The flesh of certain marine finfish contains high concentrations of the free amino acid histidine. In products of these fish species bacteria with the ability to form the enzyme histidine decarboxylase may produce high concentrations of histamine. Products with more than 500-1000 mg of histamine/kg can cause histamine fish poisoning (HFP) in otherwise healthy consumers. HFP is a common and fortunatly relatively mild seafood borne disease with symptoms including flushing, rash, headache, diarrhoea and vomiting. To form toxic concentrations of histamine in seafood bacteria must grow to high concentrations of about 1 million bacteria/g or more (Fig. 1). Consequently, to limit growth of histamine producing bacteria is an efficient way to reduce histamine formation in relevant marine finfish products.

Toxic concentrations of histamine in seafood can be formed by some mesophilic bacteria as well as by and some psychrotolerant bacteria (See  Table 1 below). Morganella morganii is probably the most important mesophilic bacteria with respect to histamine foamtion in seafood stored at temperatures above 10-15 °C but this bacterium is not producing toxic concentrations of histamine in seafood chilled to below about 7°C. In contrast, Morganella psychrotolerans can form toxic concentrations of histamine in seafood at storage temperatures as low at 0°C (Emborg & Dalgaard, 2008a,b). SSSP includes models to predict histamine formation by both Morganella psychrotolerans and Morganella morganii. This allows prediction of e.g. the effect of delayed chilling where products can be exposed to some time of storage at ambient temperature followed by chilled storage at 0-5°C.

Fig. 1. Example of growth and histamine formation by Morganella bacteria in a seafood product. Importantly, the concentration of histamine remains very close to zero for a substantial part of the products usual shelf-life and then suddenly the concentration of histamine increase to toxic levels. Consequently, products of marine finfish may test acceptable for histamine content when shipped from a company and then become toxic with high concentrations of histamine when they reach the consumers.

 Table 1 Incidents of histamine fish poisoning where the bacteria responsible for histamine formation have been identified.

Modified from Dalgaard et al. (2008).

Primary models for growth and histamine formation

The expanded Logistic model with delay is shown below in Eqn. 1 and a simulation of this primary growth model is shown in Figure 1 above. Compared to the classical Logistic models Eqn. 1 is expanded with the parameter 'm' to control dampening of growth when the cell concentration (N_t, cfu/g) approached the maximum cell concentration (N_max, cfu/g). To predict growth of  both Morganella psychrotolerans and Morganella morganii SSSP uses an fixed value of m = 0.7.  To predict histamine formation SSSP uses a simple primary model where growth is related to histamine formation by a constant yield factor (Eqn. 2) (Emborg & Dalgaard, 2008a,b).