Gentamicin is an aminoglycoside antibiotic.  It exhibits bactericidal activity against aerobic gram-negative bacteria makes gentamicin a good option to treat several common infections. Since gentamicin has a minimal gastrointestinal absorption, its administration is usually by parenteral routes, including systemic, topical and ophthalmic formulations. Although there are reports of resistant strains of gram-negative bacteria, most of these microbes, which have aerobic or facultative metabolism, are susceptible to the gentamicin and other aminoglycosides. The most common microorganisms in clinic settings that present appropriate therapeutic response are members of the Enterobacteriaceae family (e.g., Escherichia coli, Klebsiella pneumoniae, Serratia spp. and Enterobacter spp.), Pseudomonas aeruginosa, and some strains of Neisseria, Moraxella, and Haemophilus genera.[1][2][3][4] A significant percentage of coagulase-negative staphylococci and methicillin-susceptible Staphylococcus aureus isolates show inhibition by gentamicin in clinical drug concentration, although they can readily develop resistance. 

According to the Food and Drug Administration (FDA) orientations, gentamicin should be used conformed with culture and susceptibility information, whenever they are possible. However, the option for gentamicin is also appropriate when based on epidemiological data. Therefore, it has applications in several clinical scenarios, such as bacterial septicemia, meningitis, urinary tract infections, gastrointestinal tract (including peritonitis) and soft tissue infection, but always using additional information (e.g., patient age, symptoms, and signs at presentation, local antimicrobial resistance patterns) to enhance the probability to use gentamicin against susceptible germs. The combination with another antibiotic, especially beta-lactams, is reasonable in bacterial endocarditis, enterococcal bacteremia, and other severe infections, although other antibiotics are preferable in these settings.[5] The beta-lactams break the bacterial cell wall and allow gentamicin to get in the bacterial cytoplasm where it can access the ribosomal target, explaining why this combination can be useful against gram-positive bacterial infection.

Gentamicin, an aminoglycoside antibiotic, is bactericidal.  Gentamicin passes through the gram-negative membrane in an Oxygen-dependent active transport. As oxygen is required, this is why aminoglycosides are not effective in anaerobic bacteria.

Once in the cytoplasm, gentamicin and other aminoglycosides bind to the 16s rRNA at the 30s ribosomal subunit, disturbing the translation of mRNA and, thus, leading to the formation of truncated or nonfunctional proteins.[5] The mechanism of the bactericidal activity of gentamicin has not been fully elucidated yet, but some propose that truncated proteins are placed at the cell wall, compromising its impermeability, while others also suggest that accumulation of reactive oxygen species, as a consequence of depletion of proteins involved with oxidation-reduction reactions, may lead to bacterial death.[6]

Gentamicin, like all aminoglycosides, exhibit concentration-dependent killing.  Higher concentrations correlate with greater antimicrobial killing.  For these reasons, peaks and troughs should be monitored closely with systemic use.  Additionally, research has noted the synergistic effects of aminoglycosides on gram-positive bacteria when combined with other medications have been mentioned, but the mechanism is unknown.

Gentamicin is generally available in parenteral, ophthalmic, and topical preparations. The parenteral route encompasses, mainly, intramuscular and intravenous administration, whose doses of both are calculated based on the patient weight. For moderate and morbidly obese patients, the weight for dose calculation equals 0.4 multiplied by excess body weight plus estimated ideal body weight. The dose of 5 to 7mg/kg daily given intravenously (infused over 30 to 120 minutes) is the preferable way for gentamicin application in most systemic infections by sensible germs, even though the traditional dosing of 3 to 5 mg/kg/day divided into doses every 8 hours is still an option in certain scenarios. For some non-serious infections, such as pelvic inflammatory disease without sepsis, the daily dose may be administered intramuscularly. Infectious endocarditis caused by staphylococci or enterococci was previously treated with gentamicin along with beta-lactams, although this therapy has been abandoned since the increase in bacterial resistance rates.[5] Gentamicin, usually combined with an anaerobicidal antibiotic, can be applied at a single dose of 5ml/kg 60 minutes before surgical incision in gastrointestinal, urologic, and gynecologic procedures for surgical infection prophylaxis.[7]

When administered as an intramuscular injection, gentamicin achieves peak serum concentrations after 30 to 90 minutes. Because of its polar nature, the penetration into the central nervous system and general cells is minimal, as well as the binding to plasma albumin. The majority of gentamicin is excreted unmetabolized by glomerular filtration, which enables a urinary concentration almost 100-fold higher than of the serum.[8]

The ophthalmic gentamicin preparations exist as ointment and solution, both at 0.3% concentration. They are typically used for bacterial eye infections, such as keratitis and conjunctivitis.[9] The topic gentamicin ointment and cream have a concentration of 0.1% and are restricted specific skin and subcutaneous tissue infections, which usually are secondary to abrasions, cuts, and burns.

Gentamicin and aminoglycosides have some specificities about their antibacterial activity. The bactericidal capacity of aminoglycosides correlates with their peak concentration in such a way that the greater the concentration, enhances bacterial killing. The postantibiotic effect (PAE), another feature of aminoglycosides, is the characteristic for bacterial regrowth suppression that persists a few hours after antibiotic concentration falls below the minimum inhibitory concentration (MIC); high peak concentration also advantages the PAE.[10] Therefore, these properties explain the reason for gentamicin is preferable in high-dose regimens associated with extended-interval doses.

Characteristically, gentamicin reaches high concentrations in the renal cortex and the inner ear. The latter may be injured, leading to auditory and, especially, vestibular dysfunction. The first manifestation of cochlear damage is often high-pitched tinnitus, which may last a few weeks after the gentamicin is interrupted. A high-frequency hearing loss may be present in almost two-thirds of the patients receiving aminoglycosides, including gentamicin, but only a small number complains of hearing impaired.[11] The vestibular toxicity manifests as nausea, vomiting, balance disorder, and vertigo within the two first weeks. A chronic phase, which may persist for about two months, is marked for ataxia and not rarely leaves some residual dysfunctions. The gentamicin is prone to accumulate in the renal proximal tubular cells and can cause damage. Hence, mild proteinuria and reduction of the glomerular filtration rate are potential consequences of gentamicin use, achieving 14% of gentamicin users in a review.[12] Once proximal tubular cells carry regeneration capacity, renal injury and its consequences often are reversible. When compared with the high-dose, extended interval dosing approach, the divided-doses scheme implies a longer time of gentamicin serum concentration above the toxicity threshold, resulting in a higher risk for ototoxicity and nephrotoxicity.

The neuromuscular blockade, although a rare event, is a serious adverse effect of virtually all aminoglycosides. The known risk factors are concurrent conditions (e.g., myasthenia gravis) or medications (e.g., vecuronium) that interfere with the neuromuscular junction. The blockade probably results from a reduction of presynaptic release of acetylcholine and interference of acetylcholine postsynaptic receptors function, both effects mediated by aminoglycosides. The intravenous administration of calcium can overcome this toxicity.

The absolute contraindication for gentamicin use is a history of hypersensitivity reaction to it or other aminoglycosides, although this is a rare event. In cases of renal impairment, dosing adjustment should be made based on the glomerular filtration rate (GFR); for high-dose, extended interval dosing approach, the dose can be preserved, but the interval between doses should increase in line with GFR decrease. In burn patients, the systemic absorption of topical gentamicin may be enhanced, and one should be watchful for the potential repercussions. Systemic gentamicin belongs to category D of FDA pregnancy risk classification (i.e., although the evidence of human fetal risk, its use is acceptable if the possible benefits overcome the risks).