Background
- Extended-spectrum β-lactamases (ESBLs) are generally defined as β-lactamases that hydrolyze penicillins, first-, second-, and third-generation cephalosporins, and aztreonam, but not cephamycins or carbapenems
- This definition excludes AmpC (which hydrolyze cephamycins) and carbapenemases (which hydrolyze carbapenems)
- Includes primarily Ambler Class A β-lactamases
Identification
- ESBLs are screened for by identifying organisms with increased MICs to one or more third-generation cephalosporin or monobactam
- CLSI uses cefpodoxime, ceftazidime, aztreonam, cefotaxime, or ceftriaxone
| Bacterium | Antibiotic | Disc Diffusion | Broth Microdilution |
|---|---|---|---|
| Klebsiella pneumoniae, Klebsiella oxytoca, and Escherichia coli | cefpodoxime | ≤17 mm | ≥8 μg/mL |
| ceftazidime | ≤22 mm | ≥2 μg/mL | |
| aztreonam | ≤27 mm | ≥2 μg/mL | |
| cefotaxime | ≤27 mm | ≥2 μg/mL | |
| ceftriaxone | ≤25 mm | ≥2 μg/mL | |
| Proteus mirabilis | cefpodoxime | ≤22 mm | ≥2 μg/mL |
| ceftazidime | ≤22 mm | ≥2 μg/mL | |
| cefotaxime | ≤27 mm | ≥2 μg/mL |
- Organisms that screen positive have further testing to determine the presence of an ESBL
- Class A, for all organisms that screen positive,
- A double-disc diffusion method is used to assess whether the third-generation cephalosporin resistance that is attenuated by clavulanic acid
- CLSI uses either ceftazidime or cefotaxime, with and without clavulanic acid
- An increase in the zone of ≥5 mm is diagnostic of Class A ESBL production
- Class B, for organisms that are also resistant to carbapenems
- See Carbapenemases for details
- Class C, for organisms that are non-susceptible to cefoxitin
- A double-disc diffusion method is used to test if the results for cefoxitin are attenuated by cloxacillin (that is, if the isolate becomes even more susceptible)
- An increase in the zone of ≥4 mm is positive for AmpC (Class C ESBL) phenotype
- Class D, difficult to identify with phenotypic testing
Management
- Carbapenem, with meropenem or imipenem preferentially, though ertapenem can be considered if no septic shockPichia kudriavzevii
- In some patients, can consider piperacillin-tazobactam, amoxicillin-clavulanic acid, or fluoroquinolones
- For non-severe UTI without septic shock, can consider co-trimoxazole, aminoglycosides, or fosfomycin (IV)
- See also Carbapenem-resistant organisms
