Amoxicillin and Clavulanate Potassium (Page 4 of 6)

12.4 Microbiology

Amoxicillin is a semisynthetic antibiotic with in vitro bactericidal activity against Gram-positive and Gram-negative bacteria. Amoxicillin is, however, susceptible to degradation by beta-lactamases, and therefore, the spectrum of activity does not include organisms which produce these enzymes. Clavulanic acid is a beta-lactam, structurally related to the penicillins, which possesses the ability to inactivate some beta-lactamase enzymes commonly found in microorganisms resistant to penicillins and cephalosporins. In particular, it has good activity against the clinically important plasmid-mediated beta-lactamases frequently responsible for transferred drug resistance.
The formulation of amoxicillin and clavulanic acid in amoxicillin and clavulanate potassium protects amoxicillin from degradation by some beta-lactamase enzymes and extends the antibiotic spectrum of amoxicillin to include many bacteria normally resistant to amoxicillin.
Amoxicillin and clavulanic acid has been shown to be active against most isolates of the following bacteria, both in vitro and in clinical infections as described in the INDICATIONS AND USAGE section.

Gram-positive Bacteria

Staphylococcus aureus

Gram-negative Bacteria

Enterobacter species
Escherichia coli
Haemophilus influenzae
Klebsiella species
Moraxella catarrhalis

The following in vitro data are available, but their clinical significance is unknown. At least 90 percent of the following bacteria exhibit an in vitro minimum inhibitory concentration (MIC) less than or equal to the susceptible breakpoint for amoxicillin and clavulanic acid. However, the efficacy of amoxicillin and clavulanic acid in treating clinical infections due to these bacteria has not been established in adequate and well-controlled clinical trials.

Gram-positive Bacteria

Enterococcus faecalis
Staphylococcus epidermidis
Staphylococcus saprophyticus
Streptococcus pneumoniae
Streptococcus pyogenes
Viridans group Streptococcus

Gram-negative Bacteria

Eikenella corrodens
Proteus mirabilis

Anaerobic Bacteria

Bacteroides species including Bacteroides fragilis
Fusobacterium species
Peptostreptococcus species

Susceptibility Test Methods

When available, the clinical microbiology laboratory should provide the results of in vitro susceptibility test results for antimicrobial drug products used in resident hospitals 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 an antibacterial drug product for treatment.
Dilution Techniques
Quantitative methods are used to determine antimicrobial minimum 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 2,3 (broth and/or agar). The MIC values should be interpreted according to criteria provided in Table 5.
Diffusion Techniques Quantitative methods that require measurement of zone diameters can also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. The zone size provides an estimate of the susceptibility of bacteria to antimicrobial compounds. The zone size should be determined using a standardized test method 3,4. This procedure uses paper disks impregnated with 30 mcg of amoxicillin and clavulanate potassium (20 mcg amoxicillin plus 10 mcg clavulanic acid) to test the susceptibility of bacteria to amoxicillin and clavulanic acid. The disc diffusion interpretive criteria are provided in Table 5.

Table 5: Susceptibility Test Interpretive Criteria for Amoxicillin and Clavulanic Acid
Minimum Inhibitory Concentrations (mcg/mL) Disk Diffusion (zone diameters in mm)
Pathogen S I R S I R
Enterobacteriaceae 8/4 16/8 32/16 ≥18 14-17 ≤13
Haemophilus influenzae and Staphylococcus aureus 4/2 8/4 ≥20 ≤19

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 2,3,4. Standard amoxicillin and clavulanic acid powder should provide the following range of MIC values noted in Table 6 for the diffusion technique using the 30 mcg amoxicillin and clavulanate potassium (20 mcg amoxicillin plus 10 mcg clavulanic acid) disk, the criteria in Table 6 should be achieved.

Table 6: Acceptable Quality Control Ranges for Amoxicillin and Clavulanic Acid
QC Strain Minimum Inhibitory Concentration (mcg/mL) Disk Diffusion (zone diameter in mm)
Escherichia coli ATCC 25922 2/1 to 8/4 18 to 24
Escherichia coli ATCC 35218 4/2 to 16/8 17 to 22
Haemophilus influenzae ATCC 49247 2/1 to 16/8 15 to 23
Staphylococcus aureus ATCC 29213 0.12/0.06 to 0.5/0.25
Staphylococcus aureus ATCC 29523 28 to 36

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