Predatory bacteria impact C. elegans life-history traits by modulating microbiota community dynamics and thereby vitamin B12 availability

Predatory bacteria such as Bdellovibrio are emerging as ecological modulators in microbial communities by restructuring community composition, yet their roles in host-associated microbiomes remain poorly understood. Using Caenorhabditis elegans as model host and its defined microbiota, we investigated how two Bdellovibrio strains with distinct prey ranges (B. tiberii MYbb2 and B. krueschi MYbb4) affect microbial community composition and host life-history traits. Both strains consistently altered microbiome composition, with MYbb4 causing more pronounced alpha-diversity shifts and MYbb2 selectively enriching strains of the genus Ochrobactrum which coincided with higher host median lifespan. Genome-based predictions indicate that de novo vitamin B12 synthesis by Ochrobactrum underlies the observed host phenotype, which was confirmed through quantitative measurements of the vitamin in mono-cell cultures. Employing the acdh-1p::GFP transcriptional reporter strain, we confirmed that a diet of B12-producing bacteria suppresses the B12-independent propionate detoxification pathway in the host, demonstrating that bacterially produced B12 is bioavailable to C. elegans. Exogenous B12 supplementation assays further confirmed the lifespan-extending effect. Together, these results suggest that predation-driven enrichment of B12-producing bacteria maintains B12 levels sufficient to detoxify propionyl-CoA via the B12-dependent pathway, preventing the accumulation of toxic metabolic byproducts that would otherwise arise under B12-limiting conditions and reduce host lifespan. Our findings demonstrate that predatory bacteria are important drivers of microbiome structure with direct consequences for host physiology, representing an underappreciated ecological mechanism for microbiome modulation.