Bacteria embedded in a biofilm are

Bacteria embedded in a biofilm are surrounded by a self produced EPS matrix, which is a dynamic complex mixture consisting of rhamnolipids, exopolysaccharides, extracellular DNA (eDNA) and several proteins.42 P. aeruginosa produces three exopolysaccharides, Pel, Psl and Alginate.43 These polysaccharides effect the stability and architecture of the biofilm and have a high impact on adherence, cell-cell interaction mechanisms and virulence of the bacteria.42,44 The production and release of extracellular non-enzymatic proteins contributes considerably to biofilm formation of P. aeruginosa and leads to stability of the biofilm.42 Several adhesins such as flagellar FliD,45 type IV pili,46 CdrA,47 and the lectins LecA48,49 and LecB50,51 play an important role in host recognition, contribute to initial attachment and stabilize the EPS matrix by interfering with the polysaccharides. The resulting sticky EPS hydrogel can act as a physical barrier by lowering the diffusion of some antibiotics and host immune defense mechanisms.42 Furthermore, the same EPS matrix lowers the concentration of nutrients (e.g., oxygen or carbon source) within the biofilms.52 Due to this nutritional gradient bacteria in deeper layers of the biofilm reduce their metabolic activity. Antibiotics, e.g., ciprofloxacin, gentamycin or meropenem, which interfere with metabolic processes as replication, translation or cell wall synthesis, are therefore less active towards bacteria in a biofilm.53,54 In contrast, it has been reported that metabolically active P. aeruginosa in the upper layer of a biofilm can adapt to some antibiotics interfering with the membrane like colistin.55 Whereas this subpopulation is not affected, colistin can kill dormant bacteria in a biofilm.55 However, a combined antimicrobial treatment is not able to eradicate all bacteria and the