To isolate from bacteria and the like, visitors are not being permitted.

Literal

Bacteria [etc.-など] [from-から] isolate [purpose-ため], visitation-refusal [formal become-となっています].

面会謝絶 (no visitors) is a four-character medical/legal compound — 面会 (meeting) + 謝絶 (refusal). ~となっています is the formal version of ~になっている, indicating a current state that was decided: visitors have been refused. ~ため marks the purpose of the isolation.