Oral Microbiome & Saliva

• Saliva contains components that interact with microorganisms to help control the composition of the oral microbial communities
• Proteins and glycoproteins are contained in saliva
• These adsorb to the tooth surface, forming acquired pellicle, which determines bacteria attachment
• Primary source of nutrients
• Inhibits growth and facilitates the clearance of exogenous microorganisms
• Antimicrobial factors are contained in saliva
• These control microbial colonization
• Chewing and saliva flow remove non-attached microorganisms
• Lysozyme is contained in saliva
• This is a protein that can lyse bacteria
• Saliva contains lactoferrin
• This is an iron-biding protein which sequesters iron, essential for microbial growth
• Gingival crevicular fluid is a serum-like fluid flowing through the junctional epithelium into the gingival sulcus
• Flow can remove non-attached microorganisms
• A high protein concentration in gingival crevicular fluid acts as a nutrient source
• Gingival crevicular fluid contains host-defence components that regulate microorganisms
• Some microorganisms require haem-containing molecules needed for growth contained in gingival crevicular fluid

Adherence

• Via specific interactions between adhesins on microorganism surface and receptors in the conditioning film
• Salivary molecules cover oral cavity surfaces which form a conditioning film- acquired pellicle
• Organisms must adhere to a conditioning film, or other microorganisms to colonize the oral cavity

Colonization

• The mouth is highly selective with the host promoting colonization and growth of certain bacteria
• Facilitated by inter-bacteria interactions like initial adherence, coaggregation, nutritional syntropy/symbiosis, antagonism, and communication
• Facilitated by heritable component, saliva and host defence

Factors Influencing Microbial Colonization of the Oral Cavity

• Non-microbial factors include host genetics, tooth eruption, antibiotics, orthodontics, dentures, diet, and smoking
• Microbial factors are metabolic activity providing change in local environment (pH, O2, redox potential), Provision of receptors for attachment and Provision of nutrients
• People tend to get oral microorganisms from their mother where mother and child share a similar microbial community
• No difference between adaptive or biological mother
• Genes influence the human oral microbiome; monozygotic twins have more similar oral microbiome, enzymes, saliva flow rate, mucins, proline rich proteins, and immune molecules

Early Influencers

• Birth Mode:
• Caesarean vs vaginal delivery
• Collects different microorganisms on the way out
• Difference dissipates quickly
• Early feeding:
• Breastmilk v formula result in different microorganisms at 2 months
• This difference disappears at 8 months as teeth erupt and diet changes
• The introduction of solid food changes the oral community
• Tooth eruption
• Pacifier usage
• Clean vs sterilised show no difference

Laboratory Techniques Used to Study Bacteria

• DNA sequencing
• Procedure: sequence bacterial DNA fragments and identify microorganisms by comparison of sequence obtained with sequence databases
• Pros: viable microorganism not required, detects uncultivable species, large amounts of information, assessment of biodiversity
• Cons: subject to bias and contamination, high error rate, computationally intensive
• DNA probes (microarrays, checkerboard hybridisation)
• Procedure: uses DNA probes complementary to particular microorganisms
• Pros: viable microorganisms not required, can detect multiple species
• Cons: only targeted microorganisms can be detected
• Bacterial culture
• Procedure: growth of bacteria on specific medium followed by characterisation
• Pros: can investigate physiological, biochemical properties, antimicrobial resistance
• Cons: must have viable microorganism, many oral species are fastidious and yet to be cultured