Overview
Host–microbe interaction describes the continuum from mutualistic or commensal coexistence and colonization to contamination artifacts and true infection, including opportunistic disease in compromised hosts.
Classification
| Term | Meaning | Example framing |
|---|---|---|
| Colonization | Present, no disease | Asymptomatic bacteriuria |
| Contamination | Not from true site flora/pathogen | Skin flora in poorly prepped blood culture |
| Infection | Disease process | Bacteremia with clinical sepsis signs |
| Opportunistic | Pathogenic mainly if host impaired | Many skin flora in neutropenia |
Morphologic Features
Interaction concepts are interpretive, not morphologic. Smear findings (organisms + host inflammatory cells) can support but not alone prove infection.
Laboratory Characteristics
- Sterile-site recovery raises concern pending correlation
- Mixed nonsterile growth often reflects microbiota
- Local contaminant algorithms and comments vary, verify SOPs
Reference Intervals
Quantitative culture thresholds (e.g., urine CFU cutoffs) are method- and institution-specific and must be verified locally. They support, but do not replace, clinical correlation.
| Clue | Favors | Caveat |
|---|---|---|
| Sterile site + credible collection | True presence | Still needs clinical correlation |
| Mixed dermal flora / poor prep | Contamination risk | Policy-dependent workup |
| Compromised host | Higher opportunist concern | Not automatic pathogen label |
Clinical and Laboratory Significance
Accurate interaction framing prevents over- and under-calling culture results and improves communication with clinical teams.
Differential Considerations
Differentiate colonization, contamination, and infection before assigning clinical disease labels to an isolate.
Comparison Tables
Colonization vs infection
| Feature | Colonization | Infection |
|---|---|---|
| Microbe present | Yes | Yes |
| Host disease attributable | No | Yes |
| Lab role | Report per policy | Report + correlate / notify per policy |
Classification Frameworks
Interaction terms are laboratory-clinical concepts, not a WHO taxonomy system.
Laboratory Notes
- Never provide treatment advice in interpretive comments unless protocolized under laboratory medical direction
- Document notification and recollection requests per institutional policy
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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