The biofilm contains both metabolically active regions (brown) and nutrient-limited or starved regions (red).  If starved cells are less susceptible to the antimicrobial agent being used, then at least part of the biofilm will survive treatment.

The biofilm contains cells that resemble those grown in conventional suspension cultures (brown), and it contains cells that adopt a phenotype that is distinct to the biofilm state (red).  It is possible that cells in the biofilm phenotype are inherently less susceptible to antimicrobial agents.