Background
The human microbiota is the collection of a wide array of microorganisms such as bacteria, archaea, fungi and viruses that inhabit various body sites including skin, saliva and the gut [
1]. The microbiome, defined as the collection of microbiota and their genes, plays an important role in human health and disease [
2]. The development in the field of sequencing and bioinformatic tools in the last decade, has brought an unprecedented attraction to the microbiome field [
3]. The microbiome composition varies from one person to another, as well as across different body sites [
4].
Saliva is a biofluid secreted by the major and minor salivary glands [
5]. It contains several components such as electrolytes, proteins, immunoglobulins, enzymes and microbes [
6]. The main role of the saliva is to protect the mucus from pathogens, to maintain tooth integrity in addition to its role in taste and digestion [
7]. Being highly available, saliva is considered as an easy to collect sample that does not require hospitalization or special preparation [
8,
9].
Located at the opening of the gastrointestinal tract, the oral cavity provides a convenient, accessible site for collecting and analyzing microbial samples in the saliva [
10]. It is also worth noting that, the salivary microbiome mirrors the gut microbiome in terms of complexity and diversity [
10]. The salivary microbiome exhibits long-term stability and does not fluctuate according to the circadian rhythm, indicating that the time of the saliva sampling is not critical [
11‐
16]. Collection of saliva can be achieved by several methods, including spitting, swabbing and draining [
17], and the method of collection exerts a minimal effect on the extraction of microbial DNA [
18]. A healthy adult human mouth hosts a complex and resilient ecosystem of hundreds of different microbial species [
19]. These microbes reside in different sites of the oral cavity, which is mainly composed of a soft mucosa that is constantly shedding, and a hard surface which comprises the teeth [
20]. The saliva is a representing constituent of both locations but more profoundly of the soft mucosa [
20]. According to many studies, the salivary microbiome consists of more than 700 species mainly belonging to the
Bacteroidetes,
Firmicutes,
Proteobacteria,
Actinobacteria and
Fusobacteria phyla [
21‐
24].
Streptococcus,
Prevotella,
Neisseria,
Haemophilus,
Porphyromonas and
Rothia are the common genera members of salivary microbiome observed in healthy adults [
25‐
27]. The salivary microbiome composition is influenced by several pre and post-natal factors including host genetics, the mode of delivery at birth; the method of infant feeding; teeth eruption, the use of medications, especially antibiotics; smoking, intraoral pH, oral hygiene and diet among others [
28]. The salivary microbiome plays a major role in regulating the immune-inflammatory balance in the host [
29]. In a large American cohort study, Wu et al. compared the salivary microbiome composition in current smokers and non-smokers [
30]. They observed that the salivary microbiome of smokers reflected a decrease in the abundance of the phylum
Proteobacteria, and in
Capnocytophaga,
Peptostreptococcus and
Leptotrichia genera; while the genera
Atopobium and
Streptococcus were found to be elevated in smokers compared to non-smokers [
30]. Another study examined the oral microbiome of smokers and non-smokers in addition to the levels of cytokines in saliva samples, where they found that smoking altered the cytokine levels and the salivary microbiome composition [
31].
Dysbiosis of the oral microbiome has been implicated in various oral disorders such as periodontitis, tooth decay or loss of teeth, where it promotes pathogenic bacterial growth and enables the dissemination of the oral bacteria systemically [
32]. Several studies were conducted in order to assess the microbiome composition and its role in dental and periodontal health, and showed that, healthy individuals have a greater microbial diversity and a greater abundance of
Neisseria,
Haemophilus, and
Fusobacterium. This is in contrast to individuals who suffer from dental caries, where Streptococcus was the most abundant genus detected [
33,
34]. In a comparative study of healthy Finnish adults with and without caries,
Corynebacterium,
Fusobacterium,
Capnocytophaga,
Porphyromonas,
Prevotella, and
Leptotrichia were significantly more abundant in healthy volunteers as compared to those with dental caries [
35].
Porphyromonas gingivalis,
Tannerella forsythia,
Treponema denticola,
Prevotella intermedia and
Aggregatibacter actinomycetemcomitans were shown to be higher in Moroccan patients with periodontitis [
36] and
P. gingivalis,
P. intermedia,
T. forsythia and
Fretibacterium were higher in Japanese patients with periodontitis [
37,
38]. Moreover, oral microbial dysbiosis is usually observed in patients with systemic diseases such as obesity, diabetes, cancer, rheumatoid arthritis, Parkinson’s disease, type 2 diabetes (T2D) and cardiovascular diseases among others [
39‐
45].
Based on its potential role in health and disease, the salivary microbiome harbors a great potential for being used as a health monitor or disease diagnostic tool. However, the degree of variation at the population level has been assessed in very few studies [
12,
46,
47], none of them reflecting the Arab population. The aim of this study is to characterize the salivary microbiome of the Qatari population and to assess the role of gender, age, oral health, smoking and some dietary habits in the salivary microbiome composition.
Discussion
The purpose of this study was to examine the salivary microbiome composition in the Qatari population, and assess its association with gender, age, oral health, smoking coffee and tea consumption. We characterized the salivary microbiome of 997 Qatari participants including 442 males and 555 females. To the best of our knowledge, this is one of the largest population-based studies assessing the salivary microbiome and is the first study to characterize the salivary microbiome in an Arab population like Qataris.
We show that the bacterial profile, in spite of its high diversity, was dominated by the phylum
Bacteroidetes, which is different compared to other populations like Bangladesh, UK, Japan, South Korea and Brazil, where
Firmicutes was the most predominant phylum. In this study, we show that
Prevotella,
Porphyromonas,
Streptococcus,
Veillonella,
Haemophilus,
Gemella, and
Neisseria were the most common members of the Qatari salivary microbiome, with
Prevotella being the most predominant genus. In contrast,
Streptococcus is the most abundant genus in the UK, South Korea, Japan and US populations [
46,
48‐
53]. The differences observed in the Qatari salivary microbiome compared to other populations, may be influenced by various factors, including host genetics, diet and environmental factors [
49].
In this study, we show that the observed species richness index was significantly higher among Qatari males than females, with a significant shift in particular genera like
Bergeyella,
Tannerella in males and
Treponema,
Mycoplasma and
Corynebacterium in females. This may be influenced by many factors including hormones and body mass index as previously reported [
50,
54‐
56].
Our data show that the salivary microbial composition is associated with age, which is in accordance to previously reported results [
57]. The senior participants included in our cohort showed a reduced bacterial diversity and significantly increased
Prevotella, when compared to the younger adults. Proteolytic bacteria such as
Prevotella are known to degrade proteins and peptides and are associated with periodontitis [
28,
58]. It is known that the saliva composition changes with age due to the slower salivary flow [
59], inadequate oral care, increase of several inflammatory mediators [
60], systemic diseases and other additional environmental factors that together will affect the microbial composition in the saliva [
28,
61]. The adult salivary microbial population this cohort was more diverse including microbes like
Streptococcus, Haemophilus,
Rothia and Veillonella and Lautrophia, some are known to degrade carbohydrates [
50].
We also show that participants that suffer from various oral conditions including bleeding or painful gum, loose teeth or mouth ulcers have lower salivary microbial diversity; with
Prevotella being the most predominant member. Previous studies have shown an association between increased abundance of
Prevotella, aphthous ulcers and periodontal disease (more specifically gingivitis) [
62,
63]. Our data indicates that the alpha diversity of the salivary microbiome in the control groups was significantly higher in comparison to those suffering from poor oral health, which is similar to previously reported results [
64]. Our data show that usage of denture is positively correlated with age and is closely linked with the oral health status [
65]. In this study, we show that using denture reduces the diversity of the salivary microbiome with an enrichment of both
Proteobacteria and
Actinobacteria phyla while
Streptococcus and
Neisseria were enriched at the genus levels. This result is supported by other studies comparing biofilms forming on natural teeth against those forming on denture teeth [
66,
67].
Smoking modulates the microbial composition of various body sites including upper gut, respiratory tract and the oral microbiome [
68‐
71]. Prior studies had shown that smoking disrupts the microbial homeostasis leading to various oral disease such as gingivitis and dental loss [
72]. Our analysis revealed that smoking reduced the salivary microbial diversity and
Bacteroidetes was the most abundant phylum observed, which has been reported previously [
31]. Moreover, the genus
Prevotella was more abundant in the smokers compared to non-smokers, suggesting therefore an increased vulnerability of the smokers to develop oral diseases such as gingivitis [
31]. This suggests that smoking has to always be considered in the future when assessing the oral microbiome composition, as it clearly affects the salivary microbiome composition.
Coffee and tea are commonly consumed beverages in most populations, and both were heavily studied to assess their health benefits [
73‐
75]. In our cohort, around 11% of the Qatari participants reported drinking coffee and 23% reported drinking tea. While some papers reported that both coffee and tea affect the microbial composition of the saliva [
73,
76,
77]; our data show that a significant increase in
Granulicatella,
Gemella,
Streptococcus and
Lautrophia along with an increased microbial richness and diversity is observed in the coffee but not in the tea drinkers. Higher abundance of
Granulicatella in the saliva of coffee drinkers was previously reported [
73].
Our data showed that
Prevotella is the most abundant bacteria observed in the salivary microbiome of the Qatari adult population.
Prevotella is one of the commonly reported members of the oral microbiome [
78] and it has been linked previously to various inflammatory conditions such as rheumatoid arthritis, metabolic disorders and periodontal infections among others [
79]. Based on gender, we did not observe any significant difference in the abundance of
Prevotella between males (55%) and females (53.69%) in this cohort. We show that,
Prevotella was more abundant in subjects with mouth ulcers (54.76%), bleeding gum (50.97%) as compared to the healthy individuals (41.04%, 41.10%).
Prevotella is higher in the group of smokers (57.32%) as compared to non-smokers (46.82%). Various
Prevotella species can play different roles in health and disease [
79‐
81]. In a recent study of 161 healthy Italian participants, the salivary microbiome was classified into
Prevotella-dominant type,
Streptococcus/
Gemella-dominant type and
Neisseria/
Fusobacterium-dominant types [
82]. The microbial co-occurrence/exclusion pattern was explained by the microorganisms need to nutrients that can be provided by a selective group of bacteria [
83,
84]. Our study divulges the decrease in Co-occurring
Prevotella/
Porphyromonas shifts in healthy controls and increase in diseased cases. In another study
Prevotella histicola was shown to have a boosting effect of Copaxone, used to treat patients with multiple sclerosis [
85]. Most of the members of
Prevotella remain to be considered as commensals in healthy participants, which then turn to pathogens in oral infections and immunocompromised patients.
Prevotella is a beneficial microbe that is associated with plant-rich diet and the diverse
Prevotella species will have differences in responding to the diet and health status of hosts [
86]. More research is needed in order to look further deeper into
Prevotella’s potential and its interactions with its host and other bacteria for therapeutic use in clinical practice.
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