Quantifying the bioprotective effect of Lactobacillus sakei CTC494 against Listeria monocytogenes on vacuum packaged hot-smoked sea bream

Highlights

• The bioprotective potential of L. sakei CTC494 was investigated on hot-smoked sea bream.

• The simultaneous growth of L. sakei CTC494 and L. monocytogenes was modelled.

• L. sakei CTC494 did not increase sensory deterioration of smoked sea bream at an initial level ≤4 log CFU/g.

• L. sakei CTC494 limited the pathogen growth at 5 °C and dynamic conditions ranging from 3 to 12 °C.

• Modified Jameson and Lotka-Volterra models properly described microbial interaction.

Abstract

In this study, the bioprotective potential of Lactobacillus sakei CTC494 against Listeria monocytogenes CTC1034 was evaluated on vacuum packaged hot-smoked sea bream at 5 °C and dynamic temperatures ranging from 3 to 12 °C. The capacity of three microbial competition interaction models to describe the inhibitory effect of L. sakei CTC494 on L. monocytogenes was assessed based on the Jameson effect and Lotka-Volterra approaches. A sensory analysis was performed to evaluate the spoiling capacity of L. sakei CTC494 on the smoked fish product at 5 °C. Based on the sensory results, the bioprotection strategy against the pathogen was established by inoculating the product at a 1:2 ratio (pathogen:bioprotector, log CFU/g). The kinetic growth parameters of both microorganisms were estimated in mono-culture at constant storage (5 °C). In addition, the inhibition function parameters of the tested interaction models were estimated in co-culture at constant and dynamic temperature storage using as input the mono-culture kinetic parameters. The growth potential (δ log) of L. monocytogenes, in mono-culture, was 3.5 log on smoked sea bream during the experimental period (20 days). In co-culture, L. sakei CTC494 significantly reduced the capability of L. monocytogenes to grow, although its effectiveness was temperature dependent. The LAB strain limited the growth of the pathogen under storage at 5 °C (<1 log increase) and at dynamic profile 2 (<2 log increase). Besides, under storage at dynamic profile 1, the growth of L. monocytogenes was inhibited (<0.5 log increase). These results confirmed the efficacy of L. sakei CTC494 for controlling the pathogen growth on the studied fish product. The Lotka-Volterra competition model showed slightly better fit to the observed L. monocytogenes growth response than the Jameson-based models according to the statistical performance. The proposed modelling approach could support the assessment and establishment of bioprotective culture-based strategies aimed at reducing the risk of listeriosis linked to the consumption of RTE hot-smoked sea bream.

Introduction

Smoked fish products are sold as ready-to-eat (RTE) foods characterized by a relatively long refrigerated shelf-life when packaged under vacuum (Hwang, 2007). These seafood commodities are popular, but they are also considered among the top risk foodstuffs since they can be contaminated with foodborne pathogens and no cooking is applied before consumption (Ghanbari et al., 2013). At present, the microbiological concerns in the EU associated with extended shelf-life refrigerated RTE foods are focused on psychrotrophic foodborne pathogens such as Listeria monocytogenes (EFSA BIOHAZ, 2018).

Biopreservation, also called bioprotection, is a biocontrol approach to enhance product safety and shelf-life using microorganisms selected for their antimicrobial properties, so called protective cultures (Leroi et al., 2015). Lactic-acid bacteria (LAB) are considered a new generation of food additives and the basis of food biopreservation (Said et al., 2019). Protective cultures are considered by the regulatory agencies as ‘new’ food additives, meaning that they require market authorization for their technological use in foods. However, most LAB are Generally Recognized as Safe (GRAS) and many LAB species (including Lactobacillus sakei) have been granted by EFSA with the Qualified Presumption of Safety (QPS) status (EFSA (European Food Safety Authority), 2018). In the EU, microorganisms with the latter food-grade standard do not need to undergo a further safety assessment other than to provide evidence of efficacy and to satisfy the specified qualifications, if applicable, for its market approval. Two recent studies have proved that the antilisterial sakacin K-producing Lactobacillus sakei strain CTC494 (from meat origin) is effective to inhibit L. monocytogenes in filleted sea bream and cold-smoked salmon under refrigerated storage (Aymerich et al., 2019; Costa et al., 2019). Nevertheless, the inhibitory capacity of this bioprotective LAB strain has not been tested in other fish products where the differences in product's characteristics and formulations might either favor its inhibition thanks to the antimicrobial hurdle combinations (Leistner, 2000) or hinder the ability of the strain to inhibit L. monocytogenes (Tahiri et al., 2009; Vasilopoulos et al., 2010).

Quantifying microbial interaction in food can be highly complex and often overlooked in predictive microbiology studies (Powell et al., 2004). Most of the competitive growth models available in literature are based on two approaches: one based on the Jameson effect phenomenon (i.e. nutrient competition) (Jameson, 1962) and the other using the general Lotka-Volterra competition model (i.e. predator-prey model) (Powell et al., 2004; Valenti et al., 2013). Both mathematical models represent a simultaneous deceleration of bacterial populations. The inhibition of L. monocytogenes by endogenous LAB usually responsible for spoilage has been studied and modelled in minimally processed fish products (Mejlholm et al., 2015; Mejlholm and Dalgaard, 2015, 2007). In this regard, most of the published microbial interaction models aim at describing competition between background microbiota and microbial pathogens, rather than to characterize the performance of bioprotective bacteria with specific antagonistic activities, that are normally added at higher levels than the natural background (spoilage) microbiota (Cornu et al., 2011). To the best author's knowledge, studies having quantified the bioprotective effect of bacteriocin-producing LAB cultures through the development and implementation of predictive models are scarce. The first attempt to model the inhibitory effect of a bacteriocinogenic LAB strain against Listeria in fish was made by Costa et al. (2019), in which model parameters were derived from experiments in a fish-based broth and then validated on fresh filleted sea bream.

The objective of this study was (i) to evaluate the bioprotective potential of L. sakei CTC494 against L. monocytogenes on hot-smoked sea bream under constant and dynamic storage temperature conditions and (ii) to evaluate the capacity of three microbial interaction models based on the Jameson effect and Lotka-Volterra approaches to describe the inhibitory effect of L. sakei CTC494 on L. monocytogenes.