Modelling the interactions between Pseudomonas putida and Escherichia coli O157:H7 in fish‐burgers: use of the lag‐exponential model and of a combined interaction index

Aims: The objective of the current study was to examine the interactions between Pseudomonas putida and Escherichia coli O157:H7 in coculture studies on fish‐burgers packed in air and under different modified atmospheres (30 : 40 : 30 O₂ : CO₂ : N₂, 5 : 95 O₂ : CO₂ and 50 : 50 O₂ : CO₂), throughout the storage at 8°C. Methods and Results: The lag‐exponential model was applied to describe the microbial growth. To give a quantitative measure of the occurring microbial interactions, two simple parameters were developed: the combined interaction index (CII) and the partial interaction index (PII). Under air, the interaction was significant (P < 0·05) only within the exponential growth phase (CII, 1·72), whereas under the modified atmospheres, the interactions were highly significant (P < 0·001) and occurred both in the exponential and in the stationary phase (CII ranged from 0·33 to 1·18). PII values for E.  coli O157:H7 were lower than those calculated for Ps. putida. Conclusions: The interactions occurring into the system affected both E. coli O157:H7 and pseudomonads subpopulations. The packaging atmosphere resulted in a key element. Significance and Impact of the Study: The article provides some useful information on the interactions occurring between E. coli O157:H7 and Ps. putida on fish‐burgers. The proposed index describes successfully the competitive growth of both micro‐organisms, giving also a quantitative measure of a qualitative phenomenon.