Mechanistic and metabolic basis of bacterial cross-feeding

In their natural habitat, microorganisms interact with a variety of micro- as well as macroorganisms. Such interactions result in either positive or negative effects on the growth and survival of involved species. Negative effects on growth, can be mostly attributed to competition for limited resources and space, while positive effects on growth are challenging to justify. Metabolite cross-feeding is one such interaction that describes the transfer of primary or secondary metabolites from one organism to another. Considering that metabolites are costly and impose a significant energetic cost to the cell producing them, it is intriguing to know how the process of cross-feeding is favourable. Bacteria employ different mechanisms to carry out the exchange of metabolic by-products, intermediates, and electrons between each other during the process of cross-feeding. Contact-dependent mechanisms of exchange (such as direct cell-cell contact, type secretion systems, and pili), provide the following advantages: (i) protection of the exchanged product from environmental degradation or modification, (ii) provision of the product in a concentrated form, and (iii) prevention of uptake of the product by unintended recipients. The role of contact-dependent mechanisms in the transfer of genetic material (bacterial conjugation) and toxins (contact-dependent inhibition or killing) has been studied for years. However, the importance of similar contact-dependent mechanisms during the cross-feeding of essential nutrients is not fully understood. This thesis aimed at identifying a contact-dependent mechanism for amino acid cross-feeding in bacteria.