The gastrointestinal tract is highly colonized by different microorganisms. Within the mucous layer, the close contact enables biofilm formation. Also, the ability of Clostridium difficile to form biofilms hinders treatment with antibiotics [19].
Modern medicine incorporates biomaterial into the human body, either temporary as central venous catheters or permanently as a joint arthroplasty. These artificial surfaces are immediately coated by conditioning layers and predestined to be colonized by microbial biofilms. Gram-negative rods such as Pseudomonas sp. and Acinetobacter sp., and also oral bacteria such as streptococci and Neisseria sp. are frequently found in biofilms formed in endotracheal tubes [20]. Prosthetic joint infections are the main reason for periprosthetic joint infection; they are the most common reason for revision of total knee arthroplasties and the third most common reason for revision of total hip arthroplasties [21]. The infections occur up to 24 months after surgery as a possible contamination during surgery and thereafter as a hematogenous spreading of microorganisms from other parts of the body. Most identified microorganisms are Staphylococcus aureus and gram-negative bacilli [22].
Antibiofilm Therapy
Biofilm-associated infections are extremely difficult to treat. For example, in the case of an orthopedic infection a two-stage revision is made. The infected artificial joint is removed and intermediately replaced by a spacer made of PMMA (polymethylmethacrylate) bone cements highly loaded with antibiotics, which should prevent renewed biofilm formation (Fig. 2).
The biofilm matrix serves as a barrier not only to antimicrobial agents but also to immune cells. Even if phagocytes can penetrate they are ineffective, and the release of intracellular compounds increases the density and integrity of the biofilm matrix [3]. In biofilms, bacteria may become more virulent through the exchange of genes, including resistance genes [3]. Most importantly, there are subpopulations of dormant bacteria with metabolically low activity that are non-subdividing and, following resistance against antimicrobials, require metabolically active cells [3].
According to the increased awareness of biofilm-associated infections, different approaches are under investigation. For several years, attempts have been made to prevent or to retard biofilm formation on medical devices. Whilst taking the side effects into account, antibiotics are prophylactically used to kill planktonic bacteria before they are able to form biofilms [23]. Antiadhesive biomaterials are developed by modification of the surface with biosurfactants, cold plasma, or the incorporation of antimicrobial-acting agents which might be in part released into the environment; however, the effectiveness is generally very limited [23]. Promising approaches seem to be the inhibition of quorum sensing or interfering with matrix constituents. Inhibitors of quorum-sensing molecules have been developed, and inhibitors of acyl-homoserine lactones significantly reduced P. aeruginosa biofilm formation in vitro [24]. Other possibilities are the dispersion of the biofilm matrix and targets are alginate, extracellular DNA, and proteins which might be degraded by polysaccharides, DNases, or proteinases [24].
Oral Biofilms
As mentioned above, Costerton [1] was the first who defined the term “biofilm.” Among others, he described the formation of a glycocalyx by Streptococcus mutans and he published a graph of several oral bacteria in dental plaque [1], establishing that dental plaque is a biofilm. In PubMed, the term “dental plaque” gives the earliest articles dating from 1946. Interestingly, the very first one showed an association of dental plaque with both periodontal disease and caries in an animal model [25]. In the years thereafter the major focus was the association of dental plaque with caries. In 1965, Löe et al. [26] demonstrated that plaque formation induces gingivitis in man. Nowadays, it is well known that oral biofilms are formed not only on natural teeth but also on restorative materials, fixed and removable prosthetic constructions, dental implants, as well as to a certain extent on epithelium.
Dental Biofilm Formation
First, before bacteria attach to the tooth surface, a pellicle is formed. The adsorption of proteins to the enamel surface is selective, very fast saliva proteins (acidic proline-rich protein, cystatin, statherin, and protein S100-A9 proteins) attach, while serum proteins attach more slowly [27]. In the subgingival region, more serum-derived proteins are attached to the root surface [28], which can be seen in association with the different surface (cementum) or the flow of the gingival crevicular fluid.
Colonization of microorganisms in the oral cavity and in particular the biofilm formation on teeth depends on many factors, including age, diet, oral hygiene, and the immune response [29]. A few Gram-positive bacteria are able to adhere to pellicle-coated surfaces via specific adhesion-receptor interactions; other microorganisms subsequently attach to them and drive biofilm formation [29]. Analysis of early stages of dental biofilm formation by microarray analysis showed oral streptococci to be most prominent, together with Gemella haemolysans, Haemophilus parainfluenzae, Actinomyces sp., Rothia sp., Neisseria sp., Kingella oralis, Slackia exigua, and Veillonella sp. [30]. Fusobacterium sp. and Parvimonas micra were also present, whereas Porphyromonas gingivalis was not detected [30]. A low presence of Filifactor alocis and of the TM7 complex was linked with a potential role of these bacteria in developing periodontal disease, and the detection of S. mutans in about 25% of individuals can be correlated with potential caries development [30].
Already decades ago, attempts were made to characterize the microbial composition of mature dental biofilms in more detail. In 1993, Kolenbrander and London [31] published a scheme of organization of microorganisms in a subgingival biofilm. The later modified scheme shows that the first oral streptococci, among them S. oralis, S. sanguinis, and S. gordonii, and a few actinomyces (A. oris, A. naeslundii) adhere to the salivary pellicle-coated tooth surface. Then, several other Gram-positive rods and certain Gram-negative bacteria, such as F. nucleatum and Veillonella spp., can attach and provide receptors for late colonizers, including Aggregatibacter actinomycetemcomitans, Treponema denticola, and Tannerella forsythia [32]. A hallmark to describe networking in subgingival biofilm was the creation of different colored complexes by Socransky et al. [33]. By using a checkerboard technique allowing the determination of 40 different bacterial species, bacteria were grouped according to their joined presence, the red complex (P. gingivalis, T. forsythia, and