Читать книгу Oral Biofilms - Группа авторов - Страница 16

Mature biofilms have a complex structure, consisting of a polysaccharide matrix and fluid-filled channels [2]. A fully hydrated biofilm is composed of about 15% microbial cells and 85% matrix material [3]. The matrix is of importance as it provides stability and protects against environmental insults; it consists of polysaccharides, water, lipids, proteins, and considerable amounts of extracellular DNA [4]. The exopolysaccharides mask bacterial ligands and decrease the ability of phagocytes to recognize and to phagocytose bacteria in a biofilm [10]. Microorganisms synthesize different exopolysaccharides. For example, for Pseudomonas aeruginosa three different polysaccharides have been described, an alginate being a mucoid negatively charged polymer, a glucose-rich polysaccharide being of importance at the liquid-air interface, and a pentasaccharide acting as scaffold for other bacterial cells [10]. The proteins of the matrix originate from bacterial outer membrane vesicles and from cytoplasmic proteins; after being recycled, they polymerize into amyloid-like structures like curli fibers and flagella [5]. Proteins may also form a hydrophobic layer around a mature biofilm [5] which protects the biofilm against environmental insults. The extracellular DNA in the biofilm matrix originates from lysed bacterial cells or is actively released from membrane vesicles at bacterial cell surfaces; interestingly, not all bacterial DNases are able to degrade it [11]. Its functions in a biofilm is discussed as a nutrient reservoir, the dissemination of genes between microorganisms, and a strengthening of the biofilm structure [11].

Fig. 2. Specimens of PMMA loaded with gentamicin and contaminated in vitro with an S. aureus strain for 2 days (photograph: Center for Electron Microscopy, University Hospital of Jena, Jena, Germany).