P. aeruginosa persisters surviving in a biofilm treated with ofloxacin (Oflox). (A) Biofilms were formed on pegs of a Calgary Biofilm Device (14) and were then treated with a given concentration of antibiotic in Mueller-Hinton broth for 6 h, rinsed, and dislodged by sonication. Live cells were then counted by plating. The number of live cells recovered from a single peg is expressed as the number of CFU per peg. A strain that overexpressed the main MDR pump that extrudes fluoroquinolones (MexAB⁺⁺) and a strain that lacked the pump (34) were used in this experiment. The contribution of the pump to resistance is evident at low concentrations of the antibiotic but has little effect on the survival of persisters. (B) Planktonic cells were treated similarly with ofloxacin and plated for determination of the cell count. The apparent absence of persisters is due to the low density of the population and the detection limit of the experiment; at higher densities, persisters are evident at low levels in a planktonic population (A. Spoering and K. Lewis, unpublished data). Adopted from reference 12, with permission.

Model of biofilm resistance based on persister survival. An initial treatment with antibiotic kills planktonic cells and the majority of biofilm cells. The immune system kills planktonic persisters, but the biofilm persister cells are protected from host defenses by the exopolysaccharide matrix. After the antibiotic concentration drops, persisters resurrect the biofilm and the infection relapses (33).