Acid tolerance in streptococcal biofilm cells

Biofilm cells of Streptococcus mutans differ from their planktonic counterparts in many ways with, for instance, adhesion to a surface leading to increased acid tolerance. Acid tolerance in plaque bacteria is thought to be a pre-requisite for the development of caries. The presence of oral streptococci such as Streptococcus gordonii, Streptococcus sanguinis and Streptococcus oralis in plaque is associated with health but these species are also capable of fermenting carbohydrates to metabolic acids thus lowering plaque pH.

Objective: To compare the acid tolerance in biofilm cells of S. gordonii, S. oralis and S. sanguinis with that of S. mutans.

Methods: Evaluation of acid tolerance was undertaken in a flow-cell system. Bacteria were grown for 3 hours in minimal medium and then exposed to pH 3.0 for 2 hours. After removal, the number of surviving bacteria was calculated by plating on blood agar.

Results: For all species, 3-hour biofilm cells were more tolerant to a pH 3.0 challenge than corresponding planktonic cells. For S. mutans, survival rate was 5% while for S. gordonii, S. sanguinis and S. oralis the values were 0.2%, 0.7% and 2.7% respectively. The differences in survival rates of S. oralis and S. sanguinis compared to those of S. mutans were not significant but S. gordonii showed a significantly lower level of survival than S. mutans.

Conclusions: In S. oralis and S. sanguinis- strains of streptococci not traditionally associated with caries- surface adhesion induced equivalent levels of acid tolerance to those seen in S. mutans biofilms. This suggests that events associated with surface adhesion which lead to increased resistance to low pH may be similar for bacteria associated with caries and those more often associated with oral health.

Supported by The Swedish Research Council and The Knowledge Foundation, Sweden.