Despite its weak offensive output, MSU has only one loss this year, mainly due to its aggressive, physical defense. However, MSU's aggressive, physical defense has led them to becoming the 117th worse NCAA team (out of 125) for penalty yardage [70 yards/gm].
At ND, MSU's aggressiveness on defense led to numerous key defensive penalties which gave away the game to the Irish.
After the ND game, MSU AD Mark Hollis made an inquiry to Big Ten officials regarding the penalty calls against the Spartans. Mark Dantonio was quoted by the Detroit Free Press as questioning whether the penalty calls against his DB's reflected a change in standards for pass defenders. Many media members questioned the calls against MSU. In MSU's most recent home game vs. Purdue, MSU was only called for 2 penalties for 14 yards.
I think the key determinant in Michigan's upcoming game at Michigan State will be whether the refs fairly call MSU penalties, especially for pass interference/defensive holding and roughing the passer. If Big Ten officials continue to experience a "backlash" effect and exhibit reluctance to fairly call MSU penalties in East Lansing, it will doom Michigan's chances for a big road win. In addition, excessively rough play by MSU (as has occurred in past match-ups) could also result in injuries to Michigan's key players.