Amikacin Sulfate (Page 2 of 6)

Microbiology

Mechanism of Action

Amikacin, 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.

Mechanism of Resistance

Aminoglycosides are known to be ineffective against Salmonella and Shigella species in patients. Therefore, in vitro susceptibility test results should not be reported.

Amikacin resists degradation by certain aminoglycoside inactivating enzymes known to affect gentamicin, tobramycin, and kanamycin.

Aminoglycosides in general have a low order of activity against Gram-positive organisms other than Staphylococcal isolates.

Interaction with Other Antimicrobials

In vitro studies have shown that amikacin sulfate combined with a beta-lactam antibiotic acts synergistically against many clinically significant Gram-negative organisms.

Antimicrobial Activity

Amikacin has been shown to be active against the following bacteria, both in vitro and in clinical infections [see INDICATIONS AND USAGE ].

Gram-positive Bacteria

Staphylococcus species

Gram-negative Bacteria

Pseudomonas species
Escherichia coli
Proteus species (indole-positive and indole-negative)
Klebsiella species
Enterobacter species
Serratia speciesAcinetobacter species

Amikacin has demonstrated in vitro activity against the following bacteria. The safety and effectiveness of amikacin in treating clinical infections due to these bacteria have not been established in adequate and well-controlled trials.

Citrobacter freundii

Susceptibility Test Methods

When available, the clinical microbiology laboratory should provide cumulative results of the in vitro susceptibility tests for antimicrobial drugs used in local hospitals and practice areas to the physician as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid the physician in selecting the most effective antimicrobial.

Dilution Techniques

Quantitative methods are used to determine antimicrobial minimal inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standardized test method.1, 3 Standardized procedures are based on a dilution method (broth or agar) or equivalent with standardized inoculum concentrations and standardized concentrations of amikacin powder. The MIC values should be interpreted according to the criteria provided in Table 1.

Diffusion Techniques

Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure requires the use of standardized inoculum concentrations and paper disks impregnated with 30 mcg of amikacin.2, 3 The disk diffusion values should be interpreted according to the criteria provided in Table 1.

Table 1: Susceptibility Test Interpretive Criteria for Amikacin
*

For Salmonella and Shigella spp., aminoglycosides may appear active in vitro but are not effective clinically; the results should not be reported as susceptible.

For staphylococci that test susceptible, aminoglycosides are used only in combination with other active agents that test susceptible.

Pathogen Minimum InhibitoryConcentrations(mcg/mL) Disk DiffusionZone Diameters(mm)
S I R S I R
Enteriobacteriaceae * ≤ 16 32 ≥ 64 ≥ 17 15-16 ≤ 14
Pseudomonas aeruginosa ≤ 16 32 ≥ 64 ≥ 17 15-16 ≤ 14
Acinetobacter spp. ≤ 16 32 ≥ 64 ≥ 17 15-16 ≤ 14
Other Non-Enterobacteriaceae ≤ 16 32 ≥ 64 - - -
Staphylococcus spp. ≤ 16 32 ≥ 64 ≥ 17 15-16 ≤ 14

S = susceptible, I = intermediate, R = resistant

A report of “Susceptible” indicates that the antimicrobial is likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentration at the infection site necessary to inhibit growth of the pathogen. A report of “Intermediate” indicates that the result should be considered equivocal, and if the microorganism is not fully susceptible to alternative clinically feasible drugs, the test should be repeated. This category implies possible clinical applicability in body sites where the drug is physiologically concentrated. This category also provides a buffer zone that prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of “Resistant” indicates that the antimicrobial is not likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentrations usually achievable at the infection site; other therapy should be selected.

Quality Control

Standardized susceptibility test procedures require the use of laboratory controls to monitor and ensure the accuracy and precision of supplies and reagents used in the assay, and the techniques of the individuals performing the test.1, 2, 3 Standard amikacin powder should provide the following range of MIC values provided in Table 2. For the diffusion technique using the 30-mcg amikacin disk the criteria provided in Table 2 should be achieved.

Table 2: Acceptable Quality Control Ranges for Amikacin
Quality ControlOrganism Minimum Inhibitory Concentrations (mcg/mL) Disk Diffusion Zone Diameters (mm)
Escherichia coli ATCC 25922 0.5-4 19-26
Pseudomonas aeruginosa ATCC 27853 1-4 18-26
Staphylococcus aureus ATCC 25923 Not Applicable 20-26
Staphylococcus aureu s ATCC 29213 1-4 Not Applicable
Enterococcus faecalis ATCC 29212 64-256 Not Applicable

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