Effects of vitamin B12 supply on cellular processes of the facultative vitamin B12 consumer Vibrio campbellii.

Abstract

Vitamin B₁₂ (cobalamin, herein B₁₂) is a key cofactor for most organisms being involved in essential metabolic processes. In microbial communities, B₁₂ is often scarce, largely because only few prokaryotes can synthesize B₁₂ de novo and are thus considered B₁₂-prototrophs. B₁₂-auxotrophy is mostly manifested by the absence of the B₁₂-independent methionine synthase, MetE. Here, we focus on bacteria that we classified as facultative B₁₂ consumers as they encode both B₁₂-independent and -dependent (MetH) methionine synthases yet largely cannot synthesize B₁₂ de novo. The genus Vibrio belongs to this group, and our work shows that upon B₁₂ supply growth of Vibrio campbellii is accelerated and autoinducer-2 (AI-2) concentrations are enhanced. We speculate that methionine synthesis efficiency, dependent on B₁₂ availability, is linked to AI-2 synthesis. The precursor for AI-2 synthesis is S-adenosyl-L-homocysteine (SAH), which in turn requires methionine as a precursor. In almost all Vibrio species studied, btuF (B₁₂-binding protein), which is required for B₁₂ uptake, and cobD (Adenosylcobinamide-phosphate synthase), which enables remodeling of B₁₂-like compounds, are encoded on the same operon as pfs (or mtnN, Adenosylhomocysteine nucleosidase), the first enzyme in the two-step AI-2 synthesis reaction. Transcriptomic analyses show that virulence factors, such as toxin synthesis, fimbriae formation, and activation of the type-6 secretion system, which have been shown to be regulated by quorum sensing via AI-2, are significantly upregulated in V. campbellii when B₁₂ is available. Our results demonstrate that V. campbellii is a facultative B₁₂ consumer and indicate that B₁₂ availability affects AI-2 levels and thus potentially virulence factor regulation.IMPORTANCEMetabolites play a key role in microbial metabolism and communication. While vitamin B₁₂ is an essential cofactor for important enzymatic reactions, autoinducer-2 mediates interspecies signaling and can regulate the expression of genes that are crucial for virulence and survival. In our study, we hypothesize and present findings how these two important metabolites are linked in Vibrio species. Vibrio campbellii grows without B₁₂ but at an accelerated rate when B₁₂ is present, and we detect higher AI-2 values in cultures with B₁₂ amendment. Transcriptome analyses show how vitamin B₁₂ availability significantly upregulates gene expression of virulence factors such as toxin synthesis, fimbrial formation, and activation of the type-6 secretion system in V. campbellii.