Histamine fish poisoning occurs when fish containing high amount of histamine are consumed. Since histamine is thermally stable, control of histamine forming bacteria in seafood is a good strategy to prevent the formation of histamine. One of the potential ways to do this is to apply gamma radiation on the histamine formers. In order to understand the effect of gamma radiation on the histamine forming bacteria, laboratory isolates of prolific histamine forming bacteria namely Morganella morganii, Klebsiella variicola and Proteus vulgaris were exposed to different doses of gamma radiation in nutrient broth and in tuna muscle spiked with histamine formers. None of the test bacteria survived in tuna muscle radiated at 2.0 kGy. Among the selected histamine formers, Klebsiella variicola was highly sensitive to gamma radiation and was eliminated at a dose of 1.5 kGy. Histamine production levels were also reduced as radiation dose increased. These results suggest that gamma irradiation can effectively eliminate histamine forming bacteria and the threat of histamine poisoning in seafood.
Foods that are extensively handled during preparation and stored without refrigeration are often associated with staphylococcal food poisoning. This problem is more confounding when contaminating strains belong to the methicillin-resistant Staphylococcus aureus (MRSA) group. In this study, we investigated the survivability of methicillin-resistant S. aureus (MRSA) in two different seafood matrices at different storage conditions. MRSA was inoculated at 6 and 3 log cfu /g levels into all sample groups comprised of peeled shrimp ( Parapeneopsis stylifera ) stored at -20 o C, Bombay duck ( Harpadon nehereus ) stored in ice and dried Bombay duck stored at room temperature. The populations of MRSA in frozen peeled shrimp showed 1.52 log reduction in 6 log cfu /g inoculated sample, while in 3 log cfu/g inoculated sample, the counts remained more or less stable after 60 days of storage. In fresh Bombay duck inoculated with 6 log cfu /g and stored in ice for 18 days, MRSA count decreased by 2.75 log cfu /g . In contrast, MRSA counts remained stable in 3 log cfu /g inoculated Bombay duck stored in ice. However, the total viable count (TVC) in ice-stored Bombay duck increased by 3.02 log cfu /g over 16 days of storage. In dried Bombay duck stored at room temperature, MRSA count declined by 3.27 log cfu /g in 6 log cfu /g inoculated sample and by 0.91 log cfu/g in 3 log cfu /g inoculated sample. The results suggest that the survival of methicillin-resistant S. aureus depends on the temperature of storage and the inoculum size. In our study, MRSA exhibited better survival when inoculated at 3 log cfu /g irrespective of seafood matrix and storage temperature.
ABSTRACT Histamine poisoning occurs when temperature-abused marine fish containing elevated levels of histamine are consumed. Histamine-producing bacteria found in fish can colonize processing surfaces and form biofilms. In this study, the biofilm-forming abilities of histamine-producing bacteria from Indian mackerel ( Rastrelliger kanagurta ) and the effect of hypochlorite treatment on biofilm formation were studied. The isolates of this study produced histamine in the range of 471 to 2,126 ppm. The histidine decarboxylase gene hdc was detected in all isolates producing histamine except in one strain each of Psychrobacter pulmonis and Proteus vulgaris . All isolates tested in this study produced moderate biofilms under control conditions, whereas exposure to 1 and 3 ppm of sodium hypochlorite significantly enhanced biofilm formation. However, exposure to 5 ppm of sodium hypochlorite showed an inhibitory effect on biofilm formation by all the isolates except Klebsiella variicola . The results of this study suggest that histamine-producing bacteria can form stable biofilms and that this activity may be enhanced by the application of low levels of sodium hypochlorite, a phenomenon that might influence the persistence of histamine-producing bacteria in fish processing areas. HIGHLIGHTS Bacteria isolated from Indian mackerel produced histamine in the range of 471 to 2,126 ppm. Histamine-producing bacteria isolated from the same fish can vary in the levels of histamine produced. The hdc gene was not detected in one strain each of Psychrobacter pulmonis and Proteus vulgaris . All histamine-producing bacteria formed moderate biofilms under control conditions. Exposure to 1 and 3 ppm of sodium hypochlorite increased biofilm formation by histamine-producing bacteria.