Dental plaque biofilms: communities, conflict and control

Periodontology 2000, Vol. 55, 2011, 16–35 Printed in Singapore. All rights reserved

 2011 John Wiley & Sons A/S

PERIODONTOLOGY 2000

Dental plaque biofilms: communities, conflict and control P H I L I P D. M A R S H , A N N E T T E M O T E R & D E I R D R E A. D E V I N E

From the very beginning of the discipline of microbiology, the dogma has been to isolate bacteria in pure culture in order to be able to define their individual properties. This process also involved the use of conventional broth (planktonic) culture to prepare biomass and to determine the phenotype of particular species. This approach provided a sound foundation for contemporary investigations of classical infectious diseases. Recently, however, there has been a renaissance in our understanding of microbial behaviour in natural habitats, and a recognition that chronic diseases can have a complex aetiology. It is now accepted that, in nature, bacteria exist for the most part attached to a surface as a biofilm, often as a member of a polymicrobial community (or consortium) of interacting species. If biofilms were merely planktonic-like cells that had adhered to a surface and the properties of a multi-species microbial community were just the sum of the constituent populations, then the scientific and clinical imperative for their study would be low. However, application of novel imaging (confocal or epifluorescence microscopy, fluorescence in situ hybridization, live ⁄ dead stains, etc.) and molecular techniques (16S rRNA gene amplification and sequence comparison, proteomics, transcriptomics, reporter gene technology, etc.) has radically altered our understanding of the biology of multi-species biofilms (Table 1), and key developments that are pertinent to the control of dental plaque are highlighted in this review. Another major shift in our understanding of microbial behaviour has come from our increased knowledge of microbial ecology (3), and recognition of the intimate relationship between the resident human microflora and the host. Changes in the host environment have a direct impact on gene expression, and thereby influence the metabolic activity,

16

competitiveness and composition of the microflora, while the action of resident microorganisms can have consequences for the host. An appreciation of this dynamic relationship is critical to fully understand the relationship between the oral microflora and the host in health or disease.

The mouth as a microbial habitat The human body is estimated to be composed of more than 1014 cells, of which only 10% are mammalian (125, 161). The majority are the microorganisms that make up the resident microflora found on all environmentally exposed surfaces of the body, and this Ôhuman microbiomeÕ is reported to have a metabolic capacity equivalent to that of the human liver. The microflora of the skin, mouth, digestive and reproductive tracts, etc. are distinctive because of the characteristic biological and physical properties of each site (161), despite the potential movement of microorganisms between sites. This observation illustrates a key concept; namely, that the properties of the habitat are selective and dictate which organisms are able to colonize, grow and be minor or major members of the community. The mouth is similar to other habitats within the body in having a characteristic microbial community that provides benefits for the host. The mouth is warm and moist, and is able to support the growth of a distinctive collection of microorganisms (viruses, mycoplasma, bacteria, Archaea, fungi and protozoa) (90). Bacteria are the most numerous group and initially were characterized using cultural approaches. Over time, it became clear that there was a discrepancy between the number of bacteria in a sample that could be grown by these conventional

Plaque biofilms and communities

Table 1. Properties of biofilms and microbial communities (adapted from Ref. 90) General property

Dental plaque example

Open architecture

Presence of channels and voids

Microbial protection

Production of extracellular polymers to form a functional matrix; physical protection from phagocytosis

Host protection Enhanced tolerance to antimicrobials* Neutralization of inhibitors

Colonization; resistance Reduced sensitivity to chlorhexidine and antibiotics; gene transfer b-lactamase production by neighbouring cells to protect sensitive organisms

Novel gene expression*

Synthesis of novel proteins on attachment or on binding to host molecules; upregulation of gtfBC in mature biofilms

Coordinated gene responses

Production of bacterial cell-to-cell signalling molecules (e.g. CSP, AI-2)

Communication with host

Downregulation of pro-inflammatory responses by resident oral bacteria; remodelling of the cytoskeleton of epithelial cells

Spatial and environmental heterogeneity Broader habitat range More efficient metabolism Enhanced virulence

pH and O2 gradients; co-adhesion Obligate anaerobes in an overtly aerobic environment Complete catabolism of complex host macromolecules (e.g. mucins) by microbial consortia (food chains and food webs) Pathogenic synergism in periodontal diseases

*One consequence of altered gene expression may be increased tolerance to antimicrobial agents.

approaches and those that were observed directly by microscopy (27, 110). It is estimated that