Inhibitory efficacy of various antibiotics on matrix and viable mass of Staphylococcus aureus and Pseudomonas aeruginosa biofilms

Both bacteria and the matrix are essential for the development of biofilms. Antimicrobials should therefore be tested against both components. The aim of this study was to determine the structure–activity relationships of different antibiotics against biofilm-forming Staphylococcus aureus and Pseudomonas aeruginosa strains using in vitro biofilm discriminatory assays. Only four of twelve antibiotics showed efficacy against S. aureus biofilms. Rifampicin had a 50% inhibitory activity both against the matrix and bacteria at 16× the minimum bactericidal concentration (MBC). Polymyxin B killed nearly all bacteria at 8× MBC, but left the matrix undisturbed. Both P. aeruginosa biofilms responded differently to antibiotic treatment. Rifampicin showed the greatest activity, with 100% killing of microorganisms combined with 91% destruction of the matrix at the MBC. In conclusion, rifampicin showed the highest activity on biofilm matrix and bacteria in S. aureus and P. aeruginosa biofilms. Our results also indicated that biofilm viable mass was more susceptible to treatment than the biofilm matrix, which is mainly responsible for biofilm persistence. Future research should specifically focus on compounds destroying the matrix that can be used as an adjunct to antibiotic therapy.