Bacterial histamine formation in mackerel and albacore was studied by inducing histamine in the muscles under controlled storage conditions. The optimum temperature for histamine formation was 25°C. The highest level of histamine detected was 283 mg/100 g in the 2-day stored mackerel; and 67.1 mg/100 g in the 6-day stored albacore. To identify the bacteria crucial to histamine formation, histamine formers were isolated using the conventional culture method. Enteric bacteria were most frequently isolated from the fish. Weak histamine formers were found in the gill and skin of fresh fish, and they required the enrichment step. Prolific histamine formers were mostly isolated from the decomposed muscles during storage at 25°C. Morganella morganii was the most prolific histamine former, producing >3,000 ppm in culture broth. M. morganii was the most prevalent histamine former in mackerel. In albacore, however, the most prevalent species was Hafnia alvei, a weak histamine former, resulting in less histamine accumulation than mackerel. Weak histamine formers, identified as natural bacteria in the marine environment, were found in mackerel during storage at 4°C after fish became unsuitable for human consumption. At 0°C, neither histamine-forming bacteria nor histamine was detected in fish. M. morganii formed significant amounts of histamine (>200 mg/100 g) in artificially contaminated fresh and frozen mackerel, albacore, and mahi-mahi when the fish were improperly stored at ambient temperatures (25°C). Growth of M. morganii was controlled by storage of fish at 4°C or below, but histamine formation was controlled only during frozen storage. For rapid detection of M. morganii, a PCR assay was developed by designing 16S rDNA targeted primers. Unique primers found for M. morganii were: the forward primer, 5'-CTCGCACCATCAGATGAACCCATAT-3'; and the reverse primer, 5'-CAAAGCATCTCTGCTAAGTTCTCTGGATG-3'. Nine CFU/ml of M. morganii inoculated in albacore homogenate were detected with a 6 h-enrichment of samples in TSB at 37°C. It would be necessary to monitor the presence of M. morganii in fish during handling and storage due to its high histamine-producing capability and prevent its contamination and proliferation after capture. The PCR assay developed in this study would be helpful to routinely monitor its presence in fish. / Graduation date: 2002