Aggregation of enzymatically modified Streptococcus mitis indicating involvement of lectin-ligand type interaction

The aggregation properties of Streptococcus mitis ATCC 903 cells modified by treatment with heat or different enzymes was investigated. Bacteria that had the ability to aggregate spontaneously lost this capacity by treatment with proteolytic enzymes, beta-galactosidase or heat. Cells subjected to different types of modification were mixed in various proportions and their aggregation properties were recorded. To discriminate between the two kinds of cells in the suspension, one partner in the aggregation reaction was labelled with 14C-palmitic acid. Bacteria treated with beta-galactosidase co-aggregated with spontaneously aggregating cells (not modified) and with cells treated with heat. Heat-treated cells co-aggregated with spontaneously aggregating cells and with cells treated with beta galactosidase. Cells treated with trypsin did not co-aggregate either with spontaneously aggregating cells or cells treated with heat or beta-galactosidase. These findings are consistent with the hypothesis that two surface components are required for specific aggregation of S. mitis cells. We suggest that both components are degraded or released from the bacterial surface by treatment with trypsin (and other proteolytic enzymes) as shown by the inability of these cells to take part in any co-aggregation with spontaneously aggregating cells. Treatment with beta-galactosidase degrades a carbohydrate receptor constituting the terminal part of a glycoprotein. Heat treatment inactivates a protein lectin. The fact that heat-treated bacteria and bacteria treated with beta-galactosidase aggregate when mixed supports the assumption that two components take part in the aggregation reaction.