It appears there is no consensus among social scientists (or any other academics) on the definition of the "middle class," and, moreover, certainly it is the case that most of them do _not_ mean the bourgeoisie by this term as Marx did. "Research methods" (if we can use such a fancy term here) in social sciences as they exist in academy are plural, and it depends on individual researchers as well as schools of thought how the "middle class" gets defined (if it gets defined with any degree of precision at all).
Yoshie