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Abstract
Biofilms of the opportunistic pathogen Pseudomonas aeruginosa are implicated in the pathology of numerous infections. These biofilms are resistant to multiple antibiotics, and several mechanisms have been proposed to explain this phenomenon. This study investigated whether the resistance of P. aeruginosa PAO1 biofilms to gentamicin and ciprofloxacin can be explained by the inability of these antibiotics to penetrate the extrapolymeric substances (EPS) layer of the biofilm. Disrupted and undisrupted biofilms were treated with antibiotic or Tryptic Soy Broth (TSB) media for four hours. Disrupted samples were prepared by subjecting biofilms to tissue homogenization prior to antibiotic treatment in order to model complete antibiotic penetration, whereas undisrupted samples were prepared by simply leaving biofilms intact. In addition, qualitative pellet size observations were made of undisrupted, treated biofilms. Results indicated that 1- and 5-day old disrupted biofilms had similar cell concentrations to their respective undisrupted counterparts. We also observed that gentamicin effectively reduced cell concentrations in 1- and 5-day old biofilms, while ciprofloxacin was only effective against 5-day old biofilms. Antibiotics appeared to reduce overall biofilm size. We conclude that resistance of P. aeruginosa biofilm cells to gentamicin and ciprofloxacin after four hours of incubation cannot be explained solely by an inability of the antibiotics to penetrate the EPS layer, and that other resistance mechanisms must also be contributing.