Chronic Mycoplasma pneumonia and Chlamydia pneumonia infections are associated with the onset and exacerbation of asthma. These microbial infections result in chronic lower airway inflammation, impaired mucociliary clearance, an increase in mucous production and eventually asthma. Furthermore, children who experience severe viral respiratory infections early in life have a high possibility of having asthma later in their childhood. These viral respiratory infections are mostly caused by respiratory syncytial virus (RSV) and human rhinovirus (HRV). Although RSV infections increase the risk of asthma in early childhood, the association between asthma and RSV decreases with increasing age. HRV on the other hand is an important cause of bronchiolitis and is strongly associated with asthma development. In children and adults with established asthma, viral upper respiratory tract infections (URIs), especially HRVs infections, can produce acute exacerbations of asthma. Thus, Chlamydia pneumoniae, Mycoplasma pneumoniae and human rhinoviruses are microbes that play a major role in non-atopic asthma.
Asthma
According to Hertzen (2002), a common characteristic of asthmatic patients is to have epithelial cells that respond to injury by enhancing the capability of producing proinflammatory and profibrogenic cytokines, instead of repairing the injured epithelial layer. As a result, inflammation and associated healing process leads to scar formation and tissue remodelling, which are symptoms that can be found in almost all asthmatics patients. Thus, asthma is a chronic inflammatory disorder of the airways. Asthma is divided into two subgroups: atopic (extrinsic) and non-atopic (intrinsic). The atopic subgroup is closely associated with family history of the disease, whereas the non-atopic subgroup has its onset in adulthood and it is not caused by inheritance. It is known that non-atopic asthma has a more severe clinical course than atopic asthma. Non-atopic asthma may be caused by chronic viral, bacterial infections, or colonization with pathogenic bacteria.
The distinction between atopic and non-atopic asthma is nuanced. "Atopic" is defined as having one or more positive skin tests to a battery of common aeroallergens, however, compared to people without asthma both atopic and non-atopic asthmatics have elevated levels of serum IgE (the "allergic" antibody) that is believed to be more directly responsible for asthma symptoms. A recent meta-analysis reported that the overall population attributable risk for C. pneumoniae-specific IgE in chronic asthma was 47% and was strongly and positively associated with disease severity. Population-attributable risk is the proportion of disease that is potentially attributable to the risk factor under investigation, indicating the potential for infection with this "stealth pathogen" as a significant contributor to asthma incidence and prevalence.
Associated microorganisms
Chlamydophila pneumoniae
General descriptions
Chlamydophila pneumoniae, formerly known as Chlamydia pneumoniae, is a bacterium that belongs to the phylum Chlamydiae, order Chlamydiales, and genus Chlamydophila. It is rod-shaped and Gram-negative. It has a characteristic pear-shaped elementary body (EB) that is surrounded by a periplasmic space, which makes it morphologically distinct from the round EBs of C. trachomatis and C. psittaci. C. pneumoniae is non-motile and utilizes aerobic respiration. As an obligate intracellular bacterium, C. pneumoniae is both parasitic and mesophilic.