Hydroxyapatite in general has a strong propensity to bind with protein, including all forms of bacteria. This probably includes «good» bacteria as well but in the oral cavity, tends to bind strongly to gram positive cariogenic bacteria such as streptococcus mutans. One reason is because they can only collect and multiply on the tooth surface, not in saliva or on the oral mucosa. Thus, when the tooth surface is directly brushed with toothpaste containing hydroxyapatite, these bacteria in particular become attached to the hydroxyapatite during brushing, facilitating their removal on rinsing from the mouth.