Antimicrobial Defense Systems in Saliva

Abstract

The oral cavity is one of the most heavily colonized parts of our body. The warm, nutrient-rich and moist environment promotes the growth of a diverse microflora. One of the factors responsible for the ecological equilibrium in the mouth is saliva, which in several ways affects the colonization and growth of bacteria. In this paper, we discuss the various mechanisms by which the composition of the oral microflora is modulated by saliva. Saliva covers the oral hard and soft tissues with a conditioning film which governs the initial attachment of microorganisms, a crucial step in the setup of the oral microflora. It furthermore contains proteins which in the soluble phase bind to bacteria, blocking their adherence to surfaces. When the supply of nutrients is diminished, bacteria use salivary glycoproteins, especially high-molecular-weight mucins, as a source of complex carbohydrates, requiring a consortium of microorganisms for breakdown. In this way saliva promotes the complexity of the oral microflora, which in itself protects against overgrowth by few pathogenic species. Finally, saliva harbors a large panel of antimicrobial proteins which directly and indirectly inhibit uncontrolled outgrowth of bacteria. These include lactoferrin, lactoperoxidase, lysozyme and antimicrobial peptides. Under pathological conditions serum leakage occurs, and saliva mobilizes the humoral and cellular defense mechanisms in the blood. In sum, saliva favors the establishment of a highly diverse microflora, rather than a semisterile environment.

Glycan recognition at the saliva – oral microbiome interface

“…These include most prominently lysozyme, lactoferrin, salivary peroxidase, histatins, cystatins, and defensins. These important components of saliva are reviewed elsewhere [37–40]. The antimicrobial activity of salivary proteins may even extend beyond the oropharynx into the proximal digestive tract, there exerting specific antimicrobial activity against enteric pathogens [41].…”

The microbiome–systemic diseases connection

“…Saliva is one of the major factors responsible for microbial homoeostasis in the oral cavity which is illustrated by the fact that reduced salivary secretion in patients with SS leads to an increase in microbial‐related diseases, such as dental caries and oral candidiasis (Arendorf and Walker, ; Christensen et al , ). Homoeostasis of the oral microbiome is maintained by saliva through modulation of bacterial attachment, modulation of bacterial growth and inhibition of bacterial growth (van't Hof et al , ). Salivary agglutinin and SIgA both bind to bacteria and prevent bacterial attachment to the oral mucosa.…”

Quorum sensing of Streptococcus mutans is activated by Aggregatibacter actinomycetemcomitans and by the periodontal microbiome

“…Mucins are glycoproteins contributing to the antimicrobial activity of saliva [47], to which we can now add their ability to inhibit the QS of S. mutans . This might be caused by blockage of the Opp permease [39] that can then not import XIP anymore.…”