Identification of bacteria crucial to histamine accumulation in Pacific mackerel during storage

Bacterial growth and histamine formation in Pacific mackerel during storage at 0, 4, 15, and 25 degreesC were monitored. To identify bacterial species contributing to histamine formation, several groups of bacteria were isola ted by using selective media under temperatures corresponding to the variou s storage conditions. Initially, low counts of bacteria were found in the g ill, skin, and intestine of fresh fish, and only weak histamine formers wer e found in the gill. Histamine was found in the muscle when fish were store d above 4 degreesC, and aerobic plate counts reached 10(6) CFU/g. When fish became unsuitable for human consumption by abusive storage, toxicological levels of histamine were always found. The highest level of histamine forme d was 283 mg/100 g in 2 days. The optimum temperature for supporting growth of prolific histamine formers was 25 degreesC. The most prolific and preva lent histamine former was Morganella morganii, followed by Proteus vulgaris , both of which were isolated on violet red bile glucose (VRBG) agar. At 15 degreesC, a significant level of histamine was still produced in fish musc le, although prolific histamine formers were less frequently detected than at 25 degreesC. The isolates on thiosulfate citrate bile salts sucrose (TCB S) agar were weak histamine formers and identified as Vibrio parahaemolytic us and Vibrio alginolyticus. At 4 degreesC, less than 57.4 mg/100 g of hist amine was found in fish stored for 14 days. Most isolates were natural bact erial flora in the marine environment and identified as weak histamine form ers. At 0 degreesC, neither histamine former nor histamine production was d etected up to 14 days of storage.