Sourdough lactic acid bacteria (LAB) need to be adapted to a highly acidic and, therefore, challenging environment. Different mechanisms are employed to enhance competitiveness, among which conversion of arginine into ornithine through the arginine deiminase (ADI) pathway is an important one. A combined molecular and kinetic approach of the ADI pathway in Lactobacillus fermentum IMDO 130101, a highly competitive sourdough LAB strain, identified mechanisms with advantageous technological effects and quantified the impact of these effects. First, molecular analysis of the arcBCAD operon of 4.8 kb revealed the genes encoding the enzymes ornithine transcarbamoylase, carbamate kinase, arginine deiminase, and an arginine/ornithine (A/O) antiporter, respectively, with an additional A/O antiporter 702.5 kb downstream of the ADI operon. The latter could play a role in citrulline transport. Second, pH-controlled batch fermentations were carried out, generating data for the development of a mathematical model to describe the temporal evolution of the three amino acids involved in the ADI pathway (arginine, citrulline, and ornithine) as a result of the activity of these enzymes and transporter(s). Free arginine in the medium was converted completely into a mixture of citrulline and ornithine under all conditions tested. However, the ratio between these end-products and the pattern of their formation showed variation as a function of environmental pH. Under optimal pH conditions for growth, citrulline release and some further conversion into ornithine was observed. When growing under sub-optimal pH conditions, ornithine was the main product of the ADI pathway. These kinetic data suggest a role in adaptation of L. fermentum IMDO 130101 to growth under sub-optimal conditions.