Antibiotics, on the Basis of Mechanism of Action

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Antibiotics are, no doubt, something you must have heard about or possibly used over the cause of your life. As humans, we are prone to bacteria infection compare to other types of infection. Antibiotics are medication, used to destroy (treat) or inhibit the growth of bacteria, causing either Bacteriocidal or Bacteriostatic conditions for the bacteria. It is some worth impossible to talk about antibiotics, and not look into their mechanism of action.

https://commons.wikimedia.org/wiki/File:Antibiotic_suceptible_bacteria

Antibiotics, on the Basis of Mechanism of Action

To identify the mechanism of action for antibiotics, understanding their action on the cell of a bacterium is important. Bacteria cells can be identified as either gram positive bacteria, or gram negative bacteria, based on their cell envelopes. Gram Positive Bacteria are surrounded by thick layers of peptidoglycan that makes its cell envelope rigid and tough. Gram positive Bacteria lacks an outer membrane, while Gram Negative bacteria have a thin peptidoglycan cell wall which is surrounded by a lipid membrane (lipopolysaccharide).

In the mechanism of action of antibiotic, they attack the cell structure and component of the bacteria. These mechanisms of action are

The cell wall of bacteria are made of peptidoglycans, which are cross-linked by several transpeptide, transglycosylase, and/or carboxypeptidase (penicillin Binding protein / transpeptidase). In other, for these set of antibiotics to work, they have to function in two ways, which are; to inhibit the enzymes that synthesize the peptidoglycan layers of the cell wall, thereby causing the cell wall to be inhibited, which can be done using Vancomycin (glycopeptide antibiotics) and Fosfomycin (phosphonic antibiotics). Also, the antibiotics could reduce the cross-linking of peptidoglycans by attacking the penicillin binding proteins (DD-transpeptidase) thereby causing the bacteria to be susceptible to cell death. Antibiotics in this category are beta-lactam Penicillin g, Penicillin V, Dicloxacillin, Oxacillin, Nafcillin, Amoxicillin, Ampicillin, Piperacillin, Doripenem, Imipenem, Meropenem, Ertapenem (Carbapenems), Aztreonam (Monobactam) and Cephalosporins which has different generations ranging from 1st to 5th. In the first generation are Cefazolin, Cefalotitin, and Cephalexin. The Second generation, which aren't that common, are cefaclor (I saw this antibiotics one time with the name Ceclor), Cefoxicitin, and Cefotetan. Third generation Cephalosporins includes; Ceftriaxone, Ceftazidime, and Cefotaxime. The Fourth generation Cephalosporins include Cefepime and in the fifth generation is ceftaroline. It is important to know that some of the drugs mentioned are broad spectrum drugs.

Some bacteria have been able to resist these drugs with enzymes known as Beta-Lactamase that break the beta-lactam ring in the antibiotics, thereby making them inactive/ineffective. In other, for these drugs to be function against the Beta-Lactamase enzymes produced by the bacteria, Beta-Lactamase inhibitors are introduced. These drugs include; Amoxicillin (Clavulanate), Ampicillin (Subactam), Piperacillin (Tazobactam) and Avibactam.

Just as the name implies, these drugs alter the cell membrane of Bacteria. In Gram-negative bacteria, the lipopolysaccharide, and in Gram-Positive bacteria, phosphatidylglycerol. The drugs break up the cell membrane of the bacteria, causing the bacteria membrane to leak out cellular molecules and causing death. Drugs such as Daptomycin which makes the cell membrane more porous, Polymyxins and Polyenes (they are used to target fungi also).

Folic acid is important to synthesize the Nucleic acids which makes up the bacteria DNA. Folic Acid Metabolism inhibitors, like Sulfonamides, and Trimethoprim, inhibits folate synthesis by targeting Dihydropteroate synthase (DHPS). Normally, Bacteria metabolises p-aminobenzoic acid (PABA) to dihydrofolate (DHF), then Tetrahydroolate. Folic Acid inhibitors inhibits folic acid mechanism, thereby reducing the synthesis of DNA and RNA, making it impossible for the bacteria cells to divide. Folic Acid metabolism inhibitors are Bacteriostatic. .

DNA Synthesis inhibitors target the DNA synthesis in bacteria. Drugs such as Metronidazole causes the strand breakage and death of the bacteria by inhibiting protein synthesis (nucleic acids) after interacting with the DNA of the bacteria. Nitrofurantoin drugs get metabolised in the bacteria, which disrupt the ribosomal RNA, DNA, thereby damaging them.

Bacteria processes the enzymes topoisomerase that allows DNA strand transcription, chromosome segregation and replication. With DNA Gyrase inhibitors, which include the Fluoroquinolones which inhibits the DNA Gyrase (topoisomerase enzyme).

In Bacteria, antibiotics inhibit the protein synthesis in the 50s and the 30s ribosomal subunit. In the 50s ribosome, antibiotics Macrolides inhibits protein synthesis. Drugs such as Azithromycin, Erythromycin, Clindamycin Protein is important for replication functions and the growth of the cell, and 50s Ribosome inhibitors inhibits the protein synthesis, thereby reducing growth.

The 30s ribosome subunit inhibiting protein synthesis has antibiotics such as Aminoglycosides group, and Tetracycline group. Amynoglycosides are bacteriocidal, while Tetracycline is bacteriostatic.

Conclusion

This post doesn't cover the Bacteria Coverage of Antibiotics, Mechanism of antibiotic resistance, Adverse Drug effect and contraindications, and the look into Antibiotics for common infection. This post explains the Mechanism of action of antibiotics. In future posts, every other aspect will be touched one after the other. This post doesn't explain the bacteria coverage, so it is not identifying the bacteria spectrum of the antibiotics.


Image 1 || Wikemedia commons || Antibiotics



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7 comments
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Nice post, congratulation, you are a good Doctor. It is very important topic for medical community, congratulation, bye..🙋🙋🙋

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This was a good introduction into antibiotics. I had a good read.

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Thanks a lot for reading dear

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Oh my, this is indeed a detailed explanation on antibiotics, who would have thought? Well-done

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I now understand the full meaning of antibiotics, thanks for this great article.
If you would allow me to ask, during my treatment in the hospital due to Crohn's disease, we used to take antibiotics every day, but there was a patient with colitis for whom these antibiotics did not work and he stayed for forty days until he died. What is the reason for the ineffectiveness of this treatment for some disease states?

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