Cadaverine　is　produced　in　organisms　from　the　amino　acid　lysine　in　a　decarboxylation　reaction　catalyzed　by　lysine　decarboxylase　(EC　4.1.1.18).　The　inducible　lysine　decarboxylase　CadA　plays　a　vital　role　in　acid　stress　response　for　enteric　bacteria.　Vibrio　vulnificus　is　an　extremely　virulent　human　pathogen　causing　gastroenteritis　when　the　acid　conditions　that　prevent　survival　of　V.　vulnificus　in　the　stomach　or　small　intestine　are　overcome.　A　gene　encoding　CadA　was　identified　from　V.　vulnificus.　Subsequent　analyses　showed　that　CadA　from　V.　vulnificus　(VvCadA)　is　a　decamer　with　a　82-kDa　subunit.　Homogenous　VvCadA　was　purified　from　Escherichia　coli　and　used　for　lysine　decarboxylation　with　an　optimal　pH　of　6.0　and　optimal　temperature　of　37　degrees　C.　The　apparent　V-max　and　K-m　for　lysine　were　9.45　+/-　0.24　mu　M/min　and　0.45　+/-　0.05　mM,　respectively.　Mutation　analysis　suggested　that　the　amino-acid-binding　pyridoxal　phosphate,　the　cofactor　of　the　enzyme,　plays　a　vital　role　in　the　reaction.　Mutation　of　the　negatively　charged　residues　interacting　with　lysine　also　affected　the　activity　of　the　enzyme　to　some　extent.　Quantitative　RT-PCR　showed　that　expression　of　VvcadA　was　up-regulated　under　low　pH,　low　salinity,　and　oxidative　stresses.　Furthermore,　the　concentration　of　cadaverine　released　to　the　cell　exterior　also　increased　under　these　stresses.　Protein　sequence　similarity　network　(SSN)　analysis　indicated　that　lysine　decarboxylases　with　ornithine　decarboxylases　and　arginine　decarboxylases　shared　a　common　ancestor,　and　that　lysine　decarboxylases　are　more　conserved　during　evolution.　Our　data　provide　evidence　for　the　biochemical　characteristics　and　important　roles　of　VvCadA　under　stress　conditions.