Sulphur-Acquisition Pathways for Cysteine Synthesis Confer a Fitness Advantage to Bacteria in Plant Extracts

Bacteria and plants are closely associated with human society, in fields such as agriculture, public health, the food industry, and waste disposal. Bacteria have evolved nutrient-utilisation systems adapted to achieve the most efficient growth in their major habitats. However, empirical evidence to support the significance of bacterial nutrient utilisation in adaptation to plants is limited. Therefore, we investigated the genetic and nutritional factors required for bacterial growth in plant extracts by screening an Escherichia coli gene-knockout library in vegetable-based medium. Mutants lacking genes involved in sulphur assimilation, whereby sulphur is transferred from sulphate to cysteine, exhibited negligible growth in vegetable-based medium or plant extracts, owing to the low cysteine levels. The reverse transsulphuration pathway from methionine, another pathway for donating sulphur to cysteine, occurring in bacteria such as Bacillus subtilis, also played an important role in growth in plant extracts. These two sulphur-assimilation pathways were more frequently observed in plant-associated than in animal-associated bacteria. Sulphur-acquisition pathways for cysteine synthesis thus play a key role in bacterial growth in plant-derived environments such as plant residues and plant exudates.