Gentamicin microbiology
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Microbiology
In vitro tests have demonstrated that gentamicin is a bactericidal antibiotic which acts by inhibiting normal protein synthesis in susceptible microorganisms. It is active against a wide variety of pathogenic bacteria including Escherichia coli, Proteus species, (indole-positive and indole-negative), Pseudomonas aeruginosa, species of the Klebsiella-Enterobacter-Serratia group, Citrobacter species and Staphylococcus species (including penicillin and methicillin-resistant strains). Gentamicin is also active in vitro against species of Salmonella and Shigella. The following bacteria are usually resistant to aminoglycosides: Streptococcus pneumoniae, most species of streptococci, particularly group D and anaerobic organisms, such as Bacteroides species or Clostridium species.
In vitro studies have shown that an aminoglycoside combined with an antibiotic that interferes with cell wall synthesis may act synergistically against some group D streptococcal strains. The combination of gentamicin and penicillin G has a synergistic bactericidal effect against virtually all strains of Streptococcus faecalis and its varieties (S. faecalis var. liquifaciens, S. faecalis var. zymogenes), S. faecium and S. durans. An enhanced killing effect against many of these strains has also been shown in vitro with combinations of gentamicin and ampicillin, carbenicillin, nafcillin or oxacillin.
The combined effect of gentamicin and carbenicillin is synergistic for many strains of Pseudomonas aeruginosa. In vitro synergism against other gram-negative organisms has been shown with combinations of gentamicin and cephalosporins.
Gentamicin may be active against clinical isolates of bacteria resistant to other aminoglycosides. Bacteria resistant to one aminoglycoside may be resistant to one or more other aminoglycosides. Bacterial resistance to gentamicin is generally developed slowly.
Susceptibility Testing
If the disc method of susceptibility testing used is that described by Bauer et al. (Am J Clin Path 45:493,1966; Federal Register 37:20525-20529,1972), a disc containing 10 mcg of gentamicin should give a zone of inhibition of 15 mm or more to indicate susceptibility of the infecting organism. A zone of 12 mm or less indicates that the infecting organism is likely to be resistant.
Zones of greater than 12 mm and less than 15 mm indicate intermediate susceptibility. In certain conditions it may be desirable to do additional susceptibility testing by the tube or agar dilution method; gentamicin substance is available for this purpose.[1]
References
Adapted from the FDA Package Insert.