Biofilm is characterized by heterogenous environment and the presence of a variety of subpopulations. A biofilm structure is composed of metabolically active (both resistant and tolerant) and non-active cells (viable but not culturable cells, VBNC, and persisters) as well as polymer matrix consisting of polysaccharide, extracellular DNA and proteins. Biofilm growth is associated with an escalated level of mutations and horizontal gene transfer (HGT) which is promoted in due to the packed and dense structure. Bacteria in biofilms communicate by QS, which activates genes participating in virulence factors production (modified from Hall and Mah).

• Hall, C.W. and Mah, T.F. (2017) "Molecular mechanisms of biofilm-based antibiotic resistance and tolerance in pathogenic bacteria". FEMS Microbiol. Rev. 41: 276–301