Cobamide excretion allows Salmonella to transition from its anaerobic to aerobic lifestyle

Cobamides (Cbas) are structurally complex biomolecules used by cells of all domains of life. However, de novo Cba biosynthesis occurs only in some bacteria and archaea. The uptake, salvage, and remodeling of Cbas and their precursors have been well studied in several prokaryotes, but our understanding of the mechanism(s) of Cba release into the environment is limited. In this study, we quantitatively demonstrate Cba excretion by Salmonella enterica subsp. enterica sv. Typhimurium strain LT2 (hereafter S. Typhimurium). We performed experiments in liquid and solid media using strains that produced Cbas from a precursor that can be converted to Cbas in the presence of oxygen. We show that under anaerobic conditions, S. Typhimurium synthesizes Cbas de novo and excretes this valuable cofactor into the medium. To do this, S. Typhimurium requires ≥24 h of incubation under anaerobic conditions to form microcolonies that can continue synthesizing and excreting Cbas when shifted to aerobic growth conditions. We propose that de novo Cba biosynthesis and excretion may be necessary for S. Typhimurium to transition from an anaerobic to an aerobic environment, and that Cba excretion may allow growth of Cba-requiring members of polymicrobial communities.IMPORTANCEThis work is important for several reasons. First, this is the first evidence that Salmonella enterica subsp. enterica sv. Typhimurium strain LT2 excretes cobamides, a valuable nutrient for all forms of life, including humans. Second, although de novo cobamide biosynthesis in S. Typhimurium is blocked by oxygen, we show that once a microcolony of S. Typhimurium is formed under anaerobic conditions, exposure to air does not block cobamide biosynthesis, and robust colony growth can proceed. Hence, the formation of microcolonies under anaerobic conditions may be a critical strategy for S. Typhimurium survival in its free-living lifestyle. Third, in an environment where S. Typhimurium is part of a polymicrobial community, this pathogen may allow growth of other microbes that require cobamides to survive, as it may occur in polymicrobial biofilms. Finally, this work provides the groundwork for advancing our understanding of why S. Typhimurium excretes such a valuable cofactor and how excretion is accomplished.