What Is Bacterial Colonization?

Bacterial Colonization of the Lungs Happens With Cystic Fibrosis

Image of CT brain scan of 5 year old child suffering from cystic fibrosis. Credit: Medic Image

Colonization occurs when microorganisms live on or in a host organism but do not invade tissues or cause damage.

People with cystic fibrosis (CF) are often colonized with bacteria, especially in the mucus and airways. If the organisms begin to invade tissues and cause damage or make the person sick, the colonization becomes an infection.

Respiratory Tract Changes in People With Cystic Fibrosis

In people with cystic fibrosis, excess secretion of mucus and chronic bacterial infection result in a special type of chronic obstructive pulmonary disease.

People with cystic fibrosis eventually develop bronchiectasis wherein the airways become badly damaged and scarred.

In people with cystic fibrosis, pathological changes are initially seen in the bronchioles, which are small branches of the airway located deep within the lungs.

Why Do the Lungs in People With Cystic Fibrosis Produce Excess Mucus?

It's somewhat unclear whether people with CF initially produce excess viscid mucus or do so because of bacterial colonization. However, examination of the lungs in newborns with cystic fibrosis who have yet to be colonized with bacteria shows excess production of mucus. These findings suggest that the mucus predisposes people with cystic fibrosis to colonization with bacteria.

What Types of Bacteria Colonize the Lungs of People With Cystic Fibrosis?

Many types of aerobic bacteria, anaerobic bacteria and opportunistic pathogens like fungus colonize the lungs of people with CF.

Here are some organisms that colonize lungs of people with cystic fibrosis:

  • Pseudomonas aeruginosa.
  • Staphylococcus aureus
  • Escherichia coli
  • Haemophilus influenzae.
  • Klebsiella
  • Prevotella
  • Propionibacterium
  • Veillonella
  • Aspergillus (fungus)

Of the above organisms, colonization with Pseudomonas typically predominates and about 80 percent of adults with cystic fibrosis are colonized with this type of bacteria.

In addition to the above pathogens, people with cystic fibrosis are more likely to be colonized with multidrug-resistant organisms. Multidrug-resistant organisms take hold in the lungs after a person with cystic fibrosis receives antibiotic treatment for lung infection, such as Pseudomonas. Colonization and infection with multidrug-resistant organisms is scary because these organisms resist treatment with conventional antibiotics.

Why Does Colonization With Pseudomonas Predominate?

Experts have plenty of thoughts or hypotheses as to why the lungs of people with cystic fibrosis are predominantly colonized with Pseudomonas, including the following:

  • reduced mucociliary clearance combined with the malfunction of antibacterial peptides
  • impaired defense of the lungs due to low levels of glutathione and nitrous oxide
  • reduced ingestion of bacteria by lung cells
  • increased numbers of bacterial receptors

We don't know exactly which of these mechanisms is most important in establishing colonization of Pseudomonas.

Nevertheless, it's apparent that once Pseudomonas takes hold, chronic infection persists.

People with cystic fibrosis who are colonized with Pseudomonas require daily maintenance therapy as well as periodic aggressive intravenous antibiotic therapy when infection sets in. Unfortunately even with such treatment, Pseudomonas infection results in progressive loss of lung function and leads to eventual death in most people with cystic fibrosis.

Selected Sources

Voynow JA, Mascarenhas M, Kelly A, Scanlin TF. Cystic Fibrosis. In: Grippi MA, Elias JA, Fishman JA, Kotloff RM, Pack AI, Senior RM, Siegel MD. eds. Fishman's Pulmonary Diseases and Disorders, Fifth Edition. New York, NY: McGraw-Hill; 2015. Accessed January 23, 2016.

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