Amoxicillin is similar to ampicillin in its bactericidal action against susceptible organisms during the stage of active multiplication. It acts through the inhibition of biosynthesis of cell wall mucopeptide. Amoxicillin has been shown to be active against most strains of the following microorganisms, both in vitro and in clinical infections as described in the INDICATIONS AND USAGE section.
Aerobic Gram-Positive Microorganisms
Enterococcus faecalis
Staphylococcus spp.* (β-lactamase–negative strains only)
Streptococcus pneumoniae
Streptococcus spp. (α- and β-hemolytic strains only)
*Staphylococci which are susceptible to amoxicillin but resistant to methicillin/oxacillin should be considered as resistant to amoxicillin.
Aerobic Gram-Negative Microorganisms
Escherichia coli (β-lactamase–negative strains only)
Haemophilus influenzae (β-lactamase–negative strains only)
Neisseria gonorrhoeae (β-lactamase–negative strains only)
Proteus mirabilis (β-lactamase–negative strains only)
Helicobacter
Helicobacter pylori
Susceptibility Tests
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 procedure. Standardized procedures are based on a dilution method1 (broth or agar) or equivalent with standardized inoculum concentrations and standardized concentrations of ampicillin powder. Ampicillin is sometimes used to predict susceptibility of S. pneumoniae to amoxicillin; however, some intermediate strains have been shown to be susceptible to amoxicillin. Therefore, S. pneumoniae susceptibility should be tested using amoxicillin powder. The MIC values should be interpreted according to the following criteria:
For Gram-Positive Aerobes
Enterococcus
MIC (mcg/mL) | Interpretation |
≤8 | Susceptible (S) |
≥16 | Resistant (R) |
Staphylococcusa
MIC (mcg/mL) | Interpretation |
≤0.25 | Susceptible (S) |
≥0.5 | Resistant (R) |
Streptococcus (except S. pneumoniae)
MIC (mcg/mL) | Interpretation |
≤0.25 | Susceptible (S) |
0.5 to 4 | Intermediate (I) |
≥8 | Resistant (R) |
S. pneumoniaeb from non-meningitis sources.
(Amoxicillin powder should be used to determine susceptibility.)
MIC (mcg/mL) | Interpretation |
≤2 | Susceptible (S) |
4 | Intermediate (I) |
≥8 | Resistant (R) |
NOTE: These interpretive criteria are based on the recommended doses for respiratory tract infections.
For Gram-Negative Aerobes
Enterobacteriaceae
MIC (mcg/mL) | Interpretation |
≤8 | Susceptible (S) |
16 | Intermediate (I) |
≥32 | Resistant (R) |
H. influenzaec
MIC (mcg/mL) | Interpretation |
≤1 | Susceptible (S) |
2 | Intermediate (I) |
≥4 | Resistant (R) |
a. Staphylococci which are susceptible to amoxicillin but resistant to methicillin/oxacillin should be considered as resistant to amoxicillin.
b. These interpretive standards are applicable only to broth microdilution susceptibility tests using cation-adjusted Mueller-Hinton broth
with 2 to 5% lysed horse blood.
c. These interpretive standards are applicable only to broth microdilution test with H. influenzae using Haemophilus Test Medium (HTM).1
A report of “Susceptible” indicates that the pathogen is likely to be inhibited if the antimicrobial compound in the blood reaches the concentrations usually achievable. 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 or in situations where high dosage of drug can be used. This category also provides a buffer zone, which prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of “Resistant” indicates that the pathogen is not likely to be inhibited if the antimicrobial compound in the blood reaches the concentrations usually achievable; other therapy should be selected.
Standardized susceptibility test procedures require the use of laboratory control microorganisms to control the technical aspects of the laboratory procedures. Standard ampicillin powder should provide the following MIC values:
Microorganism | MIC Range mcg/mL) |
E. coli ATCC 25922 | 2 to 8 |
E. faecalis ATCC 29212 | 0.5 to 2 |
H. influenzae ATCC 49247d | 2 to 8 |
S. aureus ATCC 29213 | 0.25 to 1 |
Using amoxicillin to determine susceptibility:
Microorganism | MIC Range (mcg/mL) |
S. pneumoniae ATCC 49619e | 0.03 to 0.12 |
d. This quality control range is applicable to only H. influenzae ATCC 49247 tested by a broth microdilution procedure using HTM.1
e. This quality control range is applicable to only S. pneumoniae ATCC 49619 tested by the broth microdilution procedure using
cation-adjusted Mueller-Hinton broth with 2 to 5% lysed horse blood.
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 procedure2 requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 10 mcg ampicillin to test the susceptibility of microorganisms, except S. pneumoniae, to amoxicillin. Interpretation involves correlation of the diameter obtained in the disk test with the MIC for ampicillin.
Reports from the laboratory providing results of the standard single-disk susceptibility test with a 10 mcg ampicillin disk should be interpreted according to the following criteria:
For Gram-Positive Aerobes
Enterococcus
Zone Diameter (mm) | Interpretation |
≥17 | Susceptible (S) |
≤16 | Resistant (R) |
Staphylococcusf
Zone Diameter (mm) | Interpretation |
≥29 | Susceptible (S) |
≤28 | Resistant (R) |
β-hemolytic streptococci
Zone Diameter (mm) | Interpretation |
≥26 | Susceptible (S) |
19 to 25 | Intermediate (I) |
≤18 | Resistant (R) |
NOTE: For streptococci (other than β-hemolytic streptococci and S. pneumoniae), an ampicillin MIC should be determined.
S. pneumoniae
S. pneumoniae should be tested using a 1 mcg oxacillin disk. Isolates with oxacillin zone sizes of ≥20 mm are susceptible to amoxicillin. An amoxicillin MIC should be determined on isolates of S. pneumoniae with oxacillin zone sizes of ≤19 mm.
For Gram-Negative Aerobes
Enterobacteriaceae
Zone Diameter (mm) | Interpretation |
≥17 | Susceptible (S) |
14 to 16 | Intermediate (I) |
≤13 | Resistant (R) |
H. influenzaeg
Zone Diameter (mm) | Interpretation |
≥22 | Susceptible (S) |
19 to 21 | Intermediate (I) |
≤18 | Resistant (R) |
f. Staphylococci which are susceptible to amoxicillin but resistant to methicillin/oxacillin should be considered as resistant to amoxicillin.
g. These interpretive standards are applicable only to disk diffusion susceptibility tests with H. influenzae using Haemophilus Test
Medium (HTM).2
Interpretation should be as stated above for results using dilution techniques.
As with standard dilution techniques, disk diffusion susceptibility test procedures require the use of laboratory control microorganisms. The 10 mcg ampicillin disk should provide the following zone diameters in these laboratory test quality control strains:
Microorganism | Zone Diameter (mm) |
E. coli ATCC 25922 | 16 to 22 |
H. influenzae ATCC 49247h | 13 to 21 |
S. aureus ATCC 25923 | 27 to 35 |
Using 1 mcg oxacillin disk:
Microorganism | Zone Diameter (mm) |
S. pneumoniae ATCC 49619i | 8 to 12 |
h. This quality control range is applicable to only H. influenzae ATCC 49247 tested by a disk diffusion procedure using HTM.2
i. This quality control range is applicable to only S. pneumoniae ATCC 49619 tested by a disk diffusion procedure using
Mueller-Hinton agar supplemented with 5% sheep blood and incubated in 5% CO2.
Susceptibility Testing for Helicobacter pylori
In vitro susceptibility testing methods and diagnostic products currently available for determining minimum inhibitory concentrations (MICs) and zone sizes have not been standardized, validated, or approved for testing H. pylori microorganisms.
Culture and susceptibility testing should be obtained in patients who fail triple therapy. If clarithromycin resistance is found, a non-clarithromycin-containing regimen should be used.
INDICATIONS AND USAGE
To reduce the development of drug-resistant bacteria and maintain the effectiveness of amoxicillin and other antibacterial drugs, amoxicillin should be used only to treat or prevent infections that are proven or strongly suspected to be caused by susceptible bacteria. When culture and susceptibility information are available, they should be considered in selecting or modifying antibacterial therapy. In the absence of such data, local epidemiology and susceptibility patterns may contribute to the empiric selection of therapy.
Amoxicillin is indicated in the treatment of infections due to susceptible (ONLY β-lactamase–negative) strains of the designated microorganisms in the conditions listed below:
Infections of the ear, nose, and throat – due to Streptococcus spp. (α- and β-hemolytic strains only), S. pneumoniae, Staphylococcus spp., or H. influenzae.
Infections of the genitourinary tract – due to E. coli, P. mirabilis, or E. faecalis.
Infections of the skin and skin structure – due to Streptococcus spp. (α- and β-hemolytic strains only), Staphylococcus spp., or E. coli.
Infections of the lower respiratory tract – due to Streptococcus spp. (α- and β-hemolytic strains only), S. pneumoniae, Staphylococcus spp., or H. influenzae.
Gonorrhea, acute uncomplicated (ano-genital and urethral infections) – due to N. gonorrhoeae (males and females).
H. pylori eradication to reduce the risk of duodenal ulcer recurrence
Triple Therapy
Amoxicillin/clarithromycin/lansoprazole
Amoxicillin, in combination with clarithromycin plus lansoprazole as triple therapy, is indicated for the treatment of patients with H. pylori infection and duodenal ulcer disease (active or 1-year history of a duodenal ulcer) to eradicate H. pylori. Eradication of H. pylori has been shown to reduce the risk of duodenal ulcer recurrence. (See CLINICAL STUDIES and DOSAGE AND ADMINISTRATION.)
Dual Therapy
Amoxicillin/lansoprazole
Amoxicillin, in combination with lansoprazole delayed-release capsules as dual therapy, is indicated for the treatment of patients with H. pylori infection and duodenal ulcer disease (active or 1-year history of a duodenal ulcer) who are either allergic or intolerant to clarithromycin or in whom resistance to clarithromycin is known or suspected. (See the clarithromycin package insert, MICROBIOLOGY.) Eradication of H. pylori has been shown to reduce the risk of duodenal ulcer recurrence. (See CLINICAL STUDIES and DOSAGE AND ADMINISTRATION.)
Indicated surgical procedures should be performed.