Overview
Resistance mechanisms are biologic strategies that reduce antimicrobial activity. AST detects the resulting phenotype under standardized conditions; mechanism naming requires validated follow-up per SOP.
Classification
| Mechanism | Lab implication | Hedge |
|---|---|---|
| Enzymatic inactivation | Class/subclass patterns | Confirmatory tests local |
| Efflux | Multi-agent shifts possible | Exclude technical error |
| Porin/permeability | Reduced susceptibility patterns | ID and method first |
| Target mod/protection | Agent-specific shifts | SOP for special tests |
Morphologic Features
Zones with inner colonies, haze, or skipped dilution wells may reflect heteroresistance or reading complexity, measure per SOP; do not improvise categories.
Laboratory Characteristics
- Prove QC pass and correct inoculum/media
- Pattern review against organism ID
- Backup disk/MIC when SOP requires
- Authorized comments only, no drug advice
Reference Intervals
Breakpoint cutoffs, QC expected ranges, and confirmatory interpretive criteria are institutional, verify adopted CLSI or EUCAST edition and local tables.
Clinical and Laboratory Significance
Valid resistant phenotypes trigger reporting and any mandated public-health or stewardship notifications defined locally. Invalid methods must not generate mechanism reports.
Differential Considerations
Differentiate true resistance from heavy inoculum, wrong medium, mixed culture, misidentification, misread endpoints, and QC failure.
Comparison Tables
Resistance pattern triage
| Clue | Suggests | Do not |
|---|---|---|
| QC fail + “resistant” patient | Method problem | Release + name enzyme |
| QC OK + class pattern | Possible mechanism | Recommend therapy |
| Colonies in zone | Read per SOP / confirm | Ignore or guess |
Classification Frameworks
Not applicable.
Laboratory Notes
- No treatment/dosing/drug selection language
- Hedge breakpoints and QC ranges as institutional
- Inoculum, media, reading, QC before mechanism claims
References
Authoritative textbooks, guidelines, and reviews supporting this reference entry. Verify reference intervals, critical limits, and reflex criteria against institutional protocols and current guideline editions.
Textbooks
- Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott's Diagnostic Microbiology. 15th ed. Elsevier; 2020.
- Mahon CR, Lehman DC, Manuselis G. Textbook of Diagnostic Microbiology. 6th ed. Elsevier; 2018.
- Carroll KC, Hobden JA, Miller S, et al. Jawetz, Melnick, & Adelberg's Medical Microbiology. 28th ed. McGraw-Hill; 2019.
- Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 9th ed. Elsevier; 2021.
CLSI and Professional Guidelines
- Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing. CLSI document M100 (verify current edition and institutional adoption).
- Centers for Disease Control and Prevention / NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL). Current edition (verify institutional adoption).
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