Amoxicillin Resistance
Amoxicillin resistance occurs when the antibiotic is not able to treat certain bacterial infections because the pathogens causing these infections have developed mechanisms to prevent the drug from functioning.
Susceptibility Testing
When possible, antibacterial substances, such as amoxicillin, are tested for their effectiveness against various infectious pathogens. These test results allow clinicians to choose the antibiotic likely to result in the most effective treatment of a particular bacterial infection. For instance, one such susceptibility test provides minimum inhibitory concentration (MIC) values that can then be used to identify a pathogenic bacterial strain as susceptible, intermediate, or resistant to a certain antibiotic (see Table 1). Bacterial strains are classified as "Susceptible" when the antibiotic is likely to inhibit its growth; "Intermediate" or ambiguous when it is recommended that additional tests should be done to see if there are alternative, clinically feasible drugs that the pathogen is fully susceptible to; and "resistant' when it is not likely that the antibiotic tested will inhibit bacterial growth, and alternative therapy should be selected.
Table 5. Minimum inhibitory concentrations (MIC) in µg/mL that would classify the pathogenic bacterial strain as susceptible to, intermediate, or resistant to amoxicillin. Adapted from (FDA, 2015). These values may not be the latest approved by the US FDA.
| Pathogen | MIC (µg/mL) for Susceptible (S) strains | MIC (µg/mL) for Intermediate (I) strains | MIC (µg/mL) for Resistant (R) strains |
|---|---|---|---|
| S. pneumoniae | ≤2 | 4 | ≥8 |
Resistance Mechanism(s)
For amoxicillin specifically, the main mechanisms of bacterial resistance against the drug are:
* Antibiotic target alteration
* Antibiotic inactivation
* Reduced permeability to antibiotic
Antibiotic Target Alteration
One mechanism of antibiotic resistance is the mutational alteration or enzymatic modification of the target of the drug. In Streptococcus pneumoniae, the targets of amoxicillin - PBP1a, PBP2x, PBP2b, can be altered to allow bacteria to confer resistance to the drug. Enzymatic modifications in the transpeptidase domain and C-terminal domain strongly correlate with increases in the MICs of amoxicillin.
Antibiotic Inactivation
When a β-lactam antibiotic, like amoxicillin, enters the bacterial cell, it may also bind to an enzyme known as a β-lactamase in addition to its intended PBP target. The β-lactamase will then inactivate the drug by breaking the amide bond of the functional β-lactam ring to open up. As a result of this chemical modification, the antibiotic is no longer able to bind to and inhibit its target PBP enzymes, thus losing its antibacterial properties.
Learn more about β-lactamases.
TEM-1 is a class A β-lactamase, found in many bacteria. It hydrolyzes amoxicillin to allow the bacteria to develop resistance. Other class A β-lactamses, conferring resistance to amoxicillin include: TEM-30 in E. coli, ACI-1 in Acidaminococcus fermentans (a gram-negative anaerobic cocci), SCO in Acinetobacter spp. isolates and E. coli, and blaF in Mycolicibacterium fortuitum.
Class D β-lactamases that confer resistance to amoxicillin include: OXA-1 in E. coli and OXA-2 in the Enterobacteriaceae family (CARD, 2017).
A broad-spectrum class A β-lactamase, blaC, is coded in the chromosome of Mycobacterium tuberculosis. It has been shown to hydrolyze a large number of β-lactam antibiotics and is a major obstacle in the treatment of tuberculosis with such drugs. The β-lactam ring of the antibiotic opens up when it covalently binds to the Ser 70 side chain in the enzyme (Figure 5, Mire et al., 2013). Several other amino acids in the binding pocket make non-covalent interactions.
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| Figure 5. Structure of BlaC and amoxicillin Acyl-Intermediate Complex (PDB ID 4EBN). Inset shows the antibiotic in CPK colors, while the amino acids forming covalent or non-covalent interactions with the antibiotic are shown in orange ball and stick representation. |
Reduced permeability to Antibiotic
The outer membrane protein M35 of Moraxella catarrhalis is an antigenically conserved porin. Downregulation of this porin reduces the bacteria's permeability to amoxicillin and significantly increases the minimum inhibitory concentration of amoxicillin. In other words, the downregulation of these porins, M. catarrhalis can develop resistance to amoxicillin.
Learn more about porins.
Mechanisms Against Resistance
Amoxicillin is often used in combination with a β-lactamase inhibitor, clavulanic acid, to prevent the drug from being degraded by β-lactamases and increase its antibacterial properties. Click here to learn more about clavulanic acid.
Back to the article on amoxicillin.
References
Mire, J. A., Pai, P., Siddiqi, N., Russell, D. H., Rubin, E. J., Sacchettini, J. C. (2013), https://doi.org/10.2210/pdb4EBN/pdb
