Bacteria have 70S ribosomes, made of a small 30S subunit and a large 50S subunit. Ribosomes in the cytoplasm of human cells are larger 80S ribosomes. Because the two types differ in structure, some antibiotics bind to bacterial ribosomes but not to human cytoplasmic ribosomes. This gives them selective toxicity.By binding to the ribosome, these antibiotics stop translation, so the bacteria cannot make the enzymes and structural proteins they need to grow and divide. Examples:
• Tetracyclines bind to the 30S subunit and stop tRNA molecules binding to the ribosome, so no new amino acids can be added.
• Chloramphenicol binds to the 50S subunit and inhibits the formation of peptide bonds between amino acids.
• Erythromycin binds to the 50S subunit and stops the ribosome moving along the mRNA.
• Streptomycin binds to the 30S subunit and causes the mRNA to be misread, so faulty proteins are made.
Most protein synthesis inhibitors are bacteriostatic: they stop growth, and the immune system removes the bacteria. Streptomycin is an exception and is bactericidal.These antibiotics are not completely harmless to humans. Mitochondria have their own ribosomes, which are similar to bacterial ones. This is one reason why some of these drugs have side effects, and why chloramphenicol is now used only for serious infections or as eye drops.Resistance can arise if a mutation changes the shape of the ribosome's binding site, so the antibiotic no longer fits.
• Tetracyclines bind to the 30S subunit and stop tRNA molecules binding to the ribosome, so no new amino acids can be added.
• Chloramphenicol binds to the 50S subunit and inhibits the formation of peptide bonds between amino acids.
• Erythromycin binds to the 50S subunit and stops the ribosome moving along the mRNA.
• Streptomycin binds to the 30S subunit and causes the mRNA to be misread, so faulty proteins are made.
Most protein synthesis inhibitors are bacteriostatic: they stop growth, and the immune system removes the bacteria. Streptomycin is an exception and is bactericidal.These antibiotics are not completely harmless to humans. Mitochondria have their own ribosomes, which are similar to bacterial ones. This is one reason why some of these drugs have side effects, and why chloramphenicol is now used only for serious infections or as eye drops.Resistance can arise if a mutation changes the shape of the ribosome's binding site, so the antibiotic no longer fits.