Microbial interactions in building of communities

Abstract

Establishment of a community is considered to be essential for microbial growth and survival in the human oral cavity. Biofilm communities have increased resilience to physical forces, antimicrobial agents and nutritional variations. Specific cell-to-cell adherence processes, mediated by adhesin-receptor pairings on respective microbial surfaces, are able to direct community development. These interactions co-localize species in mutually beneficial relationships, such as streptococci, veillonellae, Porphyromonas gingivalis and Candida albicans. In transition from the planktonic mode of growth to a biofilm community, microorganisms undergo major transcriptional and proteomic changes. These occur in response to sensing of diffusible signals, such as autoinducer molecules, and to contact with host tissues or other microbial cells. Underpinning many of these processes are intracellular phosphorylation events that regulate a large number of microbial interactions relevant to community formation and development.

Fig. 1

Bacterial cell-to-cell interactions in the dental plaque biofilm. One example of mutualism is displayed between P. gingivalis and T. denticola. Each releases a metabolic factor that the other can utilize, thus enhancing the growth of both species. Lines linking the microorganisms represent adhesive interactions. Aa = Aggregatibacter actinomycetemcomitans

Fig. 2

The interaction of P. gingivalis with S. gordonii depends upon two sets of adhesin-receptor pairs. The major and minor fimbriae of P. gingivalis bind glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and SspA/B on the surface of S. gordonii, respectively. The domains of SspA/B that interact with the minor fimbriae subunit, Mfa1, are highlighted in the yellow ovals. These domains reside within the SspA/B adherence region (BAR) and are required to maintain the contacts between the two species in the oral cavity.

Fig. 3

Phosphorylation events within a bacterial cell are complex and dynamic. The two-component system in A. actinomycetemcomitans is induced by AI-2, with the loss of this TCS resulting in diminished biofilm formation. Tyrosine kinases in Gram-negative bacteria such as Wzc in E. coli have been shown to regulate a variety of targets including DNA-binding proteins, capsule synthesis genes and the heat shock response. The phosphatase Ltp1 in P. gingivalis has been shown to affect a number of cellular activities including dual-species community formation, reduced EPS synthesis and hemin uptake.

Fig. 4

Communication circuits between C. albicans and oral streptococci. The diagram depicts self- or non-self control of the yeast to hypha transition, with bacterial products variously impacting on morphogenesis and biofilm formation, and C. albicans products positively or negatively influencing bacterial growth or biofilm formation (see text for discussion).