In this context, BMCs can migrate toward regions of higher substrate concentration for their metabolic pathways. The solute-BMC surface interaction causes local depletion of substrate, leading to a chemical potential gradient and net fluid flow that drives the BMC toward higher concentrations. For instance, Steinkühler et al. showed that Salmonella enterica BMCs move up gradients of propanediol, an important substrate for the propendiol utitlization (Pdu) pathway. In experiments, they were able to fabricate Pdus with and without active enzymes that consumed propanediol. The non-consuming Pdus aggregate, while Pdus with consumption help sustain motion by preventing largescale aggregation.