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Edwardsiella tarda  is a facultative intracellular pathogen in humans and animals. The Gram-negative bacterium is widely considered a potentially important bacterial pathogen. Adaptation to acid stress is important for the transmission of intestinal microbes, so the acid-resistance (AR) system is essential. However, the AR systems of  E. tarda  are totally unknown. In this study, a lysine-dependent acid resistance (LDAR) system in  E. tarda , CadBA, was characterized and identified. CadB is a membrane protein and shares high homology with the lysine/cadaverine antiporter. CadA contains a PLP-binding core domain and a pyridoxal phosphate-binding motif. It shares high homology with lysine decarboxylase.  cadB  and  cadA  are co-transcribed under one operon. To study the function of the  cadBA  operon, isogenic  cadA ,  cadB  and  cadBA  deletion mutant strains TX01Δ cadA , TX01Δ cadB  and TX01Δ cadBA  were constructed. When cultured under normal conditions, the wild type strain and three mutants exhibited the same growth performance. However, when cultured under acid conditions, the growth of three mutants, especially TX01Δ cadA , were obviously retarded, compared to the wild strain TX01, which indicates the important involvement of the  cadBA  operon in acid resistance. The deletion of  cadB  or  cadA , especially  cadBA , significantly attenuated bacterial activity of lysine decarboxylase, suggesting the vital participation of  cadBA  operon in lysine metabolism, which is closely related to acid resistance. The mutations of  cadBA  operon enhanced bacterial biofilm formation, especially under acid conditions. The deletions of the  cadBA  operon reduced bacterial adhesion and invasion to Hela cells. Consistently, the deficiency of  cadBA  operon abated bacterial survival and replication in macrophages, and decreased bacterial dissemination in fish tissues. Our results also show that the expression of  cadBA  operon and regulator  cadC  were up-regulated upon acid stress, and CadC rigorously regulated the expression of  cadBA  operon, especially under acid conditions. These findings demonstrate that the AR CadBA system was a requisite for the resistance of  E. tarda  against acid stress, and played a critical role in bacterial infection of host cells and in host tissues. This is the first study about the acid resistance system of  E. tarda  and provides new insights into the acid-resistance mechanism and pathogenesis of  E. tarda .   Supplementary Information  The online version contains supplementary material available at 10.1186/s13567-021-00987-x.

Acid resistance system CadBA is implicated in acid tolerance and biofilm formation and is identified as a new virulence factor of Edwardsiella tarda