Antibiotic susceptibility of Staphylococcus aureus with different degrees of biofilm formation

Staphylococcus aureus is one of the most common pathogens in biofilm-associated chronic infections. S. aureus living within biofilms evades the host immune response and is more resistant to antibiotics than planktonic bacteria. In this study, we generated S. aureus with low and high levels of biofilm formation using the rbf (regulator of biofilm formation) gene and performed a BioTimer assay to determine the minimum inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) of various types of antibiotics. We showed that biofilm formation by S. aureus had a greater effect on MBC than MIC, probably due to the different growth modes between planktonic and biofilm bacteria. Importantly, we found that the MBC for biofilm S. aureus was much higher than that for planktonic cells, but there was little difference in MBC between low and high levels of biofilm formation. These results suggest that once the biofilm is formed, the bactericidal activity of antibiotics is significantly reduced, regardless of the degree of S. aureus biofilm formation. We propose that S. aureus strains with varying degrees of biofilm formation may be useful for evaluating the anti-biofilm activity of antimicrobial agents and understanding antibiotic resistance mechanisms by biofilm development.