The effect of extracellular polysaccharides from Streptococcus mutans on the bactericidal activity of human neutrophils

Extracellular polysaccharides (PS) synthesized by oral bacteria constitute one of their major virulence factors. The PS, synthesized from sucrose, fac ilitate adhesion and colonization by bacteria to tooth surfaces. The study was designed to test the effect of in situ production of extracellular PS b y Streptococcus mutans on the bactericidal activity of human neutrophils. T hese effects were tested on bacteria pre-exposed to sucrose (PS-positive St rep. mutans) and compared to bacteria not exposed to sucrose (PS-negative S trep. mutans). The interactions between neutrophils and Strep. mutans were tested in suspension and on bacteria in an experimental model of dental pla que. Viability of Strep. mutans was measured by [H-3]-thymidine incorporati on into the bacteria. Degranulation of neutrophils was evaluated by the rel ease of lysozyme, and the production of reactive oxygen products was measur ed by chemiluminescence. When neutrophils were incubated with suspended bac teria, the viability of PS-negative Strep. mutans was 20% of that of bacter ia not incubated with neutrophils (control), while the viability of PS-posi tive Strep, mutans was 40% of the control, In the experimental dental-plaqu e model, 50% of the PS-negative Strep. mutans were killed by neutrophils wh ile the viability of PS-positive Strep. mutans was not different than of th e control. Degranulation of neutrophils was not affected by the presence of extracellular PS of Strep. mutans. Artificial stimulation of neutrophils w ith phorbol myristate acetate also did not enhance the bactericidal effect of neutrophils on PS-positive Strep. mutans. However, PS-positive Strep. mu tans elicited oxygen-reactive products from neutrophils, 2-fold less than w ith PS-negative Strep. mutans. The results indicate that in situ production of bacterial extracellular polysaccharides might be a major virulence fact or of Strep. mutans, enabling PS-positive Strep. mutans in the dental-plaqu e biofilm to evade killing by human neutrophils. (C) 1999 Elsevier Science Ltd. All rights reserved.