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Learning objective

  • Describe the mechanisms of action associated with drugs that inhibit cell wall biosynthesis, protein synthesis, membrane function, nucleic acid synthesis, and metabolic pathways

An important quality for an antimicrobial drug is selective toxicity , meaning that it selectively kills or inhibits the growth of microbial targets while causing minimal or no harm to the host. Most antimicrobial drugs currently in clinical use are antibacterial because the prokaryotic cell provides a greater variety of unique targets for selective toxicity, in comparison to fungi, parasites, and viruses. Each class of antibacterial drugs has a unique mode of action (the way in which a drug affects microbes at the cellular level), and these are summarized in [link] and [link] .

An illustration of a cell is shown with a view inside. The double helix is visible in the center, and a label points to it indicating DNA synthesis, fluoroquinolones, ciprofloxacin, levofloxacin, moxifloxacin, RNA synthesis, Rifamycins, and rifampin. Another label points to the cell wall and indicates beta lactams, penicillins, cephalosporins, monobactams, carbapenems, glycopepties, vancomycin, and bacitracin. A third label points to the plasma membrane and indicates polymyxins, polymyxin B, colistin, lipopeptide, and daptomycin. Within the cytoplasm, another label points to ribosomes, which include 30s subunit, aminoglycosides, tetracyclines, 50s subunit, macrolides, lincosamides, chloramphenicol, and oxazolidinones. The final label points to the metabolic pathways and indicates folic acid synthesis, sulfonamides, sulfones, trimethoprim, mycolic acid synthesis, and izoniazid.
There are several classes of antibacterial compounds that are typically classified based on their bacterial target.
Common Antibacterial Drugs by Mode of Action
Mode of Action Target Drug Class
Inhibit cell wall biosynthesis Penicillin-binding proteins β-lactams: penicillins, cephalosporins, monobactams, carbapenems
Peptidoglycan subunits Glycopeptides
Peptidoglycan subunit transport Bacitracin
Inhibit biosynthesis of proteins 30S ribosomal subunit Aminoglycosides, tetracyclines
50S ribosomal subunit Macrolides, lincosamides, chloramphenicol, oxazolidinones
Disrupt membranes Lipopolysaccharide, inner and outer membranes Polymyxin B, colistin, daptomycin
Inhibit nucleic acid synthesis RNA Rifamycin
DNA Fluoroquinolones
Antimetabolites Folic acid synthesis enzyme Sulfonamides, trimethoprim
Mycolic acid synthesis enzyme Isonicotinic acid hydrazide
Mycobacterial adenosine triphosphate (ATP) synthase inhibitor Mycobacterial ATP synthase Diarylquinoline

Inhibitors of cell wall biosynthesis

Several different classes of antibacterials block steps in the biosynthesis of peptidoglycan , making cells more susceptible to osmotic lysis ( [link] ). Therefore, antibacterials that target cell wall biosynthesis are bactericidal in their action. Because human cells do not make peptidoglycan, this mode of action is an excellent example of selective toxicity.

Penicillin, the first antibiotic discovered, is one of several antibacterials within a class called β-lactams . This group of compounds includes the penicillins , cephalosporins , monobactams , and carbapenems , and is characterized by the presence of a β-lactam ring found within the central structure of the drug molecule ( [link] ). The β-lactam antibacterials block the crosslinking of peptide chains during the biosynthesis of new peptidoglycan in the bacterial cell wall. They are able to block this process because the β-lactam structure is similar to the structure of the peptidoglycan subunit component that is recognized by the crosslinking transpeptidase enzyme, also known as a penicillin-binding protein (PBP) . Although the β-lactam ring must remain unchanged for these drugs to retain their antibacterial activity, strategic chemical changes to the R groups have allowed for development of a wide variety of semisynthetic β-lactam drugs with increased potency , expanded spectrum of activity , and longer half-lives for better dosing, among other characteristics.

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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