Responsible for addition of the terminal mannose residues to the outer chain of core N-linked polysaccharides and to O-linked mannotriose. Implicated in late Golgi modifications (By similarity). Involved in virulence. Protein modification; protein glycosylation. Induced during biofilm formation. Leads to small cell wall defects but displays a severe attenuation of virulence in a murine infection model. Belongs to the MNN1/MNT family.