SUMMARYHistamine fish poisoning is quite common and occur in consequence of microbial decarboxylase whose activity begin early in the post-mortem but are triggered in consequence of abuse in the shelf life of fish products. In this study forty-eight samples of tuna, mackerel, anchovies, sardines, fresh or processed were sampled from fish shops and supermarkets in the City of Bologna in the period from January to July 2010. Concentration of histamine was assessed using ELISA quantitative test and presence of psicrotrophic histamine forming bacteria was searched using a modified Niven agar medium which allow detection of suspect colonies that were confirmed by PCR for detecting the presence of the histidine decarboxylase genes in their DNA. The positive colonies were then identified on the basis of their morphology, Gram reaction and biochemical characteristics with API20E. The differential capability of the Niven agar was found to be low and approximately one fifth of the suspect colonies were confirmed by the PCR test, which however included both strong and weak histamine producing strains. The presence of Morganella morganii was associated with concentration of histamine 460 mg•kg -1 above the allowed limit in a sample of tuna sampled from a fish shop. The same bacterium was found in samples of Atlantic horse mackerel (Trachurus trachurus). High histamine concentration (between 258 and > 300 mg•kg -1 ) were observed in salted European pilchard and European anchovy (228 mg•kg -1 ) sold loose in supermarkets. Because temperature abuse could occur when Tuna (fresh/defrozen) are hold on chopping board to sell fresh cuts and during shelf life of salted pilchard and pickled anchovies held in opened cans in chilled display cabinets for extended period, which might results in very high histamine concentration, controls on time and temperature at the retail, in addition to those done during the harvest and processing are needed. The studies aiming at quantitatively assess the prevalence and number of histamine producing bacteria in fish products that were not involved in histamine poisoning cases and/or do not show high level of histamine are rather complex , requiring many tests for confirming the histamine forming ability of the suspect isolated strains and further studies are needed to develop techniques for enumerating the histamine producing bacteria.KEYWORDS histamine, histidine decarboxylase, fish poisoning, shelf life, Morganella morganii. INTRODUZIONEL'intossicazione da istamina è una malattia di origine alimentare che si manifesta con eruzioni cutanee, arrossamenti, sudorazione, sensazione di bruciore in bocca, talvolta con nausea, vomito, diarrea, bruciore di stomaco, mal di testa, formicolio, gonfiore, giramenti di testa e più raramente con sintomi nervosi e shock anafilattico. Le manifestazioni sono correlate alla elevata concentrazione d'istamina che si accumula quando le carni sono contaminate da batteri produttori di istidino-decarbossilasi, raggiungendo concentrazioni tossiche rilevanti in alcune specie...