The ornamental design for a bristle bearingsurface of a toothbrush head, as shown and described.
FIG. 1 is a front-right perspective view of a bristle bearing surface of a toothbrush head according to our design;
FIG. 2 is a left-front perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left-side view thereof;
FIG. 6 is a right-side view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The portions of the toothbrush head shown in broken lines form no part of the claimed design. The grayscale shading represents a contrast in tone or color.