Management of CAP focuses particularly on early administration of the appropriate antimicrobial agent [22]. Current diagnostic methods may be limited by sampling difficulties, delayed results, and difficulties in interpretation [5]. Our research used the oral flora as the diagnostic factor of CAP, which effectively solves the problem of sampling difficulties. Our data discovered the significantly higher abundance of Prevotella, Veillonella and Campylobacter, and lower abundance of Neisseria and Fusobacterium than healthy group, which can be used as biomarkers in diagnosis of CAP.
The baselines of our study population were consistent. Specifically, the basic information and oral health status were not statistically different. We recruited the accompanying family members of the patients as a control group, and they could maintain good consistency in terms of living habits, eating habits, environment, socioeconomic conditions, etc. This can greatly reduce the interference of confounding factors on the oral flora and ensure the accuracy of the results.
The oral bacteria composition of patients with COPD also found the loss of microbiota diversity [23]. We also found that CAP group had a lower diversity than healthy controls. Due to risk factors such as increased inhalation and mechanical ventilation under abnormal conditions, oral microorganisms may transfer to the respiratory tract and aggravate respiratory disease [24, 25].
Prevotella is consistently among the core bacterial communities of the respiratory tract [26, 27], which is known to be associated with pneumonia [14, 28, 29]. Research found that the periodontopathic bacterial genera Prevotella were increased in the BALF of patients with CAP [30]. We discovered an enrichment of Prevotella, especially Prevotella_melaninogenica in oral cavity of patients with CAP. It suggests that the mouth may be a reservoir of bacteria for the lungs. In a mouse lung co-infection model, the increased abundance of Prevotella melaninogenica was found, which probably was a potentially beneficial role to reduce infection [31]. The above suggests that there was consistency in the changes of oral and pulmonary bacteria (Prevotella and Prevotella_melaninogenica) under lung inflammation.
We also observed that the abundance of Veillonella and Campylobacter were significantly higher in the CAP group. Few studies have focused on their relationship with CAP. But in a lung cancer model, lower airway dysbiosis with Veillonella led to decreased survival time and increased tumor burden [32]. Skallevold et al. found that Veillonella and Streptococcus in saliva could be valuable markers with diagnostic and prognostic potentials in lung cancer [33]. On the other hand, in COPD, asthma, and cystic fibrosis (CF) lung diseases, oral microbiota (e.g. Prevotella species, Veillonella species) were present in the airways and associated with increased host inflammatory response [26–28]. Additionally, Veillonella was identified as the most prominent biomarker for COVID-19 group sequencing analyses of oropharynx swab specimens [29, 34]. Therefore, the potential of Veillonella and Campylobacter as a diagnostic biomarker for lung disease is proposed.
In control group, Neisseria and Streptococcus were the core genus of oral flora. The abundance of Neisseria and Fusobacterium was distinctly lower in CAP group than control group. Similarly, the levels of Neisseria in the pharynx were significantly lower in COPD patients compared to healthy people [35]. Lower lung function, COPD diagnosis, and greater symptoms were recognized an association positively with Neisseria in lung microbiota [36]. LaMotte et al. reported that oral Neisseria spp. could be causative bacteria in ventilator-associated pneumonia (VAP) patients [37]. Neisseria is possibly an infection-promoting factor for lung disease. There are very few studies on the relationship between Neisseria and CAP, which is in urgent need of illustration.
Fusobacterium, recognized as a periodontal pathogen, is considered as a biomarker of lung function deterioration of COPD patients coinfected with Pseudomonas aeruginosa [38]. Similar to oral flora changes in the present study, the Fusobacterium periodonticum resulted as the most significantly reduced species in COVID-19 patients [39]. Consequently, Fusobacterium, especially Fusobacterium periodonticum was a potential specific detection indicator.
The functional analysis showed that the genera Veillonella, Prevotella and Campylobacter were positively correlated with infectious disease, while genera Neisseria and Fusobacterium were the negative correlations. The result further illustrated that these microbiotas can potentially serve CAP as biomarkers.
Anyway, our results provide baseline associations between oral microbiota and CAP, which will be important for further evaluation of treatment and prognosis.