin.
• Polymyxins and Daptomycin serve as examples.
• Polymyxins disrupt LPS in Gram-negative bacteria.
• Lipopeptides such as Daptomycin, target Gram-positive bacteria.

Protein Synthesis Inhibitors

• Erythromycin, Chloramphenicol, Clindamycin, Lincomycin, Oxazolidinones (Linezolid), Tetracyclines, Spectinomycin, Streptomycin, Gentamicin, Kanamycin, Amikacin, Nitrofurans, Mupirocin, and Puromycin.
• Targets include the 30S and 50S ribosomal subunits.
• 30S: Tetracycline, Aminoglycosides.
• 50S: Erythromycin, Chloramphenicol.
• tRNA: Mupirocin.
• Chloramphenicol, Macrolides, Lincosamides, Streptogramins, Everninomycins, and Oxazolidinones are also examples of protein synthesis inhibitors.
• 23S rRNA scaffolds the 50S subunit, and 16S rRNA scaffolds the 30S subunit.

DNA Synthesis Inhibitors

• Quinolones like Nalidixic acid and Ciprofloxacin, and Novobiocin.
• They inhibit DNA gyrase, which is essential for DNA replication.
• Fluoroquinolones target Type II topoisomerases (DNA gyrase) and DNA topoisomerase IV.
• Ciprofloxacin serves as an example of a DNA gyrase inhibitor.

RNA Synthesis Inhibitors

• Actinomycin, Rifampin, and Streptovaricins.
• Rifampin inhibits RNA polymerase.
• Rifampicin serves as an example.

Folic Acid Metabolism

• Trimethoprim and Sulfonamides.
• They interfere with folic acid metabolism, an important metabolic pathway.
• Trimethoprim and Sulfonamides inhibit unique metabolic pathways like folic acid synthesis.
• Sulfonamides inhibit the conversion of PABA to FH2, while trimethoprim inhibits the conversion of FH₂ to FH₄.

Antibiotic Resistance Mechanisms

• As resistance mounts, new antibiotics are developed to combat resistant strains.

Tigecycline

• Tigecycline (Tygacil) was introduced in 2005, exhibiting bacteriostatic activity.
• It binds to the 30S ribosome and blocks the interaction of aminoacyl-tRNA with the A site of the ribosome.
• Active against both Gram-positive and Gram-negative bacteria.

Diarylquinoline

• Diarylquinoline Bedaquiline (Sirturo), introduced in 2012, inhibits Mycobacterium by targeting essential ATP synthase.

Other Antibiotics

• Pretomanid (Nitroimidazole) was introduced in 2019 as a mycolic acid synthesis inhibitor specifically targeting M. tuberculosis.
• Fidaxomicin (Dificid) was introduced in 2011 as a macrocyclic class drug (tiacumicins) that interferes with the C. difficile RNA polymerase, poorly absorbed and has long effective concentration in the GI tract, does not target GI tract normal flora ,and reduces recurrence of CDI.
• Dalbavancin and Oritavancin were introduced in 2014 as lipoglycopeptides with a similar mechanism as vancomycin but greater affinity for membrane.
• Pleuromutilins (Lefamulin) were introduced in 2019, inhibit bacterial protein synthesis, and interact with 23s rRNA of the 50S subunit.

New Paradigms in Treating Bacterial Infections

• Phage therapy holds promise.
• Rather than killing bacteria, can we "tame" them through anti-infective strategies that disarm major virulence factors?
• MRSA Agr QS is when Hamamelitannin blocks toxin production.
• Vibrio cholerae ToxT is when Virstatin blocks toxin production.
• This approach has problems of very narrow spectrum of activity, diagnostic certainty, may not work well in immunocompromised, and faces FDA approval challenges as to being as good as what already exists.
• Can we outcompete harmful bacteria by promoting good bacteria with probiotics and prebiotics?
• Fecal transplants can cure patients with C. difficile.
• Can we target resistance mechanisms directly?
• For example, using beta-lactamases with clavulanic acid (Augmentin) or developing efflux pump inhibitors.

Cell Target and Mechanisms and Resistance Examples:

• Cell Wall: Beta-lactams inhibit transpeptidation and inactivated cell wall transpeptidases in both Gram-positive and Gram-negative bacteria, but resistance can occur through OM barrier, efflux, β-lactamase, and target modification (PBPs), Penicillin, Cephalosporin, Carbapenem, and Monobactam are examples,
• Cell Wall: Glycopeptides and lipoglycopeptides and which inhibit transpeptidation by binding to the D-ala-D-ala stem and have resistance via OM barrier and D-ala-D-lac substitution. Vancomycin, Dalbavancin, and Oritavancin are examples
• Cell Membrane: Lipopeptides depolarize the membrane of Gram-positive bacteria, but the resistance mechanisms are ??? membrane alteration with Daptomycin serving as an example
• Cell Membrane: Polymyxins bind LPS and insert into the outer membranes and inner membranes of Gram-negative bacteria resistance is caused by LPS modification and efflux and Colistin serves as an example
• Protein Synthesis: Aminoglycosides bind the 16S rRNA in the 30S ribosome, across a broad spectrum resistance is caused by antibiotic inactivation or efflux and Gentamicin serves as an example
• Protein Synthesis: Tetracyclines bind the 16S rRNA in the 30S ribosome over a broad spectrum but resistance may occur through efflux or ribosome protection with Doxycycline and Tigecycline serving as examples
• Protein Synthesis: Macrolides bind the 23S rRNA in the 50S ribosome over a broad spedtrum, where efflux and 23S rRNA methylation might be resistance factors and Erythromycin serves as an example
• Protein Synthesis: Oxazolidinones bind the 23S rRNA to the 50S ribosome in Gram-positive bacteria, where efflux and 23S rRNA methylation may be at play with Linezoid serving as an example
• DNA Replication: Fluoroquinolones bind DNA gyrase in broad spectrum bacteria where resistance may be caused by efflux or gyrase mutation and Ciprofloxacin serves as an example
• DNA Replication: Metronidazole nicks bacterial DNA in anaerobic bacteria, where failure to activate, or reduction in Flavodoxin expression can be causes of resistance and Flagyl serves as an example
• Transcription: Rifampin binds the β subunit RNA polymerase in across a broad bacterial spectrum but resistance may occur through mutation of RNA polymerase or efflux. Rifampicin serves as an example.
• Transcription: Macrocyclic antibiotics (like Difficid) inhibit RNA Polymerase only in C. difficile, but resistance factors have not been reported yet. Difficid serves as an example.
• Tetrahydrofolate: Sulfonamides inhibit PABA to FH2 (PABA mimic) over many broad spectrum bacteria, were resistance is caused by enzyme mutation and efflux. Bactrim and Septra serve as examples.
• Tetrahydrofolate: Trimethoprim inhibits FH2 to FH4 (FH2 mimic) over many broad spectrum bacteria, were resistance is caused by enzyme mutation and efflux. Bactrim and Septra serve as examples.
• Mycolic Acid: Isoniazid: (Pro-Drug) inhibits Mycolic acid synthesis only in M. tb., where a Failure to Activate KatA resistance may be an issues and Isoniazid serves as an example
• Mycolic Acid: Nitroimidazole, inhibits Mycolic acid synthesis in M. tb. with Pretomanid serving as an example.