“By bourgeoisie is meant the class of modern capitalists, owners of the means of social production and employers of wage labour.
“By proletariat, the class of modern wage labourers who, having no means of production of their own, are reduced to selling their labour power in order to live.” (Manifesto of the Communist Party. Chapter I. Footnote 1, see also)
The quote above offers a clear answer to the question in the title. A careful reading suggests the key concept is that of private property of the means of production.