Involved in the lipopolysaccharide (LPS) O-antigen biosynthesis (PubMed:20580469). Catalyzes the transfer of a formyl group to GDP-perosamine, leading to the formation of GDP-N-formylperosamine (By similarity). Is critical for full bacterial virulence (PubMed:20580469). (6S)-10-formyltetrahydrofolate + GDP-alpha-D-perosamine = (6S)-5,6,7,8-tetrahydrofolate + GDP-N-formyl-alpha-D-perosamine + H(+) Bacterial outer membrane biogenesis; lipopolysaccharide biosynthesis. Homodimer. Disruption mutant lacks lipopolysaccharide O-side chain. In vivo, mutants are attenuated in C57BL/6 and IRF-1 knockout mice. In bone marrow-derived macrophages (BMDM), mutants are attenuated, cannot reach a replicative niche and induce higher levels of IL-12 and TNF-alpha when compared to parental smooth strains. Belongs to the Fmt family.