51A-36

S. H. KIM¹, R. J. Price, D. S. Chang, and H. An. (1) OSU-Seafood Laboratory, Oregon State University, 2001 Marine Dr., Astoria, OR 97103

JUSTIFICATION: Many different bacterial species have been reported as histamine formers. However, only a few species have been isolated in fish implicated in scombroid poisoning. Little information has been known for types and levels of histamine formers contributing histamine accumulation in fish.

OBJECTIVE: The objective was to identify predominant and prolific histamine formers in Pacific mackerel during storage at various temperatures.

METHOD: Pacific mackerel was stored at 0, 4, 15, and 25°C, and histamine content in muscle was monitored. Several groups of bacteria were isolated from fish stored on each condition by selective media: enteric bacteria using VRBG; vibrios, TCBS; and pseudomonads, PI. Histamine formation was confirmed by the enzymatic histamine analysis, and the histamine formers were identified using the Vitek identification system.

RESULTS: Totally, 1,200 strains were isolated from mackerel during storage up to 14 days. High histamine formers producing >1,000 ppm histamine were mostly isolated on VRBG agar from fish stored at 15-25°C. Morganella morganii was isolated on VRBG agar and identified as the most predominant and prolific histamine former. This strain was most frequently isolated at 25°C, the optimum temperature for histamine formation in fish. At 0-4°C, the rate of bacterial growth was significantly reduced. At 4°C, the isolated histamine formers were mainly natural bacterial flora in seawater. They were weak histamine formers, and their counts in fish were not high. However, histamine accumulation in muscle was still significant, forming 57.4 mg/100 g, although fish were completely decomposed at that storage. At 0°C, neither histamine formers nor their histamine production was detected in Pacific mackerel.

SIGNIFICANT: Most prolific histamine formers were identified as mesophilic bacteria. Among them, M. morganii played the major role on histamine formation. Their growth was effectively controlled by storage at 4°C or below, but histamine formation was controlled only by storage at 0°C.