Characterization of biogenic amine-producing Stenotrophomonas maltophilia strains isolated from white muscle of fresh and frozen albacore tuna

Three strains — one of them psychrotrophic and two mesophilic — of Stenotrophomonas maltophilia, an emerging pathogen involved in an increasing number of clinical syndromes, were isolated from fresh and frozen-stored (6 months at −25°C) albacore tuna in a routine screening of histamine-forming bacteria. All the three strains showed histidine decarboxylase activity when assayed in a 2% histidine–0.0005% pyridoxal-HCl containing trypticase soy broth and further analyzed by HPLC. Although all three strains produced less than 25 ppm of histamine, they showed strong lysine-decarboxylating activity, cadaverine being produced at concentrations ranging from 1736 to 4821 ppm after 48 h in 0.0005% pyridoxal-HCl–trypticase soy broth supplemented with 1% lysine. Strong secretion of extracellular lipase and protease was also observed in all three strains. The psychrotrophic strain S. maltophilia 5PC6 was able to produce both extracellular lipase and protease even at 4 and 7°C, respectively. Phenotyping assays — including the investigation of up to 40 biochemical tests and 15 susceptibility assays — revealed slight differences among the three strains. Genotyping experiments — based on RFLP analysis and Southern blot hybridizations — indicated that strains 15MF and 25MC6 were closely related but confirmed that all three strains recovered were different.