What is the effectiveness of Gentamicin (an aminoglycoside antibiotic) against Gram-positive versus Gram-negative bacteria?

From the Guidelines

Gentamicin is primarily effective against gram-negative bacteria, with limited activity against certain gram-positive organisms. This aminoglycoside antibiotic works by binding to bacterial ribosomes and inhibiting protein synthesis, making it particularly useful for treating serious infections caused by aerobic gram-negative bacilli such as Pseudomonas aeruginosa, Escherichia coli, Klebsiella, Enterobacter, Serratia, and Proteus species. While gentamicin does have some activity against gram-positive organisms like Staphylococcus aureus (including some MRSA strains) and certain streptococci, it is not typically used as first-line therapy for gram-positive infections.

The standard dosing for gentamicin is typically 5-7 mg/kg once daily for normal renal function, though traditional dosing of 1-2 mg/kg every 8 hours is still used in some settings, as suggested by 1. When administering gentamicin, monitoring of drug levels is essential to prevent toxicity, particularly nephrotoxicity and ototoxicity, as highlighted in 1 and 1. For serious infections, gentamicin is often combined with other antibiotics like beta-lactams to provide synergistic activity and broader coverage. Due to its poor activity against anaerobes and limited gram-positive coverage, gentamicin should not be used as monotherapy for mixed infections or when gram-positive pathogens are the primary concern.

Some key considerations when using gentamicin include:

• Monitoring of drug levels to prevent toxicity, particularly nephrotoxicity and ototoxicity
• Use of single daily dosing for patients with stable normal kidney function, as suggested by 1
• Avoidance of combination with other nephrotoxic drugs, as highlighted in 1
• Consideration of alternative agents for patients with impaired renal function or those at risk of nephrotoxicity, as recommended by 1

Overall, gentamicin is a valuable antibiotic for treating serious gram-negative infections, but its use requires careful consideration of dosing, monitoring, and potential toxicity, as emphasized in 1, 1, and 1.

From the FDA Drug Label

Gentamicin, an aminoglycoside, binds to the prokaryotic ribosome, inhibiting protein synthesis in susceptible bacteria. It is bactericidal in vitro against Gram-positive and Gram-negative bacteria. The following bacteria are usually resistant to the aminoglycosides, including gentamicin: most streptococcal species (including Streptococcus pneumoniae and the Group D streptococci), most enterococcal species (including Enterococcus faecalis, E. faecium, and E durans), and anaerobic organisms, such as Bacteroides species and Clostridium species. Gentamicin has been shown to be active against most of the following bacteria, both in vitro and in clinical infections: Gram-Positive Bacteria: Staphylococcus species Gram-Negative Bacteria: Citrobacter species, Enterobacter species, Escherichia coli, Klebsiella species, Proteus species, Serratia species, Pseudomonas aeruginosa

Gentamicin is bactericidal against both Gram-positive and Gram-negative bacteria. However, its effectiveness varies among specific species.

• It is active against Staphylococcus species (Gram-positive) and several Gram-negative bacteria, including Citrobacter, Enterobacter, Escherichia coli, Klebsiella, Proteus, Serratia, and Pseudomonas aeruginosa.
• Gentamicin is usually resistant to most streptococcal species and enterococcal species, as well as anaerobic organisms like Bacteroides and Clostridium species. 2

From the Research

Effectiveness of Gentamicin Against Gram-Positive Bacteria

• Gentamicin has been shown to be effective against Gram-positive bacteria, including Staphylococcus aureus 3, 4.
• The antibiotic exhibits a partially concentration-dependent killing effect against S. aureus, with concentrations beyond four to eight times the MIC not resulting in significantly faster bacterial killing 3.
• Gentamicin is also active against other Gram-positive organisms, such as group A beta-hemolytic Streptococci 4.

Effectiveness of Gentamicin Against Gram-Negative Bacteria

• Gentamicin is effective against a broad spectrum of Gram-negative bacilli, including Pseudomonas aeruginosa 3, 5, 4.
• The antibiotic demonstrates a concentration-dependent profile in suppressing P. aeruginosa regrowth after initial decline in bacterial burden 3.
• Gentamicin is active against other Gram-negative organisms, such as Escherichia coli, Aerobacter, Klebsiella, Salmonella, and Shigella 4.

Comparison of Effectiveness Against Gram-Positive and Gram-Negative Bacteria

• Gentamicin exhibits different killing profiles against S. aureus and P. aeruginosa, with a partially concentration-dependent effect against the former and a concentration-dependent effect against the latter 3.
• The antibiotic is often used to treat severe infections caused by Gram-negative bacteria, but it can also be effective against Gram-positive organisms 6, 5, 7.
• The choice of dosing strategy for gentamicin may depend on the type of infection being treated, with thrice-daily dosing potentially being superior to once-daily dosing for S. aureus infections 3.