Clearance of mixed biofilms of Streptococcus pneumoniae and methicillin-susceptible/resistant Staphylococcus aureus by antioxidants N -acetyl- l -cysteine and cysteamine

Biofilm-associated infections are of great concern because they are associated with antibiotic resistance and immune evasion. Co-colonization by Staphylococcus aureus and Streptococcus pneumoniae is possible and a threat in clinical practice. We investigated the interaction between S. aureus and S. pneumoniae in mixed biofilms and tested new antibiofilm therapies with antioxidants N -acetyl- l -cysteine (NAC) and cysteamine (Cys). We developed two in vitro S. aureus – S. pneumoniae mixed biofilms in 96-well polystyrene microtiter plates and we treated in vitro biofilms with Cys and NAC analyzing their effect by CV staining and viable plate counting. S. pneumoniae needed a higher proportion of cells in the inoculum and planktonic culture to reach a similar population rate in the mixed biofilm. We demonstrated the effect of Cys in preventing S. aureus biofilms and S. aureus – S. pneumoniae mixed biofilms. Moreover, administration of 5 mg/ml of NAC nearly eradicated the S. pneumoniae population and killed nearly 94% of MSSA cells and 99% of MRSA cells in the mixed biofilms. The methicillin resistance background did not change the antioxidants effect in S. aureus . These results identify NAC and Cys as promising repurposed drug candidates for the prevention and treatment of mixed biofilms by S. pneumoniae and S. aureus .