Overview
Gram-negative bacteria are organisms that lose crystal violet during proper decolorization and take up counterstain, reflecting a thin peptidoglycan layer and an outer membrane containing LPS.
Classification
| Frame | Typical cues | Lab note |
|---|---|---|
| Enterobacterales-like | MacConkey growth ± LF | ID to policy level |
| Nonfermenters | NLF; oxidase variable | Algorithm differs |
| GNDC | Kidney-bean diplococci | Site-specific media |
| Fastidious GNCB | Poor MacConkey growth | Enriched media |
Morphologic Features
Report pink rods, coccobacilli, or diplococci with arrangement. Correlate with colony reactions; do not assign species from Gram morphology alone.
Laboratory Characteristics
- Selective/differential enteric media separate many GNB from gram-positives in mixed flora
- Oxidase and carbohydrate reactions refine biochemical pathways
- MALDI-TOF commonly completes ID when available
Reference Intervals
Quantitative urine thresholds and respiratory workup rules are institutional and method-specific, verify local SOP.
| Site pattern | Interpretive lean | Caveat |
|---|---|---|
| Pure significant GNR in urine | Uropathogen candidate | Thresholds local |
| Mixed GNB in wound | Possible colonizers | Limited workup often |
| Any GNB in CSF/blood | High potential significance | Urgent pathway |
Clinical and Laboratory Significance
Gram-negative recovery supports infection diagnosis when quantity, purity, and site align. LPS-associated syndromes increase communication urgency for sterile-site findings.
Differential Considerations
Differentiate over-decolorized gram-positives from true GNB; differentiate lactose fermenters within Enterobacterales by formal ID methods.
Comparison Tables
GNR vs GPR envelope cues
| Feature | Gram-negative rod | Gram-positive rod |
|---|---|---|
| Outer membrane | Present | Absent |
| Typical Gram color | Pink | Purple |
| MacConkey utility | Often useful | Usually no growth / not used |
Classification Frameworks
Gram-negative is a laboratory envelope category, not a WHO disease code.
Laboratory Notes
- Media inventories differ, treat textbook plate lists as examples
- Critical GNB notification lists are institutional
- Anaerobic GNB require anaerobic protocols not visible on aerobic plates alone
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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