Classification of photobacteria associated with spoilage of fish products by numerical taxonomy and pyrolysis mass spectrometry

Forty strains of luminous and non-luminous Photobacterium phosphoreum isolates from cod (Gadus morhua) and seven reference strains of psychrotolerant and mesophilic photobacteria were examined for 156 unit characters in a numerical taxonomic study. The fish strains were isolated from the intestines, from spoiled products and by using a specific detection method. The data were analysed using the similarity coefficient and the unweighted pair-group with arithmetic averages algorithm. In addition twenty-six of the fish isolates and five reference strains were analysed by Curie-point pyrolysis mass spectrometry. The mesophilic and psychrotolerant photobacteria were clearly separated in each of the analyses. Both procedures indicated that the spoilage isolates of P. phosphoreum had originated from the live fish as strains from the fish intestines and from the spoiled products were recovered in the same sub-groups. One sub-group of psychrotolerant P. phosphoreum strains, which was selected in modified atmosphere packed fish stored at low temperature, was also highlighted using each of the methods. The importance of classifying food spoilage bacteria has been shown and a simple key generated for the identification of luminous and nonluminous isolates of Photobacterium phosphoreum.