Beta-mannosyltransferase involved in cell wall biosynthesis. Required for the elongation of beta-mannose chains on the acid-labile fraction of cell wall phosphopeptidomannan. Expression in regulated by TSA1 and repressed in rat catheter biofilm. Impairs the elongation of beta-mannose chains on the acid-labile fraction of cell wall phosphopeptidomannan. Belongs to the BMT family.