Colonization occurs when microorganisms live on or in a host organism but do not invade tissues or cause damage. Colonization refers to the presence of microorganisms that can cause infection but not to the infection itself. Having these microorganisms present, however, increases the risk of infection if the right environment for infection occurs.
Importance in Cystic Fibrosis
People with cystic fibrosis are often colonized with bacteria, especially in the mucous 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, 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.
Bronchiectasis is a condition in which the bronchi and bronchioles (smaller branches of the airways) become thickened, widened, and scarred. Mucus accumulates in these widened airways predisposing to infection. The infection then results in further thickening, widening, and scarring of the airways repeating a vicious cycle.
Beyond the bronchi and bronchioles lie the alveoli, the small air sacs through which the exchange of oxygen and carbon dioxide occurs. When the bronchioles leading to these smallest of airways lose their elastic recoil (from the thickening and scarring) and become clogged with mucus, oxygen inhaled into the lungs cannot reach the alveoli for this gas exchange to take place.
Why the Lungs in People With Cystic Fibrosis Produce Excess Mucus
It’s somewhat unclear whether people with cystic fibrosis 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.
Types of Bacteria That Colonize the Lungs of People With Cystic Fibrosis
Many types of aerobic bacteria (bacteria which grow in the presence of oxygen), anaerobic bacteria (bacteria which grow without oxygen) and opportunistic pathogens like fungus colonize the lungs of people with cystic fibrosis. The term opportunistic pathogens are used to describe microorganisms that do not ordinarily cause infections in healthy people but may cause infections when the “opportunity” arises, such as when lung disease is present.
Here are some organisms that colonize the lungs of people with cystic fibrosis:
Pseudomonas aeruginosa Staphylococcus aureus (including MRSA) Escherichia coli Haemophilus influenzae Klebsiella Prevotella Propionibacterium Veillonella Aspergillus
Of the above organisms, colonization with Pseudomonas typically predominates and about 80% of adults with cystic fibrosis are colonized with this type of bacteria.
Multidrug-Resistant Organisms and Cystic Fibrosis
In addition to the above pathogens, people with cystic fibrosis are more likely to be colonized with “superbugs” or multidrug-resistant organisms. Multidrug-resistant organisms (organisms that are resistant to many antibiotics) take hold in the lungs of a person with cystic fibrosis receives antibiotic treatment for lung infections, such as Pseudomonas. Colonization and infection with multidrug-resistant organisms are 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 peptidesImpaired defense of the lungs due to low levels of glutathione and nitrous oxideReduced ingestion of bacteria by lung cellsIncreased 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, the 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.
