Amoxicillin (Page 4 of 6)

12.4 Microbiology

Mechanism of Action
Amoxicillin is similar to penicillin in its bactericidal action against susceptible bacteria during the stage of active multiplication. It acts through the inhibition of cell wall biosynthesis that leads to the death of the bacteria.
Method of Resistance
Resistance to amoxicillin is mediated primarily through enzymes called beta-lactamases that cleave the beta-lactam ring of amoxicillin, rendering it inactive. Amoxicillin has been shown to be active against most isolates of the bacteria listed below, both in vitro and in clinical infections as described in the INDICATIONS AND USAGE section.

Gram-Positive Bacteria

Gram-Negative Bacteria

Enterococcus faecalis Staphylococcus spp. Streptococcus pneumoniae Alpha and β-hemolytic streptococci.

Escherichia coli Haemophilus influenzae Neisseria gonorrhoeae Proteus mirabilis Helicobacter pylori

Susceptibility Test Methods: (susceptibility to amoxicillin can be determined using ampicillin powder and a 10 mcg ampicillin disk)
When available, clinical microbiology should provide the results of in vitro susceptibility test results for antimicrobial drugs 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 antimicrobial 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 procedure. Standardized procedures are based on dilution methods (broth or agar)2,3 or equivalent with standardized inoculum concentrations and standardized concentrations of ampicillin powder. The MIC values should be interpreted according to the criteria in Table 4.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 procedure3 requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 10 mcg ampicillin to test the susceptibility of bacteria to ampicillin. Interpretation involves correlation of the diameter obtained in the disk test with the MIC for amoxicillin. 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 criteria listed in Table 4.

Table 4. Susceptibility Test Interpretive Criteria for Amoxicillin
Minimum Inhibitory Concentration (mcg/mL) Disk Diffusion (zone diameter in mm)
Susceptible Intermediate Resistant Susceptible Intermediate Resistant
* 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.**A positive beta lactamase test indicates resistance to amoxicillin. Isolates that are resistant to penicillin by MIC testing are also expected to be resistant to amoxicillin.

Enterococcus spp.

≤ 8

≥ 16

≥ 17

≤ 16

Staphylococcus spp.

≤ 0.25

≥ 0.5

≥ 29

≤ 28

Streptococci, viridians group (alpha-hemolytic streptococci)

≤ 0.25

0.5 to 4

≥ 8

β-hemolytic streptococci

≤ 0.25

≥ 24

Streptococcus pneumoniae (non-meningitis isolates)*

≤ 2


≥ 8


≤ 8


≥ 32

≥ 17

14 to 16

≤ 13

Haemophilus influenzae

≤ 1


≥ 4

≥ 22

19 to 21

≤ 18

Neisseria gonorrhoeae**

A report of “Susceptible” indicates the pathogen is likely to be inhibited if the antimicrobial compound in the blood reaches concentrations that are usually achievable. A report of “Intermediate” indicates that result should be considered equivocal, and, if the microorganism is not fully susceptible to alternative, clinically feasible drugs, the test should be repeated. The intermediate 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. The intermediate category also provides a buffer zone, which prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of “Resistant” indicates the pathogen is not likely to be inhibited if the antimicrobial compound in the blood reaches concentrations that are usually achievable and other therapy(ies) are likely to be preferred.
Quality Control
Susceptibility techniques require use of laboratory control microorganisms to control the technical aspects of the laboratory standardized procedures.2,3,4 Standard ampicillin powder should provide the MIC values described below. For the diffusion technique using the 10 mcg ampicillin disk, the criteria are provided in Table 5.

Table 5. Acceptable Quality Control Ranges for Amoxicillin
Bacteria ATCC# MIC Range (mcg/mL) Disc Diffusion Zone Range (mm)
# ATCC = American Type Culture Collection

Escherichia coli


2 to 8

16 to 22

Enterococcus faecalis


0.5 to 2

Haemophilus influenzae


2 to 8

13 to 21

Staphylococcus aureus


0.5 to 2


27 to 35

Streptococcus pneumoniae


0.06 to 0.25

Susceptibility Testing for Helicobacter pylori: Amoxicillin in vitro susceptibility testing methods for determining minimum inhibitory concentrations (MICs) and zone sizes have not been standardized, validated, or approved for testing H. pylori. Specimens for H. pylori and clarithromycin susceptibility test results should be obtained on isolates from patients who fail triple therapy. If clarithromycin resistance is found, a non-clarithromycin-containing regimen should be used.

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