2.5 Colonization, Infection, Contamination, & Pseudo-Infection

2.5 Colonization, Infection, Contamination, & Pseudo-Infection

Quick Answer: Colonization = organism present and replicating, no host response. Infection = invasion plus host response (fever, leukocytosis, local inflammation, tissue damage). Contamination = organism introduced from outside the patient during collection or processing. Pseudo-infection = a positive result in a patient with no disease, caused by laboratory or equipment contamination.

A positive culture is a fact. What that fact means is an interpretation, and the a-IPC blueprint lists differentiating these states as an explicit task. Getting it wrong is expensive in both directions: treating colonization wastes antimicrobials and selects for resistance, while dismissing true infection as contamination delays care.

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The Four States Side by Side

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Colonization

Colonization is the normal state of the human body. Skin, gut, oropharynx, and vagina carry dense microbial populations continuously. In healthcare, the term usually refers to carriage of an epidemiologically significant organism without disease:

• MRSA in the anterior nares
• VRE in the gastrointestinal tract
• Carbapenem-resistant Enterobacterales (CRE) in stool
• Candida auris on the skin, axilla, and groin — notable because it colonizes persistently for months and sheds heavily into the environment
• Clostridioides difficile in the colon of a patient with formed stool

Colonized patients matter enormously to IPC even though they are not sick. They are reservoirs: they seed the environment, transfer organisms to staff hands, and can later become infected themselves when a device or surgical breach gives the organism a portal of entry. This is why active surveillance cultures, contact precautions, and decolonization protocols (nasal mupirocin plus chlorhexidine bathing for MRSA; universal chlorhexidine bathing in some ICUs) exist. It is also why a colonized patient does not get antibiotics.

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Infection

Infection requires the organism to invade tissue and the host to respond. Evidence includes:

• Systemic signs: fever >38°C or hypothermia <36°C, leukocytosis or leukopenia, tachycardia, hypotension
• Local signs: erythema, warmth, swelling, purulent drainage, pain, loss of function
• Laboratory and imaging support: elevated inflammatory markers, a new infiltrate on chest radiograph, purulence on Gram stain
• Histopathology: tissue invasion on biopsy — the definitive discriminator

Exam Tip: In immunocompromised or elderly patients the host response can be blunted or atypical. An afebrile nursing-home resident with new confusion and hypothermia may be septic. Absence of fever does not equal absence of infection.

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Contamination

Contamination is organism introduction during collection or processing, not from the patient's tissue. The canonical example is a blood culture growing coagulase-negative staphylococci in one of two sets drawn from separate sites, in a patient with no line, no fever, and no clinical deterioration. Skin flora were carried into the bottle at venipuncture.

Discriminators favouring contamination:

• Organism is typical skin flora — coagulase-negative staphylococci, Corynebacterium, Cutibacterium acnes, Bacillus species (non-anthracis), viridans streptococci in some contexts
• Only one of multiple sets is positive
• Long time-to-positivity (often >48 hours)
• Multiple organism types in a single bottle
• No clinical picture consistent with bacteremia

NHSN addresses this explicitly: for common commensal organisms, the bloodstream infection definition requires two or more blood cultures drawn on separate occasions plus signs and symptoms, precisely so that single contaminated bottles do not become reportable CLABSIs.

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Pseudo-Infection and Pseudo-Outbreaks

A pseudo-infection is a positive laboratory result in a patient who genuinely has no infection at that site, arising somewhere in the specimen-to-report pathway rather than in the patient. Sources include:

1. Contaminated equipment — automated endoscope reprocessors, bronchoscopes rinsed with tap water, blood gas analyzers
2. Contaminated reagents, media, collection tubes, or water baths
3. Cross-contamination during laboratory processing
4. Assay change — introducing a more sensitive molecular platform can raise "positives" overnight with no change in disease
5. Surveillance artifact — a new case-finding method detecting cases that were always there

When several occur together, you have a pseudo-outbreak. The signature is a cluster of positive cultures without clinical illness: several bronchoalveolar lavage specimens growing Mycobacterium abscessus in patients who are afebrile, breathing comfortably, and radiographically clear.

Why the distinction changes the investigation

In a true outbreak you look for a transmission pathway among patients. In a pseudo-outbreak you look at the device, the reagent, the water, or the laboratory bench. Chasing patients in a pseudo-outbreak wastes weeks and can lead to unnecessary treatment, unnecessary isolation, and reputational harm.

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The Highest-Yield Application: Asymptomatic Bacteriuria

Asymptomatic bacteriuria (ASB) is bacteriuria without urinary symptoms — colonization, not infection. It is extremely common in catheterized patients, where bacteriuria becomes near-universal after about 30 days of catheterization.

• Do not screen for it and do not treat it, with two exceptions: pregnancy and before invasive urologic procedures with anticipated mucosal bleeding.
• Treating ASB does not reduce catheter-associated urinary tract infection (CAUTI); it drives C. difficile infection and resistance.
• The corresponding diagnostic stewardship control is to stop reflexively culturing urine in patients without urinary symptoms, and to use reflex culture only when urinalysis suggests pyuria.

A parallel exists for C. difficile: do not test formed stool. Nucleic acid amplification testing detects the toxin gene in colonized patients, so testing a patient with formed stool or a patient on laxatives converts colonization into a reportable, treated "infection" that was never there.