Summary of EUCAST Expected Resistant Phenotypes
EUCAST Expected Resistant Phenotypes: Guide for Students
Introduction
Bacterial resistance is the ability bacteria develop to survive against antibiotics that previously inhibited or killed them. This study focuses on expected resistance phenotypes (unexpected susceptibility) according to EUCAST v1.2 (January 2023) and practical concepts for validating laboratory results and clinical decision-making.
Definition: An expected resistance phenotype means that >90% of isolates of a species are resistant to an agent; therefore, a susceptible result should be considered suspicious.
Basic Concepts Unpacked
Why do "expected" resistance phenotypes exist?
- Many species exhibit intrinsic or chromosomal mechanisms that confer uniform resistance against antibiotic families (e.g., AmpC production, efflux pumps, modifying enzymes).
- These resistances are useful for:
- Validating species identification in the lab
- Avoiding unnecessary testing
- Guiding clinical therapy
Definition: Expected Susceptible Phenotype: when >99% of isolates are susceptible; a resistant result is suspicious.
Practical Interpretation in the Lab
- If the table indicates "R" for a species/antibiotic, a susceptible result should not be trusted: check identification and method. Options:
- Do not perform the test
- Do not report the result
- Report as resistant without testing (if requested, state the reason)
Comparative Tables (EUCAST v1.2 Summary)
- The original tables classify species and antibiotics with an "R" mark when susceptibility is not expected.
Table 1: Enterobacterales and Aeromonas spp. (selection)
| Species (examples) | Ampicillin / Amoxicillin | Cefuroxime | Tigecycline | Polymyxin B / Colistin | Nitrofurantoin |
|---|---|---|---|---|---|
| Citrobacter koseri / C. amalonaticus | R | ||||
| Citrobacter freundii (group) | R | R | |||
| Enterobacter cloacae complex | R | R | |||
| Klebsiella pneumoniae complex | R | ||||
| Morganella morganii | R | R | R | R | |
| Proteus spp. (e.g.: P. mirabilis) | R | R | R | R | |
| Providencia rettgeri / stuartii | R | R | R | R |
Key Notes: Many Enterobacterales show resistance to benzylpenicillin, glycopeptides, macrolides, lincosamides, and oxazolidinones.
Table 2: Non-fermenting Gram-negative Bacilli (selection)
| Species | Piperacillin-tazobactam | Ceftazidime | Carbapenems | Aminoglycosides | Colistin |
|---|---|---|---|---|---|
| Acinetobacter baumannii / pittii / nosocomialis | R | R | Note | R | Note |
| Pseudomonas aeruginosa | R | R | R | R | |
| Stenotrophomonas maltophilia | R | R | R | R (all aminoglycosides R) | R |
| Burkholderia cepacia complex | R | R | R | R | R |
Notes: Acinetobacter may appear susceptible to ampicillin-sulbactam due to sulbactam activity. Stenotrophomonas is often susceptible to trimethoprim-sulfamethoxazole but resistant to trimethoprim alone.
Table 3: Other Gram-negatives
| Species | Fusidic Acid | Trimethoprim | Nalidixic Acid |
|---|---|---|---|
| Haemophilus influenzae | R | ||
| Campylobacter jejuni / coli | R | R | R |
| Neisseria spp. | R |
Table 4: Gram-positives (selection)
| Species | Cephalosporins (except ceftazidime) | Aminoglycosides | Macrolides | Vancomycin |
|---|---|---|---|---|
| Staphylococcus saprophyticus | R | R | R | |
| Coagulase-negative (various) | R | |||
| Streptococcus spp. (various) | R | R | R | |
| Enterococcus faecalis | R | R (LLR) | R | R |
| Enterococcus faecium | R | R (LLR and AAC(6')-I) | R | R |
Notes: Enterococcus may show low-level resistance (LLR) to aminoglycosides; Enterococcus faecium produces AAC(6')-I, which affects synergy with beta-lactams/glycopeptides.
Table 5: Anaerobes
| Species | Vancomycin |
|---|---|
| Clostridium ramosum / C. innocuum | R |
Practical Examples and Brief Clinical Cases
- Case: Isolation of Enterobacter cloacae from a urine sample reported as "susceptible" to cefuroxime. Interpretation: Suspect an error; Enterobacter cloacae complex typic
Sign up for the full summaryAlready have an account? Sign in
Bacterial Resistance
Klíčové pojmy: Expected R phenotype means >90% of isolates are resistant, If the table shows R, suspect and verify any susceptible result, Enterobacterales with AmpC (e.g., Enterobacter cloacae) are often resistant to second-generation cephalosporins, Acinetobacter may appear susceptible to ampicillin-sulbactam due to active sulbactam, Stenotrophomonas maltophilia is resistant to aminoglycosides; TMP-SMX is often active, Enterococcus faecium produces AAC(6')-I, which eliminates synergy with some aminoglycosides, Do not test or report as resistant when the phenotype is expected; document the decision, Confirm unexpected results with alternative methods or molecular tests, Selected anaerobes (C. ramosum, C. innocuum) are resistant to vancomycin, Use EUCAST tables as a quality control and clinical communication tool