Evaluation of co-aggregation among Streptococcus mitis, Fusobacterium nucleatum and Porphyromonas gingivalis

Aims: To develop a semi-quantitative method for evaluating co-aggregation reactions among three bacterial species, and to examine the influence of Fusobacterium nucleatum on the adherence of Porphyromonas gingivalis. Methods and Results: The method involves coating hydroxyapatite (HAP) discs with streptococcal cells and treatment with radio-labelled bacterial cell suspensions. The sensitivity of the method was estimated by comparison with a turbidometric co-aggregation assay. Results from the two methods were in close agreement. Streptococcus mitis-coated HAP discs were immersed in a H-3-labelled Fus. nucleatum cell suspension and then a C-14-labelled P. gingivalis cell suspension. The discs were then pyrolysed to recover and quantify the released H-3 and C-14 radioactivity. The number of Fus. nucleatum cells on the discs increased with immersion time and this, in turn, resulted in elevated adherence of P. gingivalis. Conclusions: The data indicate that the method closely reflects co-aggregation characters, and that Fus. nucleatum has a positive effect on the adherence of P. gingivalis. Significance and Impact of the Study: The present method, which is designed to mimic the oral environment, should prove useful in the semi-quantitative evaluation of co-aggregation reactions.