Obligate cross-feeding of metabolites is common in soil microbial communities

Many microorganisms are refractory to laboratory cultivation. One possible explanation, known as the great plate count anomaly, is metabolic dependencies among community members. However, systematic studies of these interactions in communities like soil are missing, hindering advances in understanding these ecologically important ecosystems. Here, we address this issue by systematically analysing 6,931 bacterial isolates of 27 soil microbial communities. We find that the growth of up to 50% of all community members depended essentially on supplementation with amino acids, vitamins, or nucleotides. In 73% of cases, supplementation with multiple amino acids was necessary. Genomic analysis of 62 strains revealed that accumulation of insertion sequences and specific gene loss was associated with the observed auxotrophies. Finally, genome-scale metabolic models, computational analyses, and cocultivation experiments demonstrated that other co-occurring genotypes complemented the metabolic needs of auxotrophs, thus facilitating their growth. Our results demonstrate that soil bacteria exist within integrated metabolic networks, which hampers their cultivation.