Caister Academic Press


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A review of scientific research into Biofilms.


Introduction to Biofilms

Adapted from An et al. in Environmental Molecular Microbiology
Many bacteria can grow and live as biofilms, in which single microbial cells individually interconnect with each other through an extracellular matrix. Biofilm-forming bacteria pose severe problems in the environment, industry and health care sector due to increased bacterial survival competence in the environment and the protective nature of biofilms that prevent effective eradication.

Technological progress in microscopy, molecular genetics and genome analysis has significantly advanced our understanding of the structural and molecular aspects of biofilms, especially of extensively studied model organisms such as Pseudomonas aeruginosa. Biofilm development can be divided into several key steps including attachment, microcolony formation, biofilm maturation and dispersion; and in each step bacteria may recruit different components and molecules including flagella, type IV pili, DNA and exopolysaccharides. The rapid progress in biofilm research has also unveiled several genetic regulation mechanisms implicated in biofilm regulation such as quorum sensing and the novel secondary messenger cyclic-di-GMP. Understanding the molecular mechanisms of biofilm formation has facilitated the exploration of novel strategies to control bacterial biofilms.

Gonococcal Biofilms

Adapted from Apicella et al. in Neisseria: Molecular Mechanisms of Pathogenesis
Neisseria gonorrhoeae is an exclusive human pathogen. Our studies have demonstrated that it utilizes two distinct mechanisms for entry into human urethral and cervical epithelial cells involving different bacterial surface ligands and host receptors. Recent studies have demonstrated that the gonococcus can form biofilms on glass surfaces and over human cells. There is evidence for formation of gonococcal biofilms on human cervical epithelial cells during natural disease and further evidence that outer membrane blebbing by the gonococcus is crucial in biofilm formation over human cervical epithelial cells.

Dental Plaque

Adapted from Russell in Bacterial Polysaccharides: Current Innovations and Future Trends
Dental plaque is a complex biofilm containing several hundred different species of bacteria. While these are normally harmless commensals, shifts in the population structure can lead to the plaque-related diseases such as dental caries and periodontal disease. Surface polysaccharides are important in coaggregation reactions that bind particular species together and aid colonisation and metabolic interaction while sucrose-derived glucans and fructans have a significant effect on plaque properties. Fructans serve as an extracellular energy store while glucans contribute to adhesion, modify the permeability of plaque and alter ion-binding capacity and thus have a powerful influence on the creation of conditions in plaque that can lead to dental caries.

Oral Microbial Communities

Adapted from Rickard et al. in Molecular Oral Microbiology
Most of the bacterial species found in the mouth belong to microbial communities, called "biofilms", a feature of which is inter-bacterial communication, mediated by two distinct phenomena. The first is through direct cell-cell contact, which is mediated by specific protein "adhesins" and often, as in the case of inter-species aggregation, by complementary polysaccharide receptors.Intra (auto-) and inter-species (co-) aggregation both promote ordered successional integration of species into the biofilm.The second method of communication invoves cell-cell signalling molecules, which are of two classes; those used for intra-species and those used for inter-species signalling. An example of the former is "quorum sensing", a process in which acyl-homoserine lactones (AHLs) induce members of the same population to produce and release specific enzymes or to initiate biofilm formation. As yet, no oral bacteria have been shown to produce AHLs. However, they can produce small peptides, such as "competence stimulating peptides", which have been shown to mediate intra-species signalling and to promote single-species biofilm formation. A common form of inter-species signalling is mediated by 4, 5-dihydroxy-2, 3-pentanedione (DPD). This spontaneously forms a family of inter-convertible compounds, collectively called "Autoinducer-2" (Al-2). With respect to oral biofilm communities, the present review will focus on the molecular basis of communication and the effects of cell-cell contact and signal molecules on gene expression. A model relating inter-species cell-cell communication and biofilm development is proposed.

Gram-positive Biofilm Infections

Adapted from Theilacker et al. in Bacterial Polysaccharides: Current Innovations and Future Trends
Biofilms seem to be the default mode of growth for most if not all bacterial species and this phenomenon has profound consequences in numerous clinical settings. For Gram-positive bacteria several mechanisms involving surface proteins and carbohydrate-containing structures have been identified. Poly-N-acetylglucosamine (PIA/PNAG) has been first described in staphylococci but recently has been shown to be present also in a large number of different bacterial species. Mutants in the gene locus responsible for the synthesis of this molecule lead to a biofilm-negative phenotype. Teichoic acids are polyanionic molecules found in the cell walls of Gram-positive bacteria with low GC content. Teichoic acids are either attached covalently to the peptidoglycan (wall teichoic acids) or inserted into the cell membrane (lipoteichoic acids) and both types have been associated with biofilm formation and adhesion to eukaryotic cells. Recent evidence indicates that extracellular DNA may also be involved in biofilm formation and primary attachment of many Gram-positve organisms and this fact may explain previous observations that autolysin-defective mutants are also impaired in biofilm formation. A more detailed understanding of the molecules involved in biofilm formation is needed to device novel treatment and preventive approaches for Gram-positive infections.

Biofilms in Pasteurellaceae

Adapted from Inzana et al. in Pasteurellaceae: Biology, Genomics and Molecular Aspects
The endotoxin of the Pasteurellaceae is an essential component of the bacterial cell surface, as it is for all gram-negative bacteria. However, the Pasteurellaceae are unusual in that some members make an endotoxin that is a lipopolysaccharide (LPS) while others make a lipooligosaccharide (LOS). The LOS can be a relatively simple antigen, or a deceptively complex molecule that can be decorated with a variety of components that influence host response and interactions, contain structures that mimic host antigens, and phase vary in the expression of these antigenic epitopes. Biofilms are produced by many members of this group, and these surface-attached biofilm communities may promote bacterial persistence in vivo, even in the face of immune effectors and antimicrobial treatment.

Biofilm Formation by Vibrio cholerae

Adapted from Yildiz et al. in Vibrio cholerae: Genomics and Molecular Biology
In nature, most bacteria grow as matrix-enclosed, surface-associated communities known as biofilms. Vibrio cholerae, the causative agent of the disease cholera, forms biofilms on diverse surfaces. This ability to form biofilms appears to be critical for the environmental survival and the transmission of V. cholerae. The molecular mechanisms utilized by V. cholerae to form and maintain biofilms are being investigated through a combination of molecular genetic and microscopic approaches. A better understanding of the life cycle of this important human pathogen should prove useful in the development of future strategies for predicting and controlling cholera epidemics.

Further reading