Biofilms: the matrix revisited

Beneficial biofilm formation by industrial bacteria Bacillus subtilis and related species

Biofilms are densely packed multicellular communities of microorganisms attached to a surface or interface. Bacteria seem to initiate biofilm formation in response to specific environmental cues, such as nutrient and oxygen availability. Biofilms undergo dynamic changes during their transition from free-living organisms to sessile biofilm cells, including the specific production of secondary metabolites and a significant increase in the resistivity to biological, chemical, and physical assaults. Bacillus subtilis is an industrially important bacterium exhibiting developmental stages. It forms rough biofilms at the air-liquid interface rather than on the surface of a solid phase in a liquid, due to the aerotaxis of the cells. Biofilm formation by B. subtilis and related species permits the control of infection caused by plant pathogens, the reduction of mild steel corrosion, and the exploration of novel compounds. Although it is obviously important to control harmful biofilm formation, the exploitation of beneficial biofilms formed by such industrial bacteria may lead to a new biotechnology.

Defining the genetic differences between wild and domestic strains of Bacillus subtilis that affect poly-gamma-dl-glutamic acid production and biofilm formation

Biofilms are communities of microbial cells that are encased in a self-produced, polymeric matrix and are adherent to a surface. For several species of bacteria, an enhanced ability to form biofilms has been linked with an increased capability to produce exopolymers. To identify exopolymers of Bacillus subtilis that can contribute to biofilm formation, we transferred the genetic determinants that control exopolymer production from a wild, exopolymer-positive strain to a domesticated, exopolymer-negative strain. Mapping these genetic determinants led to the identification of gamma-poly-dl-glutamic acid (gamma-PGA) as an exopolymer that increases biofilm formation, possibly through enhancing cell-surface interactions. Production of gamma-PGA by Bacillus subtilis was known to be dependent on the two-component regulator ComPA; this study highlighted the additional dependence on the DegS-DegU, DegQ and SwrA regulator proteins. The inability of the domestic strain of B. subtilis to produce gamma-PGA was mapped to two base pairs; a single base pair change in the promoter region of degQ and a single base pair insertion in the coding region of swrA. Introduction of alleles of degQ and swrA from the wild strain into the domestic strain was sufficient to allow gamma-PGA production. In addition to controlling gamma-PGA production, ComPA and DegSU were also shown to activate biofilm formation through an as yet undefined pathway. The identification of these regulators as affecting gamma-PGA production and biofilm formation suggests that these processes are regulated by osmolarity, high cell density and phase variation.

High-resolution visualization of Pseudomonas aeruginosa PAO1 biofilms by freeze-substitution transmission electron microscopy

High-pressure freeze-substitution and transmission electron microscopy have been used for high-resolution imaging of the natural structure of a gram-negative biofilm. Unlike more conventional embedding techniques, this method confirms many of the observations seen by confocal microscopy but with finer structural detail. It further reveals that there is a structural complexity to biofilms at both the cellular and extracellular matrix levels that has not been seen before. Different domains of healthy and lysed cells exist randomly dispersed within a single biofilm as well as different structural organizations of exopolymers. Particulate matter is suspended within this network of fibers and appears to be an integral part of the exopolymeric substance (EPS). O-side chains extending from the outer membrane are integrated into EPS polymers so as to form a continuum. Together, the results support the concept of physical microenvironments within biofilms and show a complexity that was hitherto unknown.

Dental plaque: biological significance of a biofilm and community life-style

BACKGROUND

Most microorganisms in nature attach to surfaces and form matrix-embedded biofilms. Biofilms are highly structured and spatially organized, and are often composed of consortia of interacting microorganisms, termed microbial communities, the properties of which are more than the sum of the component species. Microbial gene expression alters markedly in biofilms; organisms communicate by gene transfer and by secretion of diffusible signalling molecules. Cells in biofilms are less susceptible to antimicrobial agents.

AIM AND MATERIALS & METHODS

To comprehensively review the literature to determine whether dental plaque displays properties consistent with those of a typical biofilm and microbial community.

RESULTS

Novel microscopic and molecular techniques have demonstrated that plaque has a structured architecture with an extracellular matrix, and a diverse composition (around 50% of cells are unculturable). The constituent species communicate by gene transfer, by secreted peptides (gram-positive bacteria) and autoinducer-2 (gram-positive and gram-negative bacteria). These organisms are functionally organized for increased metabolic efficiency, greater resistance to stress and for enhanced virulence. Plaque formation has direct and indirect effects on gene expression.

CONCLUSION

Dental plaque displays properties that are typical of biofilms and microbial communities in general, a clinical consequence of which is a reduced susceptibility to antimicrobial agents as well as pathogenic synergism.