ATP synthesis by substrate level phosphorylation in the reductive branch, but an ion gradient is produced. Depending on the species, the ion gradient is generated during either fumarate reduction or methylmalonyl-CoA decarboxylation; the ion gradient in turn is used to drive ATP synthesis via a membrane-bound ATP synthase. Propionatefermentationisused,forexample,duringthe production of Swiss cheese. The holes in Swiss cheese are formedfromcarbondioxide,generatedbyPropionibacterium from lactate.