In case of which bacteria is phage therapy not possible?

Intracellular bacteria like Borrelia cannot be targeted by whole living phage particles as phages have to get in direct contact with the bacterial cell surface.

However, scientists are working on an additional therapy possibility based on phages: isolated, purified phage enzymes that can be applied instead of whole phage particles. These enzymes are responsible for bacterial lysis.
But still, there is a lot of research to do especially in the field of gram negative bacteria.