Future therapies for the treatment of dental decay have to consider the importance of preserving bacterial ecology while reducing biofilm adherence to teeth. A multi-species plaque-derived (MSPD) biofilm model was used to assess how concentrations of N-acetyl-l-cysteine (NAC) (0, 01, 1, 10%) affected the growth of complex oral biofilms. Biofilms were grown (n = 96) for 24 h on hydroxyapatite discs in BMM media with 05% sucrose. Bacterial viability and biomass formation was examined on each disc using a microtitre plate reader. In addition, fluorescence microscopy and Scanning Electron Microscopy was used to qualitatively examine the effect of NAC on bacterial biofilm aggregation, extracellular components and bacterial morphology. The total biomass was significantly decreased after exposure of both 1% (from 048, with a 95% confidence interval of (044, 057) to 035, with confidence interval (031, 038)) and 10% NAC (014 with confidence interval (011, 017)). 16S rRNA amplicon sequencing analysis indicated that 1% NAC reduced biofilm adherence while preserving biofilm ecology.