Biofilms—formation, structure, and resistance
Formation and structure
Biofilm formation embraces the early, intermediate, and mature steps. In the first stage, planktonic cells move along the surface by using flagella or they can be passively transferred by body fluids. This surface recognition is followed by the microorganisms’ adhesion (reversible attachment) forming a monolayer of cells (Taraszkiewicz et al., 2013). This attachment can be facilitated by several factors such as increased shear forces, cell motility, and electrostatic interaction between the bacteria and the surface (Percival et al., 2015; Kostakioti et al., 2013). In the early step, bacteria are still susceptible to antibiotics. In the intermediate stage, bacteria bind irreversibly to the surface, multiply, and form microcolonies, which will be responsible for the production of the extracellular polymeric matrix. At this point, biofilms begin to be distinctly different from planktonic cells and become capable to resist external factors such as mechanical forces and antimicrobials (Suleman et al., 2014). In the last step of biofilm formation the amount of extracellular material increases with the incubation time. This structure can disrupt, and microorganisms can migrate to another surface and expand the infection. There are several factors that can potentiate the dispersal, such as changes in nutrients and oxygen availability and temperature (Percival et al., 2015; Kostakioti et al., 2013). Normally, the formation of a biofilm is regulated by quorum-sensing mechanisms and is, for instance, enhanced by nutrient deprivation, surface morphology and composition, and bacterial motility (Bose and Ghosh, 2011). The quorum-sensing mechanism comprises the production, release, and detection of chemical signaling molecules that allow the communication between microbial cells (Taraszkiewicz et al., 2013).