The four antibiotics that CLSI recommends for primary susceptibility testing are:
- penicillin
- erythromycin
- gentamicin
- vancomycin
Modes of action
- Penicillin is bactericidal by inhibiting the cross-linking of peptidoglycan in the cell wall. Organisms need to be actively replicating for penicillin to be effective. Penicillin is not concentration dependent, meaning that even in lower concentrations, the antibiotic can still be effective. Penicillin is excreted through the urine, is water-soluble, and can be used for a variety of infections including osteomyelitis.
- Erythromycin is bacteriostatic through inhibition of protein synthesis. It largely binds to plasma proteins by diffusing readily into most body fluids. Erythromycin concentrates in the liver and is excreted in bile. This antibiotic is often used in respiratory or skin infections and is also used as a prophylactic antibiotic.
- Gentamicin is bactericidal and initially binds to the cell membrane which increases cell permeability for entrance into the cytoplasm and targets the 30S ribosome which results in the mistranslation of proteins and cytoplasmic membrane disruption. This antibiotic is concentration-dependent and can be used for septicemia. meningitis, urinary tract infections, peritonitis, and can be combined with a ß-lactam for endocarditis.
- Vancomycin inhibits cell wall synthesis by inhibiting the polymerization of peptidoglycan in the cell wall. This antibiotic is slowly bactericidal and can be distributed in body tissues and fluids but does not penetrate the central nervous system. Vancomycin is excreted through the urine and can cause renal failure during or after treatment, especially in patients over 65 years of age.
