High concentrations of L-arginine or L-citrulline in the growth medium provided the wine bacterium Lactobacillus brevis with a significant growth advantage. The arginine deiminase pathway (ADI) arc gene cluster of Lb. brevis contains three genes, arcD, arcE1 and arcE2, encoding putative L-arginine/L-ornithine exchangers. Uptake experiments with Lactococcus lactis cells expressing the genes showed that all three transported L-ornithine with affinities in the μM range. Similarly, ArcD and ArcE2 transported L-arginine, while ArcE1 transported L-citrulline, an intermediate of the ADI pathway. Chase experiments showed very efficient exchange of L-arginine and L-ornithine by ArcD and ArcE2, and of L-citrulline and L-ornithine by ArcE1. Low affinities (mM range) combined with low translocation rates were found for ArcD and ArcE2 with L-citrulline, and for ArcE1 with L-arginine. Resting cells of Lb. brevis grown in the presence of L-arginine and L-citrulline rapidly consumed L-arginine and L-citrulline, respectively, while producing ammonia and L-ornithine. About 10% of L-arginine degraded was excreted by the cells as L-citrulline. Degradation of L-arginine and L-citrulline was not subject to carbon catabolite repression (CCR) by glucose in the medium. At high medium pH, L-citrulline in the medium was required for induction of the L-citrulline degradation pathway. Pathways are proposed for the catabolic breakdown of L-arginine and L-citrulline that merge at the level of ornithine transcarbamylase (OTC) in the ADI pathway. L-arginine uptake is catalyzed by ArcD and/or ArcE2, L-citrulline by ArcE1. L-citrulline excretion during L-arginine breakdown is proposed to be catalyzed by ArcD and/or ArcE2 through L-arginine/L-citrulline exchange.ImportanceLactobacillus brevis, a bacterium isolated from wine as well as other food environments, expresses a catabolic pathway for the breakdown of L-citrulline in the medium that consists of the L-citrulline/L-ornithine exchanger ArcE1 and part of the catabolic arginine deiminase (ADI) pathway enzymes. The proposed pathways for L-arginine and L-citrulline breakdown provide a mechanism for L-citrulline accumulation in fermented food products that is the precursor of the carcinogen ethyl carbamate.