Amoxicillin is an aminobenzyl penicillin, a semisynthetic broad-spectrum antibiotic that has a bactericidal effect by inhibiting bacterial cell wall synthesis.
The threshold values of the minimum inhibitory concentration (MIC) for various sensitive organisms vary.
Boundaries of resistance
The boundary values of the MIC for amoxicillin, according to the European Committee for the Study of Antimicrobial Susceptibility (EUCAST), version 5.0.
Pathogen | The boundary values of the MIC (mg / L) |
Sensitive ≤ | Stable> |
Enterobacteriaceae | 81 | 8 |
Staphylococcus spp. | see note2 | see note2 |
Enterococcus spp.3 | 4 | 8 |
Streptococcus groups A, B, C and G | see note4 | see note4 |
Streptococcus pneumoniae | see note5 | see note5 |
Streptococcus groups viridans | 0,5 | 2 |
Haemophilus influenzae | 26 | 26 |
Moraxella catarrhalis | see note7 | see note 7 |
Neisseria meningitidis | 0,125 | 1 |
Gram-positive anaerobes, except Clostridium difficile8 | 4 | 8 |
Gram-negative anaerobes8 | 0,5 | 2 |
Helicobacter pylori | 0,1259 | 0,1259 |
Pasteurella multocida | 1 | 1 |
Boundary values without a specific type of bacteria10 | 2 | 8 |
1 The wild type of enterobacteria is classified as susceptible to aminopenicillin. In some countries, preference is given to a separate classification of wild types of strains E. coli and P.mirabilis as intermediate products. In this case, use the IPC boundary value S ≤0.5 mg / l.
2 Most staphylococci are resistant to amoxicillin, thanks to the ability to synthesize penicillinase. Methicillin-resistant strains, with some exceptions, are resistant to beta-lactam antibiotics.
3 Susceptibility to amoxicillin can be determined by ampicillin.
4 Susceptibility to penicillin streptococci group A, AT, FROM and G can be determined by benzylpenicillin.
5 Boundary values refer only to non-meningitis strains. If strains are classified as an intermediate product of ampicillin, oral amoxicillin treatment should be avoided. Susceptibility is determined by the value of the MIC for ampicillin.
6 Boundary values refer to intravenous use. Strains synthesizing beta-lactams should be considered as resistant.
7 Strains synthesizing beta-lactamases should be considered as resistant strains.
8 Susceptibility to amoxicillin can be determined by benzylpenicillin.
9 Boundary values refer to epidemiological clipping points that distinguish wild types of strains and strains with reduced susceptibility.
10 Boundary values without a certain type of bacteria, refer to a dosage of at least 0.5 g 3-4 times a day (from 1.5 to 2 g / day).
The sensitivity of microorganisms to amoxicillin in the laboratory
Usually sensitive pathogens
Aerobic Gram-positive microorganisms:
Enterococcus faecalis
Beta-hemolytic streptococci (groups A, B, C and G)
Listeria monocytogenes
Types of bacteria in which acquired resistance is possible
Aerobic Gram-negative microorganisms:
Escherichia coli
Haemophilus influenzae
Helicobacter pylori
Proteus mirabilis
Salmonella typhi
Salmonella paratyphi
Shigella spp.
Pasteurella multocida
Aerobic Gram-positive microorganisms:
Coagulase-negative staphylococci
Staphylococcus aureus1
Streptococcus pneumoniae
Streptococcus groups viridans
Anaerobic Gram-positive microorganisms:
Clostridium spp.
Anaerobic Gram-negative microorganisms:
Fusobacterium spp.
Other microorganisms:
Borrelia burgdorferi
Naturally resistant organisms2
Aerobic Gram-positive microorganisms:
Enterococcus faecium1
Aerobic Gram-negative microorganisms:
Acinetobacter spp.
Enterobacter spp.
Klebsiella spp.
Pseudomonas spp.
Anaerobic Gram-negative microorganisms:
Bacteroides spp. (some strains Bacteroides fragilis resistant)
Other microorganisms:
Chlamydia spp.
Mycoplasma spp.
Legionella spp.
1 Natural average sensitivity in the absence of acquired mechanisms of resistance.
2 Almost all strains S. aureus are resistant to amoxicillin, since they produce penicillinase.
Bacteria can have resistance to amoxicillin (and hence ampicillin) by production of beta-lactamase, hydrolyzing aminopenicillins (which may be inhibited by clavulanic acid), changes in penicillin-binding proteins, disturbances in permeability to the drug or due to operation of a special pump action pumps efflux drug from the cell. In one microorganism can simultaneously present multiple resistance mechanisms that explain the existence of the variable and unpredictable cross-resistance to other beta-lactam antibiotics and antibacterial agents from other groups.
The prevalence of resistant strains varies geographically and at different times, so it is advisable to focus on local information on resistance, especially when treating severe infections.