Ion interactions in the aggregation of Streptococcus mitis.

Abstract

Spontaneous aggregation of Streptococcus mitis ATCC 903 was highly dependent on various electrolytes such as sodium chloride, sodium phosphate or potassium phosphate with an optimum at 10 mM concentration. No aggregation was obtained in distilled water. Addition of a number of mono- or divalent ions to bacteria incubated in 10 mM phosphate or tris-maleate buffers resulted in an inhibition of aggregation. A 50% inhibition of aggregation was obtained at low concentrations of calcium, magnesium, sulphate and carbonate ions. Other mono- and divalent ions tested were needed in significantly higher concentrations in order to obtain the same inhibition. Aggregation inhibition by calcium or magnesium salts was reversed by chelation with EDTA. Washing bacteria previously treated with magnesium or calcium salts resulted in a complete recovery of aggregating ability. Spontaneously aggregated cells were completely dissociated at 10 mM magnesium sulphate or 100 mM sodium chloride. The results indicate that electrostatic interactions are of major importance for aggregation of S. mitis ATCC 903 whereas hydrophobic interaction plays a minor role.