Occurrence and importance of psychrotolerant histamine producing bacteria in chilled fresh tuna were demonstrated in the present study. The objective was to evaluate microbial formation of histamine and biogenic amines in chilled fresh tuna from the Indian Ocean and stored either vacuum-packed (VP) or modified atmosphere-packed (MAP). Firstly, biogenic amines and the dominating microbiota were determined in VP tuna involved in an outbreak of histamine fish poisoning in Denmark. Secondly, the microbiota of fresh MAP tuna was evaluated at the time of processing in Sri Lanka and chemical, microbial and sensory changes were evaluated during storage at 1-3 degrees C. To explain the results obtained with naturally contaminated tuna the effect of VP and MAP on biogenic amine formation by psychrotolerant bacteria was evaluated in challenge tests at 2 degrees C and 10 degrees C. The VP tuna that caused histamine fish poisoning had a histamine concentration of >7000 mg/kg and this high concentration was most likely produced by psychrotolerant Morganella morganii-like bacteria or by Photobacterium phosphoreum. Similar psychrotolerant M. morganii-like bacteria dominated the spoilage microbiota of fresh MAP tuna with 60% CO2/40% N2 and formed >5000 mg/kg of histamine after 24 days at 1.7 degrees C. These psychrotolerant bacteria were biochemically similar to M. morganii subsp. morganii and their 16S rDNA (1495 bp) showed >98% sequence similarity to the type strain of this species. Toxic concentrations of histamine were produced at 2.1 degrees C in inoculated VP tuna by both the psychrotolerant M. morganii-like bacteria (7400+/-1050 mg/kg) and P. phosphoreum (4250+/-2050 mg/kg). Interestingly, MAP with 40% CO2/60% O2, in challenge tests, had a strong inhibitory effect on growth and histamine formation by both the psychrotolerant M. morganii-like bacteria and P. phosphoreum. In agreement with this, no formation of histamine was found in naturally contaminated fresh MAP tuna with 40% CO2/60% O2 during 28 days of storage at 1.0 degrees C. To reduce current problems with histamine fish poisoning due to VP tuna it is suggested, for lean tuna loins, to replace vacuum packaging with MAP containing approximately 40% CO2 and approximately 60% O2.