Overview
Pathogenicity is the ability of a microorganism to cause disease. Laboratory significance of an isolate additionally depends on anatomic site, quantity, host factors, and specimen quality.
Classification
| Category | Typical setting | Bench implication |
|---|---|---|
| Primary pathogen | Disease in normal hosts at relevant sites | Usually full workup when recovered appropriately |
| Opportunistic pathogen | Compromised host / breached barriers | Escalate workup with host/device context |
| Colonizer | Expected microbiota without disease | Limited ID/reporting per site SOP |
| Contaminant | Introduced during collection/processing | Correlate with smear, repeats, and sterile-site rules |
Morphologic Features
Conceptual reference, morphology alone does not determine pathogenicity.
Laboratory Characteristics
- Site-specific workup tables
- Smear–culture correlation
- Host risk notes on requisition
- Standardized colonization vs pathogen comments
Reference Intervals
Not numeric intervals, significance thresholds (e.g., CFU cutoffs, mixed-flora rules) are assay-, site-, and institution-specific.
Clinical and Laboratory Significance
Correct pathogenicity framing prevents both over-calling colonizers and under-calling opportunists in high-risk hosts.
Differential Considerations
Differentiate colonization, contamination, and infection using clinical syndrome plus laboratory context before expanding identification.
Comparison Tables
Context shifts significance
| Same organism | Lower concern example | Higher concern example |
|---|---|---|
| Candida spp. | Oral swab, immunocompetent | Blood or sterile site; neutropenic pneumonia workup |
| CoNS | Single peripheral blood bottle, no devices | Multiple bottles / catheter-related bacteremia workup |
| E. coli | Perineal swab | Urine with pyuria criteria met; sterile fluid |
Classification Frameworks
Not applicable.
Laboratory Notes
- Do not equate growth with disease
- Document host and device context when available
- Follow SOP for limited vs full workup
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).
Frequently Asked Questions
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