Asthma: severity of attacks linked to nasal microbiota

The relationship between asthma and the microbiota in the respiratory tract of asthmatic children is still poorly understood. A prospective study analyzed the links between the relative abundance of bacteria in 319 nasal samples collected at two points in time–with asthma under control and at the onset of an attack–and asthma attacks suffered by 254 school-age children with Stage 2 asthma, 75.7% of whom experienced an attack during the 320 days of follow-up (with two attacks for 43.4% of the children included in the follow-up period).

Bacterial groups linked to asthma risk

The results show that the risk of attack or exacerbation varies according to the type of bacteria colonizing the nasal tract. Specifically, microbiotas in which the Corynebacterium and Dolosigranulum genera prevail are associated with a lower risk of asthma attack than microbiotas dominated by more pathogenic bacteria, particularly the Staphylococcus, Streptococcus and Moraxella genera. Furthermore, a shift to Moraxella at the onset of an attack (peak expiratory flow in the yellow zone) is associated with a higher risk of exacerbation. These results are consistent with those of previous studies showing that colonization of the upper airway by opportunistic pathogens, particularly Streptococcus, Moraxella and Haemophilus, is more common in asthmatics than in healthy subjects.

Protective bacteria

In addition, at the onset of an asthma attack, the relative abundance of Corynebacterium was inversely associated with the likelihood of severe exacerbation. It should be noted that Corynebacterium (the most abundant genus identified in nasal microbiota) less frequently prevails in the nasal microbiota of asthmatic adults, suggesting a protective effect, perhaps through competitive colonization. Corynebacterium and Dolosigranulum may indeed inhibit the growth of Streptococcus by releasing antibacterial substances.

Cause or consequence?

Therefore, the microbiota of the upper respiratory tract is linked to events that take place in the lower respiratory tract. However, the design of the study prevents from drawing any conclusion on causality relationship. It is not yet known whether changes in the microbiota I) give rise to asthmatic activity, II) are the consequence or cause of a viral infection, or III) are the result of a two-way dialogue between the microbiota and the immune response of the host at a mucosal level during attacks and exacerbation. Changes in the microbiota may also be due to poorer asthma control or an inflammation of the respiratory tract.