Aminoglycosides: An Overview

Aminiglycosides are bactericidal and active against some Gram-positive and many Gram-negative organisms. They are not absorbed from the gut (although there is a risk of absorption in inflammatory bowel disease and liver failure) and must therefore be given by injection for systemic infections.

Following active transport into the cell, they bind irreversibly to a specific aminoglycoside receptor on the bacterial 30S ribosomal subunit and interfere with the initiation complex between messenger RNA and the 30S subunit, thereby inhibiting initiation of protein synthesis, consequently leading to bacterial cell death. In addition, they induce misreading of the mRNA template causing incorrect amino acids to be incorporated into the growing polypeptide chain, consequently interfering with protein elongation.

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Carbapenems: An Overview

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The carbapenems are a class of broad-spectrum beta-lactam antibacterials with activity against many gram-positive and gram-negative bacteria and anaerobes. They penetrate cell walls and bind to penicillin-binding protein (PBP) targets.

Imipenem has the greatest affinity for PBP 1A, 1B, and 2, and its lethal effect is related to binding to PBP 2 and 1B. This antibiotic is active against a wide range of gram-positive and gram-negative organisms and is stable in the presence of beta-lactamases.

Ertapenem has a particular affinity for PBPs 2 and 3. This agent is stable against hydrolysis by a variety of beta-lactamases, including penicillinases, cephalosporinases and extended-spectrum beta-lactamases.

The carbapenems are ineffective against MRSA or Enterococcus faecium.

They commonly cause mild transient aminotransferase elevations and can rarely result in clinically apparent, cholestatic liver injury.

Cephalosporins: An Overview

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Cephalosporins are broad-spectrum antibiotics whose pharmacology is similar to that of the penicillins. They bind to and inactivate penicillin-binding proteins (PBPs) located on the inner membrane of the bacterial cell wall. PBPs are enzymes involved in the terminal stages of assembling the bacterial cell wall and in reshaping the cell wall during growth and division. Inactivation of PBPs interferes with the cross-linkage of peptidoglycan chains necessary for bacterial cell wall strength and rigidity. This results in the weakening of the bacterial cell wall and causes cell lysis.

The excretion of Cephalosporins is principally renal.

They are classed as first, second, third, fourth or fifth generation; as indicated on the table below. Their spectrum of activity will be presented in a future post.

Cephalosporin Classification (UK January 2018)
Cephalosporin Classification (UK January 2018)