Beta-mannosyltransferase involved in cell wall biosynthesis. Required for addition of the second beta-mannose residue to acid-stable fraction of cell wall phosphopeptidomannan, and in elongation of beta-mannose chains on the phosphopeptidomannan acid-labile fraction. Expression is induced in biofilm and by HAP43. Impairs the elongation of beta-mannose chains on the acid-labile fraction of cell wall phosphopeptidomannan. Belongs to the BMT family.