  Bulk boundary correspondence in topological materials allows to study their
bulk topology through the investigation of their topological boundary modes.
However, for classes that share similar boundary phenomenology, the growing
diversity of topological phases may lead to ambiguity in the topological
classification of materials. Such is the current status of bulk bismuth. While
some theoretical models indicate that bismuth possesses a trivial topological
nature, other theoretical and experimental studies suggest non-trivial
topological classifications such as a strong or a higher order topological
insulator, both of which hosts helical modes on their boundaries. Here we use a
novel approach to resolve the topological classification of bismuth by
spectroscopically mapping the response of its boundary modes to a topological
defect in the form of a screw dislocation (SD). We find that the edge mode
extends over a wide energy range, and withstands crystallographic
irregularities, without showing any signs of backscattering. It seems to bind
to the bulk SD, as expected for a topological insulator (TI) with non-vanishing
weak indices. We argue that the small scale of the bulk energy gap, at the time
reversal symmetric momentum $L$, positions bismuth within the critical region
of a topological phase transition to a strong TI with non-vanishing weak
indices. We show that the observed boundary modes are approximately helical
already on the $\mathbb{Z}_2$ trivial side of the topological phase transition.
This work opens the door for further possibilities to examine the response of
topological phases to crystallographic topological defects, and to uniquely
explore their associated bulk boundary phenomena.
