Antibiotics are used in medicine and agriculture against bacterial infections and bacterial growth in food. There are several classes of antibiotic, and this article explains the bacteriocidal or bacteriostatic activity of each.
How antibiotics work
Penicillin
Penicillin was the first antibiotic to be discovered. Alexander Fleming discovered penicillin in 1928 when he accidentally left bacterial cultures uncovered near an open window.
This lead to the contamination of the cultures with mold spores, which produced a compound that killed the bacteria. This compound was later named penicillin by Fleming. Penicillin is part of a class of antibiotics called β-lactams. These antibiotics are characterized by a beta-lactam ring in the molecule’s center, and function by interfering with the synthesis of the bacterial cell wall.
β-lactams stop peptide chains from cross-linking during the formation of a new peptidoglycan chain which is a major component of the bacterial cell wall. Thus a bacterium cannot keep its structural integrity and will burst (lyse).
The structure of the β-lactam is similar to the subunits that make up peptidoglycan. It therefore acts as a competitive inhibitor to transpeptidase, an enzyme involved in the cross-linking of peptides, also called penicillin-binding protein.
ß-Lactams: Mechanisms of Action and Resistance
Cephalosporins
Cephalosporins also belong to the β-lactam group. They are very similar to penicillin but contain a different structure, which provides increased resistance to inactivation by an enzyme which can be produced by certain bacteria called beta-lactamase.
Cephalosporins antibiotic can, therefore, be used when penicillin is ineffective. β-lactams have R groups modify the antibiotic to give a different spectrum of activity. Cephalosporins have two R groups compared to one group in penicillin, creating more opportunities for chemical modification.
Aminoglycosides
Aminoglycosides are bacteriostatic; they slow down the growth and reproduction of bacteria without killing them. These antibiotics inhibit the synthesis of proteins by binding to the 30S bacterial ribosome subunit. When these subunits bind together, they produce the proteins needed by the cell.
Ribosomes in animal cells are 80S, made of subunits of 40S and 60S, while bacterial ribosomes are 70S, so specific modification in bacterial ribosome can be achieved.
Aminoglycosides prevent effective proof-reading of the proteins produced by bacteria. They cause incorrect amino acids to be inserted into the peptide chain, creating misfolded and faulty proteins. their function. Many of these are structural proteins, so defect stops the bacterium repairing holes in the cell wall, undergoing cell growth or reproducing.
Tetracyclines
Tetracyclines inhibit synthesis of proteins by binding to the 30S ribosome subunit but have a different method of action to aminoglycosides. Instead of preventing proof-reading of the peptide produced, they stop the binding of tRNA to the ribosome, stopping protein synthesis.
Preventing the binding of tRNA to the bacterial ribosome effectively prevents proteins being produced by the bacteria, leading to its death.
Macrolides
Macrolides have a similar function to aminoglycosides and tetracyclines in that they inhibit the synthesis of proteins by binding to the bacterial ribosome, but they bind to the 50S subunit. Macrolides stop the formation of peptide bonds between amino acids, preventing protein synthesis.
Fluoroquinolones
Fluoroquinolones inhibit the activity of DNA gyrase, a type of topoisomerase found in prokaryotes, which prevents a harmful DNA modification called supercoiling.
Supercoiling occurs when DNA strands are wound together too tightly or not tightly enough. Undoing this supercoiling is essential for a bacteria’s ability to replicate, so DNA gyrase is a useful target for antibiotics. Human cells do not contain DNA gyrase and have a different type of topoisomerase instead.
Further Reading
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