# Gut Microbiome in Patients with Obstructive Sleep Apnoea
## 4 Discussion  
In this study, we analysed the bacterial composition of stool samples of patients with OSA and healthy controls. The analysis followed a stepwise process. First, we aimed to identify certain groups of bacteria that could be different in patients with OSA compared to controls. In general, the differences were subtle, but we noticed stronger correlations between bacteria within the control group than in patients with OSA. This could highlight that OSA-related processes, either due to direct (i.e., hypoxemia, sleep fragmentation) or indirect (i.e., differences in metabolism and diet) stimuli, affect particular groups of bacteria rather than gut bacteria in general. Second, we performed alpha- and beta-diversityanalyses, which again revealed only minor differences in the composition of gut bacteria between the groups. Nevertheless, the latter analysis did reveal a potential lower abundance of _Actinobacteria_ in OSA, which was confirmed in a further direct comparison even following adjustment for significant confounders. Third, we directly compared relative abundances which analysis was adjusted for potential confounders. Although the raw analysis revealed significant differences for a number of bacteria, most of them disappeared following adjustment, except for _Actinobacteria_. We found a lower abundance of _Actinobacteria_ in OSA, and this reduction was related to the degree of overnight hypoxiaemia. The latter is in line with a previous report in mice that the gut levels of _Actinobacteria_ were directly related to oxygen concentrations [4]. In the only previous study in adults with OSA, the relative abundance of _Actinobacteria_ was similar in patients and controls [11]. In contrast, [PERSON] et al. reported an inverse relationship between _Actinobacteria_ and the number of awakenings in healthy subjects [27]. In addition, sleep restriction led to decreased abundance of _Actinobacteria_ in mice [28]. _Actinobacteria_ are known to produce the sleep-promoting gamma-aminobutyric acid (GABA) [29]; therefore, low _Actinobacteria_ could contribute to increased arousals in OSA. Although we did not see any relationship between the relative abundance of _Actinobacteria_ and sleep quality indices, the lack of association could be due to the limited number of subjects who had polysomnography as a diagnostic test. We reported an increased abundance of _Proteobacteria_, including _Gammaproteobacteria_ in OSA. Both were related to disease severity and to the prevalence of hypertension, cardiovascular disease, and dyslipidemia. However, the results need to be interpreted carefully, as the difference between the OSA and control groups disappeared following adjustment. Nevertheless, the results are similar to [PERSON] et al., who found that patients with moderate OSA had a higher abundance of _Gammaproteobacteria_ compared to controls [11], and [PERSON] et al., who described similar differences in snoring vs. non-snoring children [9]. In addition, the abundance of _Proteobacteria_ was related to sleep fragmentation in children [10]. _Proteobacteria_ produce lipopolysaccharidesaccharide, which could cause systemic inflammation. Confirming this, a significant direct relationship was reported between gut _Proteobacteria_, including _Gammaproteobacteria_ and plasma lipopolysaccharidesacharide-binding protein in obese subjects [30]. Similarly, in healthy subjects, the richness of _Proteobacteria_ was related to interleukin-6 levels [27]. In line with this, we detected a direct relationship between CRP and _Gammaproteobacteria_. Both _Proteobacteria_ and _Gammaproteobacteria_ were related to BMI. Obesity is a risk factor for OSA [31]; therefore, it is not clear if the association between OSA and these bacteria are independent from obesity. Nevertheless, [PERSON] et al. reported that _Gammaproteobacteria_ is associated with metabolic syndrome, most particularly hypertension in obese subjects [30] which is in line with our results, namely that a relative abundance of _Proteobacteria_ is related to cardiovascular comorbidities.