# Treatment of Biofilm Communities: An Update on New Tools from the Nanosized World
## 3 Biofilm Properties  
The highly heterogeneous nature of EPS provides bacterial communities with many characteristics that influence their behavior and allows the formation of a peculiar microenvironment in which they can better survive and cooperate, as shown in Figure 2.  
Figure 2: Biofilm microenvironmental features ( With permission, from [14]). The presence of an extracellular matrix is first a physical barrier to external stresses. As previously said, the main component of EPS is water retained by the hydrophilic molecules of the matrix, thus allowing survival also upon desiccation. In some instances, one further layer of dehydration prevention is provided by the formation of an external skin that forms a physical barrier to water evaporation [49]. Furthermore, the \"sticky\" nature of the matrix allows non-selective sorption of substances, that may be nutrients, ions or also toxic compounds, as documented in environmental samples of biofilms [50; 51; 52]. Retained compounds can be digested by enzymes secreted by bacteria embedded into the biofilm as described above, thus rendering nutrients available for the entire community. Further nutrients are provided by died bacterial cells that can be \"cannibalized\" as documented in _Bacillus subtilis_ biofilms [53]. These sorption and retention capacities allows the growth of biofilm in environments that can be highly oligotrophic, which is typical of most natural environments. Probably the most medically relevant feature that is connected to biofilm is its intrinsic tolerance that shows to any treatment. Traditionally, planktonic bacteria become resistant to a determined antimicrobial agent thanks to genetic elements that allow them to escape the mechanism blocked by the antimicrobial. By contrast, it is known that bacterial cells within biofilm are not genetically resistant to antimicrobials but always show a high survival rate upon treatment [54]. This phenomenon can be referred to as tolerance (according to [PERSON] and coauthors [16]). It has been shown that silver nanoparticles (AgNPs) inhibited to a lower extent biofilm viability compared to the planktonic counterpart. Once suspended, biofilm-embedded _Pseudomonas aeruginosa_ cells retained their sensitivity to AgNPs, comparable to the planktonic culture [55]. Thus, an explanation of this effect may be the physical barrier formed by EPS, which can chelate and modify antimicrobial molecules, owing to the presence of secreted enzymes mentioned above. Another reason of the intrinsic tolerance of bacterial cells once embedded in EPS matrix is their metabolic state. EPS causes the formation of different gradients inside the biofilm community (nutrient, oxygen, pH) [14; 16] and this leads bacteria to respond differently based on the condition they find. Low nutrients and oxygen paucity may lead to a dormant state, in which these so-called persister cells are still viable but not actively proliferating, being therefore non-susceptible to most antimicrobial treatments [56; 57]. This way, at least two mechanisms contribute to biofilm survival upon the presence of antimicrobials. Furthermore, the close proximity in which bacteria are held thanks to the biofilm matrix enables a \"social\" behavior of the community. This phenomenon can be explained in multiple ways: the primary way of communication inside the biofilm is thought to be quorum sensing (QS). This communication system is based on chemical signals that are secreted by bacterial population. The concentration of the signal increases as the population grows, reaching the _quorum_ that indicates that the population is conspicuous enough to trigger changes in gene expression patterns [58; 59]. QS is studied through the use of batch cultures in the laboratory, allowing high concentration of bacterial cells in a relatively small volume. By contrast, the role of QS in nature is questionable since bacteria rarely reach such high concentrations in close proximity to allow diffusion and sensing of the chemical signals.