Undecaprenyl phosphate translocases confer conditional microbial fitness.

The microbial cell wall is essential for cell shape maintenance and resistance to external stressors 1 . The primary structural component of the cell wall is peptidoglycan (PG), a glycopolymer with peptide crosslinks located outside of the cell membrane 1 . PG biosynthesis and structure are responsive to shifting environmental conditions such as pH and salinity 2-6 , but mechanisms underlying such adaptations are incompletely understood. Precursors of PG and other cell surface glycopolymers are synthesized in the cytoplasm and then delivered across the cell membrane bound to the recyclable lipid carrier undecaprenyl phosphate (C55-P) 7 . The transporter protein(s) that return C55-P to the cytoplasmic face of the cell membrane have been elusive. Here, we identify the DUF368-containing and DedA transmembrane protein families as candidate C55-P translocases, filling a critical gap in knowledge of the proteins required for the biogenesis of microbial cell surface polymers. Gram-negative and -positive bacteria lacking their cognate DUF368-containing protein exhibited alkaline-dependent cell wall and viability defects, along with increased cell surface C55-P levels. pH-dependent synthetic genetic interactions between DUF368-containing proteins and DedA family members suggest that C55-P transporter usage is dynamic and modulated by environmental inputs. C55-P transporter activity was required by the cholera pathogen for growth and cell shape maintenance in the intestine. We propose that conditional transporter reliance provides resilience in lipid carrier recycling, bolstering microbial fitness within and outside of the host.