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Introduction to histamine formation by Morganella psychrotolerans and Morganella morganii

The flesh of certain marine finfish contains high concentrations of the free amino acid histidine. In products of these fin-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 food borne disease with symptoms including flushing, rash, headache, diarrhoea and vomiting. To form toxic concentrations of histamine in food bacteria must grow to high concentrations of about 1 million bacteria/g or more (Fig. 1). Consequently, to limit the growth of histamine producing bacteria is an efficient way to reduce histamine formation in relevant marine finfish products.

Toxic concentrations of histamine in finfish 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 7C. In contrast, Morganella psychrotolerans can form toxic concentrations of histamine in food at storage temperatures as low at 0C (Emborg & Dalgaard, 2008a,b). FSSP 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 0-5C.

 

 

Fig. 1. Example of growth and histamine formation by Morganella bacteria in a food 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 in products have been identified.

Modified from Dalgaard et al. (2008).

Implicated food

Bacterium

Year reported

Mesophilic bacteria

Fresh tuna

Morganella morganii

1956

Fresh tuna

Hafnia alvei

1967

Fresh tuna

Morganella morganii

1973

Fresh tuna

Raoultella planticola

1978

     

Tuna heated in flexible film

Morganella morganii

2006

Dried milkfish Raoultella ornithinolytica 2006

Psychrotolerant bacteria

Dried sardines

Photobacterium phosphoreum

2004

Tuna in chilli-sauce

Morganella psychrotolerans and/or Photobacterium phosphoreum

2005

Cold-smoked tuna

Photobacterium phosphoreum

2006

Cold-smoked tuna

Morganella psychrotolerans

2006

Fresh tuna

Photobacterium phosphoreum

2006

 

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 (Nt, cfu/g) approaches the maximum cell concentration (Nmax, cfu/g). To predict growth of  both Morganella psychrotolerans and Morganella morganii FSSP uses an fixed value of m = 0.7.  To predict histamine formation FSSP uses a simple primary model where growth is related to histamine formation by a constant yield factor (Eqn. 2) (Emborg & Dalgaard, 2008a,b).

                   

Eqn. 1. Expanded Logistic model with delay (tlag) and a parameter 'm' to control growth dampening when the cell concentration (Nt, cfu/g) approaches the maximum cell concentration (Nmax, cfu/g).
 

        

Eqn. 2.  Primary histamine formation model where histamine formation is directly related to growth by a constant yield factor (YHis/CFU). The factor of 1000 is included in Eqn. 2 to convert the cell concentrations (Nt and N0) from cfu/g to cfu/kg as the predicted histamine concentration is expressed in mg histamine/kg.