Bergeron's three-goal performance was the first of his career.
Tyler Seguin and Brad Marchand each chipped in with a goal and an assist and Blake Wheeler added a goal in the Bruins' offensive explosion.
Thomas notched his league-leading sixth shutout for Boston, which maintained its two-point advantage over Montreal atop the Northeast Division.
Brian Elliott allowed five goals on 27 shots before being lifted in favor of Mike Brodeur, who made 11 saves on 12 shots as the Senators dropped their sixth straight game.
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