Mucin Influences Mechanics and Microspray Detachment of Streptococcus mutans Biofilms

Objectives: Acidogenic bacteria such as Streptococcus mutans in dental plaque biofilms are responsible for caries development. Mucin, a salivary protein, facilitates bacterial adhesion but its influence on biofilm mechanical properties is unknown. S. mutans UA159 biofilms were used to assess the influence of mucin on stiffness (elastic modulus, E) and water microspray induced detachment, Methods: Petri plates were conditioned with mucin containing medium (BHI +2% sucrose, +1% mucin, BHI+S+M) for 24 hours to form a Mucin Conditioning Layer (MCL). Biofilms were then grown on the MCL on BHI+S+M medium for 3 days. Control biofilms were grown on non-conditioned plates on mucin free media. Uniaxial compression tests were performed using a Bose ElectroForce®. A Philips Sonicare AirFloss was used to assess detachment strength by shooting a 130±0.03 µl (n=11) burst perpendicular to the biofilm from a 1 mm diameter nozzle at a distance of 1 mm, Results: Mucin caused the biofilms microcolonies to become denser and more heterogeneous, E was also significantly greater for the mucin grown biofilms than the control biofilms (509±121 Pa and 280±350 Pa respectively, P<0.05, n=5). High speed videos (8,000 fps) showed that the AirFloss had two distinct burst phases. The first was a 4.3 ms jet-like stream with a maximum velocity (u_max) of 22.3 m/s., the second was a 70 ms spray of water droplets (u_max=23.1 m/s). The diameter of the circular clearing zone created by the burst was statisticaly lower for the mucin grown biofilm than the control (2.0±0.3 cm and 2.6±0.5 cm  respectively, P<0.05, n=6). Micrcosopy showed some remaining cells in the cleared areas, Conclusion: Mucin plays an important role in biofilm cohesive strength, however, the AirFloss burst was suffecient to remove significant amounts of biofilm with or without mucin. Future work will quantify microspray induced biofilm detachment at different impingment angles and distances.