Mode of action of antibiotics

Antibiotics are used in medicine and agriculture to prevent bacterial infection. The mode of action of antibiotics is varied based on the types of antibiotics. Some of them act by limiting the growth of bacteria and some of them act by killing the bacterial cell by inhibiting cell wall synthesis.

Now, the question is how do antibiotics work?, well the mode of action of antibiotics varies based on types of bacteria and antibiotics. The antibiotic mode of action is grouped into different categories such as;

1. Inhibition of cell wall synthesis
2. Inhibition of cell membrane functions.
3. Inhibition of protein synthesis.
4. Inhibition of DNA synthesis
5. Inhibition of RNA synthesis
6. Inhibition of Mycolic Acids 
7. Inhibition of Folic acid Synthesis

Inhibition of cell wall synthesis

• The antibiotics inhibit the cell wall synthesis by blocking the peptidoglycan synthesis within the cell wall.
• Peptidoglycan is composed of polysaccharides which are the repeating disaccharides of N- acetylglucosamine and N-acetylmuramic acid and also contain a cross-linked pentapeptide.
• The Peptide cross-link is formed between the free amine of the amino acid in the 3rd position of the peptide & the D-alanine in the 4th position of another chain.

Some example and mode of action of cell wall inhibitor antibiotics are;

a. β-lactam antibiotics

• The β-lactam antibiotics inhibit the 3rd stage of transpeptidation reaction to block peptidoglycan synthesis. As a result, the D-alanine loose from the pentapeptide.
• This type of antibiotic first binds to the Penicillin-binding proteins (PBPs) and then activate the autolytic enzymes (murein hydrolases) in the cell wall. After that it leads to the degradation of peptidoglycan and lysis of the bacterial cell.
• The lysis of bacterial cell by β-lactam antibiotics occurs in isotonic environment where involve the cell swelling and rupture of a bacterial cell or either occurs in Hypertonic environment where the microbes change to protoplasts (gram +) or spheroplasts (gram -) covered by cell membrane after that the cell swell and rupture if placed in isotonic environment.

There are present different β-lactam antibiotics such as Penicillin, Cephalosporins, carbapenems, aztreonam, etc.

Mode of action of Penicillin

• Penicillin is originated from Penicillium spp (molds).
• Penicillin act by inhibiting the final cross-linking step.
• It binds to the active site of the transpeptidase & inhibits its activity.
• Penicillin is bactericidal but kills only when bacteria are actively growing.
• It can be inactivated by β-lactamases.

Mode of action of Cephalosporins

• Cephalosporin is a bactericidal drug.
• Unlike penicillin, it has a similar structure and mode of action.
• Most are products of molds of the genus Cephalosporium.
• The mode of action of Cephalosporins is, it disrupts the synthesis of the peptidoglycan layer of bacterial cell walls by the competitive inhibition on PCB (penicllin binding proteins). The peptidoglycan layer is essential for cell wall structural integrity.

B. Other cell wall inhibitors

Except β-lactam antibiotics, there are other antibiotics that can inhibit cell wall synthesis such as;

Mode of action of Vancomycin

• The source of Vancomycin is Streptomyces orientalis. 
• This antibiotic Inhibit the second stage of peptidoglycan synthesis by two methods: a. binding directly to D-alanyl-D-alanine and block transpeptidase binding and b. By inhibiting bacterial transglycosylase.
• Basically, this antibiotic binds to the D-Ala-D-Ala terminal of the growing peptide chain during cell wall synthesis, as a result of this it inhibits the transpeptidase, which leads to the prevention of further elongation and cross-linking of the peptidoglycan matrix.

Mode of action of Cycloserine

• Cycloserine act by Inhibiting 2 enzymes such as D-alanine-D-alanine synthetase and alanine racemase which catalyze the cell wall synthesis.
• Cycloserine also inhibits the 1st stage of peptidoglycan synthesis.
• It is a structural analog of D-alanine thus it inhibit the synthesis of D-alanyl-D-alanine dipeptide.
• It is a second-line drug in the treatment of TB.

Mode of action of Isoniazid & Ethionamide

• It is Isonicotinic acid hydrazine (INH).
• Isoniazid & Ethionamide act by Inhibiting the mycolic acid synthesis. 

Mode of action of Ethambutol

• Ethambutol acts by Interfering with the synthesis of arabinogalactan in the cell wall.

Mode of action of Bacitracin

• The source of Bacitracin is Bacillus licheniformis. 
• It act by preventing the dephosphorylation of the phospholipid which helps to carries the peptidoglycan subunit across the membrane, thus block regeneration of the lipid carrier & inhibits cell wall synthesis.
• Bacitracin is too toxic for systemic use. It mainly used for treatment of superficial skin infections.

Inhibition of cell membrane function

There are present different antibiotics which are acts by inhibiting the cell membrane function in different way such as;

Mode of action of Polymixin

• The Source of Polymixin is Bacillus polymyxa. 
• The Polymixin with a positively charged free amino group acts as a cationic detergent. These bind to and disrupt the negatively charged lipopolysaccharide (LPS) of the outer membrane of Gram-negative bacteria, and thus increase the cell permeability. As a result it allows the passage of the polymyxin (and of other drugs) into the periplasmic space.

Mode of action of Polyenes (Anti-fungal)

These are anti-fungal drugs. The mode of action of Polyenes is, they bind to the ergosterol and results in disruption of the fungal cell membrane, the main sterol in the membrane. There are present different polyenes such as nystatin, natamycin, and amphotericin B.

• Mode of action of Amphotericin B: This is a polyene antifungal. It acts by binding to ergosterol in fungal cell membranes, which results in developing holes in the membrane and thus allows the cell components to leak out, causing cell death.
• Mode of action of Nystatin: This is an ionophore. It acts by binding to the ergosterol, which is a major component of the fungal cell membrane. In presence of sufficient concentrations, it develops pores in the membrane, as a results it lead to K+ leakage, acidification, and death of the fungal cell.
• Mode of action of Natamycin: It acts by binding to the ergosterol in the plasma membrane of fungal cells. Then it prevents ergosterol-dependent fusion of vacuoles, as well as membrane fusion and fission.

Mode of action of Azoles

• Azoles acts by inhibition of 14α-lanosterol demethylase, which is a key enzyme in ergosterol biosynthesis. This results in depletion of ergosterol and accumulation of toxic 14α-methylated sterols in membranes of susceptible yeast species.
• There are two groups of azoles such as; the imidazoles, these contain two nitrogens in the azole ring such as, clotrimazole, econazole, ketoconazole, miconazole, and tioconazole. And the second one is the triazoles, these contain three nitrogens in the azole ring such as; luconazole, itraconazole, posaconazole, etc.

Read More: idrugscreen.com

Inhibition of protein synthesis

The antibiotics inhibit the protein synthesis within the bacteria cell by binding to the ribosomes. There are two types of antibiotics, classified based on the binding site on the ribosome such as;

Drugs that acts on 30s subunit/Anti-30S ribosomal subunit

There are two groups of drugs which are bind to the 30s subunit of the bacterial cell and inhibits the protein synthesis such as Aminoglycosides and Tetracyclines.

• Mode of action of Aminoglycosides (gentamicin): After entering to the bacterial cell it binds to the 30s ribosomal subunit and results in the misreading of the genetic code. This subsequently leads to the arrest of normal bacterial protein synthesis. Some examples of Aminoglycosides are Gentamicin, tobramycin, amikacin, etc.
• Mode of action of Tetracyclines: Once the Tetracyclines enters into the baterial cell it reversibly adheres to receptors on the 30S ribosomal subunit of the bacterial cell, and prevents the attachment of aminoacyl-tRNA to the RNA-ribosome complex. Some examples of Tetracyclines are Chlortetracycline, oxytetracycline, etc

Drugs that acts on 50s subunit

There are several drugs which are bind to the 50s subunit of the bacterial cell and inhibit the protein synthesis such as Macrolides, Chloramphenicol, Clindamycin, Linezolid, and Streptogramins.

• Mode of action of Macrolides: Macrolides acts by preventing the peptidyltransferase from joining the growing peptide attached to tRNA to the next amino acid (similarly to chloramphenicol) as well as inhibiting bacterial ribosomal translation.
• Mode of action of Chloramphenicol: Chloramphenicol inhibits the peptidyl transferase activity of the bacterial ribosome as result it leads to the prevention of protein chain elongation.
• Mode of action of Clindamycin: After binding to the 50s subunit, it prevents protein synthesis by interfering with the transpeptidation reaction, which thus inhibits early chain elongation.
• Mode of action of Linezolid: Linezolid acts by interfering with bacterial protein translation. It attaches to a site on the bacterial 23S ribosomal RNA of the 50S subunit and prevents the formation of a functional 70S initiation complex, which is required for bacterial reproduction, thereby preventing bacteria from dividing.
• Mode of action of Streptogramins:  The streptogramins composed of mixtures of two structurally distinct compounds, such as type A (bacteriostatic) and type B (bactericidal). These antibiotics act at the level of inhibition of translation through binding to the bacterial ribosome.

Inhibition of DNA synthesis

There are several drugs which help in the Inhibition of DNA synthesis of the bacterial cell such as Fluoroquinolones and Metronidazole.

• Mode of action of Fluoroquinolones: Fluoroquinolones prevent DNA synthesis by preventing two enzymes within the bacterial cell such as DNA gyrase and the topoisomerase IV enzymes. These enzymes are essential for bacterial DNA synthesis. Both of these DNA topoisomerases that absent in human cells.
• Mode of action of Metronidazole: Metronidazole acts by interacting with DNA and results in a loss of helical DNA structure and strand breakage. 

Inhibition of RNA synthesis

The inhibition of RNA synthesis within the bacterial cell is caused by Rifampin.

Rifampin

• It is used to treat mycobacterial infections including Mycobacterium avium complex, leprosy.
• It is a semisynthetic antibiotic that is produced from Streptomyces mediterranei.
• It can act against several forms of Mycobacteriumm thus it is a  broad spectrum antibiotic.
• Mode of action of Rifampin: Rifampin acts via the inhibition of DNA-dependent RNA polymerase, which is results in the suppression of RNA synthesis and cell death.

Inhibition of Mycolic Acids synthesis

The inhibition of Mycolic Acids synthesis within the bacterial cell is caused by Isoniazid.

Mode of action of Isoniazid

Isoniazid is a prodrug and must be activated by bacterial catalase. Specifically, activation is associated with the reduction of the mycobacterial ferric KatG catalase-peroxidase by hydrazine and reaction with oxygen to form an oxyferrous enzyme complex. Once activated, isoniazid inhibits the synthesis of mycoloic acids, an essential component of the bacterial cell wall. 

Inhibition of Folic acid Synthesis

There are two antibiotics which inhibit the folic acid synthesis in bacterial cells such as Sulfonamides and Trimethoprim.

• Mode of action of Sulfonamides: It is a Broad-spectrum antibiotic. Sulfonamides Inhibit the folic acid synthesis by preventing the addition of para-aminobenzoic acid (PABA) into the folic acid molecule by competing for the enzyme dihydropteroate synthetase.
• Mode of action of Trimethoprim: Trimethoprim binds to dihydrofolate reductase and inhibits the reduction of dihydrofolic acid (DHF) to tetrahydrofolic acid (THF). THF is necessary for the biosynthesis of bacterial nucleic acids and proteins.

Classification of antibiotics based on mode of action

Based on mode of action the antibiotics are classified into two groups such as; 

1. Bactericidal antibodies
2. Bacteriostatic antibiotics

Bactericidal antibiotics 

• Those antibiotics are acted by killing the bacterial cell is known as bactericidal.
• Bactericidal antibiotics kill the bacterial cells by inhibiting cell wall syntheses. Some examples are; Beta-lactam antibiotics (penicillin derivatives (penams), cephalosporins (cephems), monobactams, and carbapenems) and vancomycin.
• They also act by inhibiting bacterial enzymes or protein translation, such as daptomycin, fluoroquinolones, metronidazole, nitrofurantoin, co-trimoxazole and telithromycin.

Bacteriostatic antibiotics 

• Bacteriostatic antibiotics act by limiting the growth of bacterial cell by interfering with bacterial protein synthesis, DNA replication, or other aspects of bacterial cellular metabolism.
• Some groups of Bacteriostatic antibiotics are tetracyclines, sulfonamides, spectinomycin, trimethoprim, chloramphenicol, macrolides, and lincosamides.

Antibiotics and their mode of action table

Antibiotic	Mode of action of antibiotic
Penicillins	Penicillin bindis to the beta-lactam ring to DD-transpeptidase, inhibiting its cross-linking activity and preventing new cell wall formation.
Cephalosporins	Cephalosporins disrupt the synthesis of the peptidoglycan layer forming the bacterial cell wall.
Vancomycin	Inhibits cell wall synthesis by binding to the D-Ala-D-Ala terminal of the growing peptide chain during cell wall synthesis, resulting in inhibition of the transpeptidase, which prevents further elongation and cross-linking of the peptidoglycan matrix.
Beta-lactamase Inhibitors	Beta-lactamase Inhibitors acts by inhibiting the beta-lactamase enzymes.
Carbapenems	Carbapenems act by binding to penicillin-binding protein (PBPs), thus, causing bacterial cell wall defect, bacterial swelling and killing bacteria.
Aztreonam	By binding to penicillin-binding protein 3 (PBP3), aztreonam inhibits the third and last stage of bacterial cell wall synthesis.
Polymycin	Polymyxins bind to and disrupt the negatively charged lipopolysaccharide (LPS) of the outer membrane of Gram-negative bacteria, allowing the passage of the polymyxin (and of other drugs) into the periplasmic space.
Bacitracin	It inhibits peptidoglycan synthesis during the second step of bacterial cell wall synthesis by interfering with the activity of phosphorylase and is bactericidal.
Polymixin	The Polymixin with a positively charged free amino group acts as a cationic detergent. These bind to and disrupt the negatively charged lipopolysaccharide (LPS) of the outer membrane of Gram-negative bacteria, and thus increase the cell permeability. As a result it allows the passage of the polymyxin (and of other drugs) into the periplasmic space.
Polyenes	The mode of action of Polyenes is, they bind to the ergosterol and results in disruption of the fungal cell membrane, the main sterol in the membrane.
Azoles	Azoles acts by inhibition of 14α-lanosterol demethylase, which is a key enzyme in ergosterol biosynthesis. This results in depletion of ergosterol and accumulation of toxic 14α-methylated sterols in membranes of susceptible yeast species.
Aminoglycosides (gentamicin)	binds to the 30s ribosomal subunit and results in the misreading of the genetic code.
Tetracyclines	prevents the attachment of aminoacyl-tRNA to the RNA-ribosome complex.
Macrolides	prevents the peptidyltransferase from joining the growing peptide attached to tRNA to the next amino acid (similarly to chloramphenicol) as well as inhibiting bacterial ribosomal translation.
Chloramphenicol	inhibits the peptidyl transferase activity of the bacterial ribosome as result it leads to the prevention of protein chain elongation.
Clindamycin	it prevents protein synthesis by interfering with the transpeptidation reaction, which thus inhibits early chain elongation.
Linezolid	attaches to a site on the bacterial 23S ribosomal RNA of the 50S subunit and prevents the formation of a functional 70S initiation complex
Streptogramins	These antibiotics act at the level of inhibition of translation through binding to the bacterial ribosome.
Fluoroquinolones	Inhibits DNA gyrase and the topoisomerase IV enzymes.
Metronidazole	interact with DNA and results in a loss of helical DNA structure and strand breakage.
Rifampin	Rifampin acts via the inhibition of DNA-dependent RNA polymerase, which is results in the suppression of RNA synthesis and cell death.
Isoniazid	Isoniazid is a prodrug and must be activated by bacterial catalase. Specifically, activation is associated with the reduction of the mycobacterial ferric KatG catalase-peroxidase by hydrazine and reaction with oxygen to form an oxyferrous enzyme complex. Once activated, isoniazid inhibits the synthesis of mycoloic acids, an essential component of the bacterial cell wall.
Sulfonamides	prevents the addition of para-aminobenzoic acid (PABA) into the folic acid molecule by competing for the enzyme dihydropteroate synthetase.
Trimethoprim	binds to dihydrofolate reductase and inhibits the reduction of dihydrofolic acid (DHF) to tetrahydrofolic acid (THF).

References

• https://www.eurl-ar.eu/CustomerData/Files/Folders/17-training-course-ast-copenhagen-march2008/221_antibiotics-mode-of-action-and-mechanisms-of-resistance-1.pdf
• https://www.omicsonline.org/blog/2015/02/06/1114-Classification-of-Antiboitics.html
• https://bio.libretexts.org/Bookshelves/Microbiology/Book%3A_Microbiology_(Boundless)/13%3A_Antimicrobial_Drugs/13.1%3A_Overview_of_Antimicrobial_Therapy/13.1E%3A_Antibiotic_Classifications
• https://watermark.silverchair.com/32-Supplement_1-S9.pdf
• https://www.slideshare.net/specialclass/antibiotics-2173921
• https://www.orthobullets.com/basic-science/9059/antibiotic-classification-and-mechanism