Letters in Applied Microbiology 1997, 24, 373–378 Importance of Photobacterium phosphoreum in relation to spoilage of modified atmosphere-packed fish products P. Dalgaard, O. Mejlholm, T.J. Christiansen and H.H. Huss Danish Institute for Fisheries Research, Department of Seafood Research, Technical University of Denmark, Lyngby, Denmark 1171/96: received 20 May 1996, revised 24 September 1996 and accepted 25 September 1996 P. DALGAARD, O. MEJLHOLM, T.J. CHRISTIANSEN AND H.H. HUSS. 1997. Occurrence and growth of Photobacterium phosphoreum were studied in 20 experiments with fresh fish from Denmark, Iceland and Greece. The organism was detected in all marine fish species but not in fish from fresh water. Growth of P. phosphoreum to high levels (×10 7 cfu g -1 ) was observed in most products and the organism is likely to be of importance for spoilage of several modified atmosphere-packed (MAP) marine fish species when stored at chill temperatures. Some microbiological methods recommended for control of fish products by national and international authorities are inappropriate for detection of psychrotolerant and heat-labile micro-organisms like P. phosphoreum. These methods have been used in many previous studies of MAP fish and this could explain why, contrary to the findings in the present study, P. phosphoreum in general was not detected previously in spoiled MAP fish. INTRODUCTION ample, in recent reviews about spoilage of MAP seafoods there was no mention of P. phosphoreum (Dixon and Kell Photobacterium phosphoreum is a well known light-organ 1989; Stammen et al. 1990; Farber 1991; Reddy et al. 1992). In symbiont of several fish species but the most common habitat contrast P. phosphoreum was identified as the specific spoilage of the organism is the intestine of marine fish where high organism (SSO) responsible for TMA production and numbers are found. The organism is widespread in the marine explaining the characteristics of spoiled MAP cod in environment and many different fish from most parts of the Denmark (Dalgaard et al. 1993; Dalgaard 1995a). The high world are contaminated with P. phosphoreum, previously also CO 2 resistance of P. phosphoreum explains the modest shelf- known as gut group vibrios and N group bacteria (Liston life extension of MAP cod (Dalgaard 1995b) and suggests the 1957; Ruby and Morin 1979; Barak and Ulitzur 1980; Ruby organism may be responsible for spoilage of other MAP et al. 1980; Yoguchi et al. 1990; Ramesh et al. 1990; Nealson seafoods (Dalgaard 1995c). and Hastings 1992; Haygood 1993). In fish products P. phos- The present study was carried out to determine the occur- phoreum was found to produce trimethylamine (TMA) from rence, growth and importance of P. phosphoreum on different trimethylamine oxide (TMAO) in aerobically stored fish and chilled fish products from Denmark, Iceland and Greece. to be of some importance in histamine production (van Spree- Marine and freshwater fish were studied and the effect of kens 1974, 1987; Morii et al. 1988). However, other studies MAP and aerobic storage was evaluated. stated that P. phosphoreum is of little or no importance in the spoilage of fish products (Shewan 1971; Abgrell and Cleret 1990). MATERIALS AND METHODS Modified atmosphere-packed (MAP) fish products are becoming increasingly important but their spoilage micro- Storage experiments biology needs further study before relevant microbiological and chemical indices of spoilage can be established. For ex- Nineteen storage experiments were carried out from May 1995 to December 1995 using fish from Denmark, Iceland Correspondence to: Dr P. Dalgaard, Danish Institute for Fisheries Research, and Greece. These products were stored in a chill room in Department of Seafood Research, Technical University of Denmark, Building 221, DK-2800 Lyngby, Denmark(e-mail: pad@ffl.min.dk). melting ice and temperature loggers confirmed the products © 1997 The Society for Applied Bacteriology