Subtitle: Understanding what it means when bacteria or fungi are found in your lungs
Introduction
People with bronchiectasis, Primary Ciliary Dyskinesia (PCD), Allergic Bronchopulmonary Aspergillosis (ABPA), or Chronic Pulmonary Aspergillosis (CPA) often have microbes detected in their sputum samples.
That doesn’t always mean there’s an infection that needs treatment.
Understanding the difference between colonisation and infection helps patients and clinicians make better decisions.
Colonisation
Colonisation means that bacteria or fungi are living in the airways but aren’t currently causing harm.
This happens because mucus clearance is reduced, allowing microbes such as Haemophilus influenzae, Pseudomonas aeruginosa, or Aspergillus fumigatus to persist.
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The microbes are “residents,” not invaders.
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Symptoms stay stable.
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Blood tests for inflammation (like CRP) are usually normal.
Treatment isn’t always needed — instead, care focuses on airway clearance, physiotherapy, hydration, and monitoring through sputum cultures.
Infection
Infection means microbes are actively causing inflammation and tissue irritation.
This happens when microbial numbers rise, new strains appear, or immune defences weaken.
Typical signs:
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Increased cough, sputum, or breathlessness
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Fever or feeling unwell
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Raised inflammatory markers
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New changes on chest X-ray or CT
Treatment involves targeted antibiotics or antifungals based on sputum results and resistance testing.
Why Colonisation Can Turn Into Infection
In chronic airways disease, colonisation and infection exist on a sliding scale — a shift in balance can push the lungs from stable to inflamed.
Common triggers include:
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Growth of a new or resistant strain
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Reduced mucus clearance
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Viral infections (e.g. influenza, COVID-19)
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Immune suppression
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Loss of “friendly” bacteria in the lung microbiome
When this balance is disrupted, inflammation rises and infection takes hold.
The Balance Model
| Factor | Colonisation (Stable) | Infection (Flare-Up) |
|---|---|---|
| Microbial strain | Stable | New or virulent |
| Microbial load | Controlled | Increased |
| Microbiome | Diverse | Reduced diversity |
| Immune status | Balanced | Suppressed or overactive |
| Symptoms | Stable | Worsening |
| CRP / WBC | Normal | Raised |
Key Takeaway
In chronic lung conditions, microbes are often part of daily life. The aim isn’t complete eradication, but balance — keeping numbers low, reducing inflammation, and treating only when infection is active.
👉 Next article: When Microbes Turn Hostile – The Evolutionary Pressures Behind Infection
(Explore how antibiotics, competition, and disrupted microbiomes drive microbes to become more aggressive.)
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