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 3H-labelled Fus. nucleatum cell suspension and then a 14C-labelled P. gingivalis cell suspension. The discs were then pyrolysed to recover and quantify the released 3H and 14C 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. CONCLUSION: 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.