Microbial structures involved in the adherence to biomaterials
The ability of pathogens to adhere to cellular/acellular substrata is mediated by structures found on their surface called adhesins (adhesive structures). The adherence ability of microbial cells is considered a virulence factor (Zarnea, 1994; Lazar, 2003).
Gram-negative bacteria lipopolysaccharides (LPS) are composed of three regions: lipid A, region R (oligosaccharide), and a polysaccharide (O Antigen). LPS is a major component of the outer membrane of Gram-negative bacteria and act as endotoxins. LPS has an amphipathic structure with properties conferred by the polar hydrophilic groups and respectively, by the hydrophobic apolar groups.
The lipid is covalently bound to the oligosaccharide region R, being inserted and anchored by chains of fatty acids of the lipids of the outer membrane. LPS molecule is oriented so that the polysaccharide O projects to the outside and determines the antigenic specificity of the bacterial cell. LPS is able to bind cations due to the negative charge of lipid A, conferring the LPS a polyanionic character. The adjacent LPS molecules are apparently electrostatically related to each other by divalent cations (Ca2+, Mg2+) and form a compact structure similar to “roof tile” on the surface of the outer membrane (Chifiriuc et al., 2011).
LPS can form hydrogen bonds with different substrates. Therefore, it is estimated that 1000 hydrogen bonds can bind irreversibly each Gram-negative bacterial cell to the inert substrates (Hori and Matsumato, 2010).
Exopolymeric substances (EPS)—are components of the extracellular bacterial structures such as glicocalix, capsule, or mucosal lining. EPS include primary polysaccharides as well as other macromolecules (proteins, lipids, DNA, and humic substances), all of which form a polymer matrix. EPS matrix or polymer matrix serves to maintain the microorganisms in the biofilms and is responsible for adhesion to various substrates, through electrostatic forces and hydrogen bonds (Hori and Matsumato, 2010). The glicocalix consists of a network of filaments attached to the LPS polysaccharide from the outer membrane of Gram-negative bacteria or to the murein of the cell walls of Gram-positive bacteria. Polysaccharide filaments form a pericellular amorphous structure, which allows cell attachment to other cells or inert substrata. The pathogenic bacteria glicocalix mediates attachment of infectious cells to the mucosal surfaces. Glicocalix formed in bacteria from the natural environment can mediate the formation of polyspecific bacterial colonies giving rise to polybacterial associations. The name behavioral glicocalix comes from the ability of this structure to disappear by cultivating bacteria in artificial environments, and to return by transferring bacteria in natural unfavorable conditions. Glicocalix has been studied in the bacterial cells (Streptococcus mutans) of the dental plaque. The phenomenon of adhesion through glicocalix is of large interest for the oral microbiology, soil ecology, and corrosion research (Lazar, 2003; Chifiriuc et al., 2011). Capsule is an accessory structure, which completely covers the bacterial cell. Depending on the degree of devel- opment,the following capsular structures are identified: microcapsule (thickness up to 0.2 |im); macrocapsule (0.2 gm thick); mucous layer (thickness uneven, and erratic distribution around the cell); and zooglea (a polysaccharide mass, which is contained in a large number of bacterial cells, incorrect, as at present this type of structures are considered biofilms). Capsular material may be represented by homopolysaccharides (dextrans and levan), or heteropolysaccharides (formed of different monomers: glucose, fructose, galactose, mannose, galacturonic acid, aminated and acetylated derivatives thereof). The capsule is a material that exhibits a negative chemotactic effect for phagocytes, rendering encapsulated pathogenic bacteria more virulent (Lazar, 2003; Chifiriuc et al., 2011).
S protein layer is a primitive parietal structure present in all eubacteria, and most of the archea. Layer S is represented by two layers of a paracrystalline protein, and is considered a virulence factor of pathogenic bacteria, such as Helicobacter pylori, Chlamydia spp., Rickettsia sp., Bacillus sp., Clostridium spp., and Treponema spp., being involved in attachment to surfaces (Lazar, 2003).
Fimbriae are the best-studied bacterial adhesins. Fimbriae are filamentous appendages composed of molecular subunits called fimbrilins. The presence of these organelles on the bacterial surface (e.g., Escherichia coli, Klebsiella pneumoniae) represents an advantage in adherence and nutrition, mediating intercellular adhesion and bonding to the inert substrate (Chifiriuc et al., 2011; Lazar, 2003). Fimbriae are common in Gram-negative and rare in Gram-positive bacteria (e.g., Corynebacterium spp. and Actinomyces sp.).
Pap fimbriae/Pili are filamentous appendices composed of identical molecules of pilin (phosphoglycoprotein). Pili can be removed by mechanical agitation, but can be resynthesized. It appears that the type IV pili, mainly found in Gram-negative bacteria, including Pseudomonas aeruginosa, are involved in bacterial adhesion to the inert substrate with the formation of biofilms, and in the specific adherence to different host tissues (Hori and Matsumato, 2010).
Flagella are filamentous appendices, single or multiple, arranged on the surface of bacteria. Their role is very complex. Their presence on the pathogenic bacterial cells represents a virulence factor being involved in the adhesion to different substrates and mobility, as well as in immunogenicity (Lazar, 2003). The flagellum is composed of 11 types of protein molecules, the best known being the flagellin, which forms the external filament (Chifiriuc et al., 2011). The flagella mobility is associated to chemotaxis phenomenon, the bacteria moving to potential sources of nutrients or other favorable chemical stimuli or getting further from potentially harmful chemicals (Herlea, 1998).
“Curli” represent the third class of surface organelles (besides flagella and fimbriae), being composed of the protein called curlin and described in Escherichia coli, Shigella spp., Citrobacter spp., and Enterobacter sp. strains. These structures have the ability to mediate the adhesion of bacteria to the cell substrates, being involved in virulence (Lazar, 2003).