SCIENCE
Oral microbiome - what we knew and what we now know What is the oral microbiome? What are probiotics? Understanding these complex systems may help us better prevent and treat oral diseases.
António Coimbra Amaral, Portugal
T
he oral cavity is home to hundreds of species of bacteria, it provides nutrients for numerous microorganisms. Oral microbiome is a term that describes a complex structure of bacteria, fungi, viruses, and protozoa that colonise hard and soft tissues of the oral cavity, making the oral cavity their habitat (Benn et al., 2018). In each of these structures, the microbes grow in a community-like form, biofilm, an ultra-organised matrix that adheres to the various surfaces of the mouth. Over time, our understanding of this complex network of microorganisms evolved, largely due to the advance in the methods of identification.
Identification solutions
The culture test was the first method for the identification of microorganisms. In this method a sample is collected and incubated in a specific culture medium at a certain temperature, which allows the microorganisms to multiply facilitating their identification. Although it was the pillar for other methods to be invented, the culture test has its limitation - it is not possible to cultivate all the species present in the sample (Benn et al., 2018).
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These limitations and the discovery of DNA’s double helical-structure became starting points for the development of nucleic acid-based technologies, followed by the unimaginable discoveries of biodiversity. Furthermore, the identification of the 16S rRNA gene, a conserved DNA sequence paved the way for a wider identification and categorisation of bacteria. The most used methods in this field are, DNA hybridisation, polymerase chain reaction (PCR) and DNA sequencing (Krishnan et al., 2016).
Health and disease
A healthy oral cavity is colonised by the microbiota that live in symbiosis with the host, providing a defence mechanism against pathogens. The oral microbiome changes during the lifetime of an individual (Wade, 2013). The first species colonising the oral cavity are the Streptococcus spp. S. salivarius is mostly found in saliva and on the tongue, S. mitis on the buccal mucosa and S. sanguinis on the teeth. Environmental changes caused by streptococcal colonisation (i.e., changes in the pH, redox potential, coaggregation and nutrient availability) make the host susceptible to further colonisa-
tion. Fusobacterium nucleatum, Veillonella, Neisseria and Prevotella spp. are the species that follow Streptococcus spp. (Belibasakis et al., 2019). When the first tooth erupts, the oral cavity is further colonised by Prevotella denticola, Leptotrichia, Campylobacter, Fusobacterium and Selenomonas spp. With the eruption of teeth, gingival crevicular fluid (GCF) and saliva play a major role in the integrity of gingival sulcus - the GCF contains active proteins, immunoglobulins and antimicrobial peptides that have the capacity to modulate the oral microbiome and maintain bacterial homeostasis (Krishnan et al., 2017). Disease is associated with species that are present in low abundance and have the ability to disrupt the symbiosis pattern and convert the microbiota associated with health to a state of disease. The most common diseases caused by oral dysbiosis are dental caries, periodontal disease, and some systemic diseases. The most common microorganisms associated with dental caries are Streptococcus mutans and lactobacilli. Additionally, it is now known that other species, such as Bifidobacterium, Propionibacterium, and Scardovia spp., can
