Effect of immobilization and salt concentration on the growth dynamics of Escherichia coli K12 and Salmonella typhimurium

Unravelling the impact of fat content on the microbial dynamics and spatial distribution of foodborne bacteria in tri-phasic viscoelastic 3D models

The aim of the current study is to develop and characterise novel complex multi-phase in vitro 3D models, for advanced microbiological studies. More specifically, we enriched our previously developed bi-phasic polysaccharide (Xanthan Gum)/protein (Whey Protein) 3D model with a fat phase (Sunflower Oil) at various concentrations, i.e., 10%, 20%, 40% and 60% (v/v), for better mimicry of the structural and biochemical composition of real food products. Rheological, textural, and physicochemical analysis as well as advanced microscopy imaging (including spatial mapping of the fat droplet distribution) of the new tri-phasic 3D models revealed their similarity to industrial food products (especially cheese products). Furthermore, microbial growth experiments of foodborne bacteria, i.e., Listeria monocytogenes, Escherichia coli, Pseudomonas aeruginosa and Lactococcus lactis on the surface of the 3D models revealed very interesting results, regarding the growth dynamics and distribution of cells at colony level. More specifically, the size of the colonies formed on the surface of the 3D models, increased substantially for increasing fat concentrations, especially in mid- and late-exponential growth phases. Furthermore, colonies formed in proximity to fat were substantially larger as compared to the ones that were located far from the fat phase of the models. In terms of growth location, the majority of colonies were located on the protein/polysaccharide phase of the 3D models. All those differences at microscopic level, that can directly affect the bacterial response to decontamination treatments, were not captured by the macroscopic kinetics (growth dynamics), which were unaffected from changes in fat concentration. Our findings demonstrate the importance of developing structurally and biochemically complex 3D in vitro models (for closer proximity to industrial products), as well as the necessity of conducting multi-level microbial analyses, to better understand and predict the bacterial behaviour in relation to their biochemical and structural environment. Such studies in advanced 3D environments can assist a better/more accurate design of industrial antimicrobial processes, ultimately, improving food safety.

On the evaluation of the antimicrobial effect of grape seed extract and cold atmospheric plasma on the dynamics of Listeria monocytogenes in novel multiphase 3D viscoelastic models

The demand for products that are minimally processed and produced in a sustainable way, without the use of chemical preservatives or antibiotics have increased over the last years. Novel non-thermal technologies such as cold atmospheric plasma (CAP) and natural antimicrobials such as grape seed extract (GSE) are attractive alternatives to conventional food decontamination methods as they can meet the above demands. The aim of this study was to investigate the microbial inactivation potential of GSE, CAP (in this case, a remote air plasma with an ozone-dominated RONS output) and their combination against L. monocytogenes on five different 3D in vitro models of varying rheological, structural, and biochemical composition. More specifically, we studied the microbial dynamics, as affected by 1 % (w/v) GSE, CAP or their combination, in three monophasic Xanthan Gum (XG) based 3D models of relatively low viscosity (1.5 %, 2.5 % and 5 % w/v XG) and in a biphasic XG/Whey Protein (WPI) and a triphasic XG/WPI/fat model. A significant microbial inactivation (comparable to liquid broth) was achieved in presence of GSE on the surface of all monophasic models regardless of their viscosity. In contrast, the GSE antimicrobial effect was diminished in the multiphasic systems, resulting to only a slight disturbance of the microbial growth. In contrast, CAP showed better antimicrobial potential on the surface of the complex multiphasic models as compared to the monophasic models. When combined, in a hurdle approach, GSE/CAP showed promising microbial inactivation potential in all our 3D models, but less microbial inactivation in the structurally and biochemically complex multiphasic models, with respect to the monophasic models. The level of inactivation also depended on the duration of the exposure to GSE. Our results contribute towards understanding the antimicrobial efficacy of GSE, CAP and their combination as affected by robustly controlled changes of rheological and structural properties and of the biochemical composition of the environment in which bacteria grow. Therefore, our results contribute to the development of sustainable food safety strategies.

WHEELER AND. Fate of Shiga Toxin-Producing Escherichia coli (STEC) and Salmonella during Kosher Processing of Fresh Beef

Traditional kosher meat processing involves the following steps after slaughtering: soaking with water to remove blood, salting to help draw out more blood, and rinsing to remove salt. However, the impact of the salt used on foodborne pathogens and beef quality is not well understood. The objectives of the current study were to determine effectiveness of salt in reducing pathogens in a pure culture model, on surfaces of inoculated fresh beef during kosher processing, and the effect of salt on beef quality. The pure culture studies indicated that the reduction of E. coli O157:H7, non-O157 STEC, and Salmonella increased with increasing salt concentrations. With salt concentrations from 3 to 13%, salt reduced E. coli O157:H7, non-O157 STEC, and Salmonella ranging from 0.49 to 1.61 log CFU/ml. For kosher processing, the water soaking step did not reduce pathogenic and other bacteria on the surface of fresh beef. Salting and rinsing steps reduced non-O157 STEC, E. coli O157:H7, and Salmonella ranging from 0.83 to 1.42 log CFU/cm², and reduced Enterobacteriaceae, coliforms, and aerobic bacteria by 1.04, 0.95, and 0.70 log CFU/cm², respectively. The salting process for kosher beef resulted in reducing pathogens on the surface of fresh beef, color changes, increased salt residues, and increased lipid oxidation on the final products.

The Inclusion of the Food Microstructural Influence in Predictive Microbiology: State-of-the-Art

Predictive microbiology has steadily evolved into one of the most important tools to assess and control the microbiological safety of food products. Predictive models were traditionally developed based on experiments in liquid laboratory media, meaning that food microstructural effects were not represented in these models. Since food microstructure is known to exert a significant effect on microbial growth and inactivation dynamics, the applicability of predictive models is limited if food microstructure is not taken into account. Over the last 10-20 years, researchers, therefore, developed a variety of models that do include certain food microstructural influences. This review provides an overview of the most notable microstructure-including models which were developed over the years, both for microbial growth and inactivation.

Co-culturing microbial consortia: approaches for applications in biomanufacturing and bioprocessing

The application of microbial co-cultures is now recognized in the fields of biotechnology, ecology, and medicine. Understanding the biological interactions that govern the association of microorganisms would shape the way in which artificial/synthetic co-cultures or consortia are developed. The ability to accurately predict and control cell-to-cell interactions fully would be a significant enabler in synthetic biology. Co-culturing method development holds the key to strategically engineer environments in which the co-cultured microorganism can be monitored. Various approaches have been employed which aim to emulate the natural environment and gain access to the untapped natural resources emerging from cross-talk between partners. Amongst these methods are the use of a communal liquid medium for growth, use of a solid-liquid interface, membrane separation, spatial separation, and use of microfluidics systems. Maximizing the information content of interactions monitored is one of the major challenges that needs to be addressed by these designs. This review critically evaluates the significance and drawbacks of the co-culturing approaches used to this day in biotechnological applications, relevant to biomanufacturing. It is recommended that experimental results for a co-cultured species should be validated with different co-culture approaches due to variations in interactions that could exist as a result of the culturing method selected.

Εvaluation of oxygen availability on growth and inter-strain interactions of L. monocytogenes in/on liquid, semi-solid and solid laboratory media

The coexistence and interactions among Listeria monocytogenes strains in combination with the structural characteristics of foods, may influence their growth capacity and thus, the final levels at the time of consumption. In the present study, we aimed to evaluate the effect of oxygen availability in combination with substrate micro-structure on growth and inter-strain interactions of L. monocytogenes. L. monocytogenes strains, selected for resistance to different antibiotics (to enable distinct enumeration), belonging to serotypes 4b (C5, ScottA), 1/2a (6179) and 1/2b (PL25) and were inoculated in liquid (Tryptic Soy Broth supplemented with Yeast Extract - TSB-YE) and solid (TSB-YE supplemented with 0.6% and 1.2% agar) media (2-3 log CFU/mL, g or cm²), single or as two-strain cultures (1:1 strain-ratio). Aerobic conditions (A) were achieved with constant shaking or surface inoculation for liquid and solid media respectively, while static incubation or pour plated media corresponded to hypoxic environment (H). Anoxic conditions (An) were attained by adding 0.1% w/v sodium thioglycolate and paraffin overlay (for solid media). Growth was assessed during storage at 7 °C (n = 3 × 2). Inter-strain interactions were manifested by the difference in the final population between singly and co-cultured strains. Τhe extent of suppression increased with reduction in agar concentration, while the impact of oxygen availability was dependent on strain combination. During co-culture, in liquid and solid media, 6179 was suppressed by C5 by 4.0 (in TSB-YE under H) to 1.8 log units (in solid medium under An), compared to the single culture, which attained population of ca. 9.4 log CFU/mL or g. The growth of 6179 was also inhibited by ScottA by 2.7 and 1.9 log units, in liquid culture under H and An, respectively. Interestingly, in liquid medium under A, H and An, ScottA was suppressed by C5, by 3.3, 2.4 and 2.3 log units, respectively, while in solid media, growth inhibition was less pronounced. Investigating growth interactions in different environments could assist in explaining the dominance of L. monocytogenes certain serotypes.

Resistance Profiles of Salmonella Isolates Exposed to Stresses and the Expression of Small Non-coding RNAs

Salmonella can resist various stresses and survive during food processing, storage, and distribution, resulting in potential health risks to consumers. Therefore, evaluation of bacterial survival profiles under various environmental stresses is necessary. In this study, the resistance profiles of five Salmonella isolates [serotypes with Agona, Infantis, Typhimurium, Enteritidis, and a standard strain (ATCC 13076, Enteritidis serotype)] to acidic, hyperosmotic, and oxidative stresses were examined, and the relative expressions of non-coding small RNAs were also evaluated, including CyaR, MicC, MicA, InvR, RybB, and DsrA, induced by specific stresses. The results indicated that although all tested strains displayed a certain resistance to stresses, there was great diversity in stress resistance among the strains. According to the reduction numbers of cells exposed to stress for 3 h, S. Enteritidis showed the highest resistance to acidic and hyperosmotic stresses, whereas ATCC 13076 showed the greatest resistance to oxidative stress, with less than 0.1 Log CFU/ml of cell reduction. Greater resistance of cells to acidic, hyperosmotic, and oxidative stresses was observed within 1 h, after 2 h, and from 1 to 2 h, respectively. The relative expression of sRNAs depended on the isolate for each stress; acidic exposure for the tested isolates induced high expression levels of DsrA, MicC, InvR, RybB, MicA, and CyaR. The sRNA RybB, associated with sigma E and outer membrane protein in bacteria, showed a fold change of greater than 7 in S. Enteritidis exposed to the tested stresses. CyaR and InvR involved in general stress responses and stress adaptation were also induced to show high expression levels of Salmonella exposed to hyperosmotic stress. Overall, these findings demonstrated that the behaviors of Salmonella under specific stresses varied according to strain and were likely not related to other profiles. The finding also provided insights into the survival of Salmonella subjected to short-term stresses and for controlling Salmonella in the food industry.

Molecular detection of Salmonella spp., Listeria spp. and Brucella spp. in fresh artisanal cheese marketed in the city of Barranquilla: A pilot study

Introduction: Each year approximately 3 million people die as the result of foodborne diseases. The fresh artisan (handmade) cheese produced and distributed in the Colombian Caribbean region is a native product from the departments of Córdoba, Sucre, Bolívar, Atlántico, Magdalena, Cesar, and La Guajira. Its mass consumption increases the risk of infection with Salmonella spp., Listeria spp., and Brucella spp., as it is made with a very rustic technology, with unpasteurized cow milk, without standardized and hygienic procedures and its storage is inadequate. Objective: To detect the presence of Salmonella spp., Listeria spp., and Brucella spp. in samples of fresh artisan cheese from the Colombian Caribbean region. Materials and methods: Twenty-seven samples of cheese from five departments of the Caribbean Region (Atlántico (n=6), Bolívar (n=2), Córdoba (n=1), Magdalena (n=16), and Sucre (n=2)) were analyzed by real-time polymerase chain reaction (qPCR). Seventeen of the samples corresponded to soft cheese, five to semi-hard cheese and five to hard cheese. Results: In 62.9% (17/27) of the samples we detected Salmonella spp., in 70.4% (19/27), Listeria spp., and in 22.2% (6/27), Brucella spp., mainly from the department of Magdalena. In 62.5% (10/16) of the samples we detected Salmonella spp. and Listeria spp. while in the department of Atlántico, 50% (3/6) of the samples corresponded to Brucella spp. Conclusion: The results confirmed the presence of these microorganisms in all the samples of soft cheese from the Colombian Caribbean region.

Modelling the microbial dynamics and antimicrobial resistance development of Listeria in viscoelastic food model systems of various structural complexities

Minimal processing for microbial decontamination, such as the use of natural antimicrobials, is gaining interest in the food industry as these methods are generally milder than conventional processing, therefore better maintaining the nutritional content and sensory characteristics of food products. The aim of this study was to quantify the impact of (i) structural composition and complexity, (ii) growth location and morphology, and (iii) the natural antimicrobial nisin, on the microbial dynamics of Listeria innocua. More specifically, viscoelastic food model systems of various compositions and internal structure were developed and characterised, i.e. monophasic Xanthan gum-based and biphasic Xanthan gum/Whey protein-based viscoelastic systems. The microbial dynamics of L. innocua at 10 °C, 30 °C and 37 °C were monitored and compared for planktonic growth in liquid, or in/on (immersed or surface colony growth) the developed viscoelastic systems, with or without a sublethal concentration of nisin. Microscopy imaging was used to determine the bacterial colony size and spatial organisation in/on the viscoelastic systems. Selective growth of L. innocua on the protein phase of the developed biphasic system was observed for the first time. Additionally, significant differences were observed in the colony size and distribution in the monophasic Xanthan gum-based systems depending on (i) the type of growth (surface/immersed) and (ii) the Xanthan gum concentration. Furthermore, the system viscosity in monophasic Xanthan gum-based systems had a protective role against the effects of nisin for immersed growth, and a further inhibitory effect for surface growth at a suboptimal temperature (10 °C). These findings give a systematic quantitative insight on the impact of nisin as an environmental challenge on the growth and spatial organisation of L. innocua, in viscoelastic food model systems of various structural compositions/complexities. This study highlights the importance of accounting for system structural composition/complexity when designing minimal food processing methods with natural antimicrobials.

Metabolic Responses of Bacterial Cells to Immobilization

In recent years immobilized cells have commonly been used for various biotechnological applications, e.g., antibiotic production, soil bioremediation, biodegradation and biotransformation of xenobiotics in wastewater treatment plants. Although the literature data on the physiological changes and behaviour of cells in the immobilized state remain fragmentary, it is well documented that in natural settings microorganisms are mainly found in association with surfaces, which results in biofilm formation. Biofilms are characterized by genetic and physiological heterogeneity and the occurrence of altered microenvironments within the matrix. Microbial cells in communities display a variety of metabolic differences as compared to their free-living counterparts. Immobilization of bacteria can occur either as a natural phenomenon or as an artificial process. The majority of changes observed in immobilized cells result from protection provided by the supports. Knowledge about the main physiological responses occurring in immobilized cells may contribute to improving the efficiency of immobilization techniques. This paper reviews the main metabolic changes exhibited by immobilized bacterial cells, including growth rate, biodegradation capabilities, biocatalytic efficiency and plasmid stability.

Influence of food intrinsic complexity on Listeria monocytogenes growth in/on vacuum-packed model systems at suboptimal temperatures

Food intrinsic factors e.g., food (micro)structure, compositional and physicochemical aspects, which are mutually dependent, influence microbial growth. While the effect of composition and physicochemical properties on microbial growth has been thoroughly assessed and characterised, the role of food (micro)structure still remains unravelled. Most studies on food (micro)structure focus on comparing planktonic growth in liquid (microbiological) media with colonial growth in/on solid-like systems or on real food surfaces. However, foods are not only liquids or solids; they can also be emulsions or gelled emulsions and have complex compositions. In this study, Listeria monocytogenes growth was studied on the whole spectrum of (micro)structure, in terms of food (model) systems. The model systems varied not only in (micro)structure, which was the target of the study, but also in compositional and physicochemical characteristics, which was an inevitable consequence of the (micro)structural variability. The compositional and physicochemical differences were mainly due to the presence or absence of fat and gelling agents. The targeted (micro)structures were: i) liquids, ii) aqueous gels, iii) emulsions and iv) gelled emulsions. Furthermore, the microbial dynamics were studied and compared in/on all these model systems, as well as on a compositionally predefined canned meat, developed in order to have equal compositional level to the gelled emulsion model system and represent a real food system. Frankfurter sausages were the targeted real foods, selected as a case study, to which the canned meat had similar compositional characteristics. All systems were vacuum packed and incubated at 4, 8 and 12°C. The most appropriate protocol for the preparation of the model systems was developed. The pH, water activity and resistance to penetration of the model systems were characterised. Results indicated that low temperature contributes to growth variations among the model systems. Additionally, the firmer the solid system, the faster L. monocytogenes grew on it. Finally, it was found that L. monocytogenes grows faster on canned meat and real Frankfurters, as found in a previous study, followed by liquids, aqueous gels, emulsions and gelled emulsions. This observation indicates that all model systems, developed in this study, underestimated L. monocytogenes growth. Despite some limitations, model systems are overall advantageous and therefore, their validation is always recommended prior to further use.

Effect of stabilizers, oil level and structure on the growth of Zygosaccharomyces bailii and on physical stability of model systems simulating acid sauces

The effect of xanthan gum, guar gum, oil and the structure promoted by these compounds on the growth of Zygosaccharomyces bailii and on physical stability of emulsified systems simulating acid sauces was studied. Furthermore, the effect of yeast growth on physical stability of emulsions was also evaluated. Yeast growth was evaluated by plate count and modeled by the modified Gompertz equation. Emulsions characteristics and their stability were determined by droplet size, zeta potential and rheological measurements. The latter was also used to evaluate structure's effect on yeast growth. Physical characteristics of emulsions depended on system composition. Yeasts slightly affected droplet size. Z. bailii growth was satisfactorily modeled by the modified Gompertz equation. The specific growth rate (μ_m) and the asymptotic value (A) obtained depended on xanthan gum, guar gum and oil content. Furthermore, the structure promoted by these compounds exerted a significant effect on growth. In general, an increase in the solid character and yield stress through the addition of xanthan gum promoted a decrease in A parameter. On the contrary, a decrease in the solid character through the addition of guar gum promoted an increase in the A parameter. The results obtained stressed that stabilizers, oil and their structuring ability play an important role on Z. bailii growth.

Colonial vs. planktonic type of growth: mathematical modeling of microbial dynamics on surfaces and in liquid, semi-liquid and solid foods

Predictive models are mathematical expressions that describe the growth, survival, inactivation, or biochemical processes of foodborne bacteria. During processing of contaminated raw materials and food preparation, bacteria are entrapped into the food residues, potentially transferred to the equipment surfaces (abiotic or inert surfaces) or cross-contaminate other foods (biotic surfaces). Growth of bacterial cells can either occur planktonically in liquid or immobilized as colonies. Colonies are on the surface or confined in the interior (submerged colonies) of structured foods. For low initial levels of bacterial population leading to large colonies, the immobilized growth differs from planktonic growth due to physical constrains and to diffusion limitations within the structured foods. Indeed, cells in colonies experience substrate starvation and/or stresses from the accumulation of toxic metabolites such as lactic acid. Furthermore, the micro-architecture of foods also influences the rate and extent of growth. The micro-architecture is determined by (i) the non-aqueous phase with the distribution and size of oil particles and the pore size of the network when proteins or gelling agent are solidified, and by (ii) the available aqueous phase within which bacteria may swarm or swim. As a consequence, the micro-environment of bacterial cells when they grow in colonies might greatly differs from that when they grow planktonically. The broth-based data used for modeling (lag time and generation time, the growth rate, and population level) are poorly transferable to solid foods. It may lead to an over-estimation or under-estimation of the predicted population compared to the observed population in food. If the growth prediction concerns pathogen bacteria, it is a major importance for the safety of foods to improve the knowledge on immobilized growth. In this review, the different types of models are presented taking into account the stochastic behavior of single cells in the growth of a bacterial population. Finally, the recent advances in the rules controlling different modes of growth, as well as the methodological approaches for monitoring and modeling such growth are detailed.

Effect of cell immobilization on the growth dynamics of Salmonella Typhimurium and Escherichia coli at suboptimal temperatures

Predictive microbiology has recently acknowledged the impact of the solid(like) food structure on microbial behavior. The presence of this solid(like) structure causes microorganisms to grow as colonies and no longer planktonically as in liquid. In this paper, the growth dynamics of Salmonella Typhimurium and Escherichia coli were studied as a function of temperature, considering different growth morphologies, i.e., (i) planktonic cells, (ii) immersed colonies and (iii) surface colonies. For all three growth morphologies, both microorganisms were grown in petri dishes. While E. coli was grown under optimal pH and water activity (aw), for S. Typhimurium pH and aw were adapted to 5.5 and 0.990. In order to mimic a solid(like) environment, 5% (w/v) gelatin was added. All petri dishes were incubated under static conditions at temperatures in the range [8.0°C-22.0°C]. Cell density was determined via viable plate counting. This work demonstrates that the growth morphology (planktonic vs. colony) has a negligible effect on the growth dynamics as a function of temperature. The observation of almost equal growth rates for planktonic cultures and colonies is in contrast to literature where, mostly, a difference is observed, i.e., μplanktonic cells≥μimmersed colonies≥μsurface colonies. This difference might be due to shaking of the liquid culture in these studies, which results in a nutrient and oxygen rich environment, in contrast to the diffusion-limited gel system. Experiments also indicate that lag phases for solid(like) systems are similar to those for the planktonic cultures, as can be found in literature for similar growth conditions. Considering the maximum cell density, no clear trend was deducted for either of the microorganisms. This study indicates that the growth parameters in the suboptimal temperature range do not depend on the growth morphology. For the considered experimental conditions, models previously developed for liquid environments can be used for solid(like) systems.

Recent trends in non-invasive in situ techniques to monitor bacterial colonies in solid (model) food

Planktonic cells typically found in liquid systems, are routinely used for building predictive models or assessing the efficacy of food preserving technologies. However, freely suspended cells often show different susceptibility to environmental hurdles than colony cells in solid matrices. Limited oxygen, water and nutrient availability, metabolite accumulation and physical constraints due to cell immobilization in the matrix, are main factors affecting cell growth. Moreover, intra- and inter-colony interactions, as a consequence of the initial microbial load in solid systems, may affect microbial physiology. Predictive food microbiology approaches are moving toward a more realistic resemblance to food products, performing studies in structured solid systems instead of liquids. Since structured systems promote microbial cells to become immobilized and grow as colonies, it is essential to study the colony behavior, not only for food safety assurance systems, but also for understanding cell physiology and optimizing food production processes in solid matrices. Traditionally, microbial dynamics in solid systems have been assessed with a macroscopic approach by applying invasive analytical techniques; for instance, viable plate counting, which yield information about overall population. In the last years, this approach is being substituted by more mechanistically inspired ones at mesoscopic (colony) and microscopic (cell) levels. Therefore, non-invasive and in situ monitoring is mandatory for a deeper insight into bacterial colony dynamics. Several methodologies that enable high-throughput data collection have been developed, such as microscopy-based techniques coupled with image analysis and OD-based measurements in microplate readers. This research paper provides an overview of non-invasive in situ techniques to monitor bacterial colonies in solid (model) food and emphasizes their advantages and inconveniences in terms of accuracy, performance and output information.

Effect of microstructure on population growth parameters of Escherichia coli in gelatin-dextran systems

Current literature acknowledges the effect of food structure on bacterial dynamics. Most studies introduce this "structure" factor using a single gelling agent, resulting in a homogeneous environment, whereas in practice most food products are heterogeneous. Therefore, this study focuses on heterogeneous protein-polysaccharide mixtures, based on gelatin and dextran. These mixtures show phase separation, leading to a range of heterogeneous microstructures by adjusting relative concentrations of both gelling agents. Based on confocal microscope observations, the growth of Escherichia coli in gelatin-dextran systems was observed to occur in the dextran phase. To find a relation between microscopic and population behavior, growth experiments were performed in binary and singular gelatin-dextran systems and culture broth at 23.5°C, with or without adding 2.9% (wt/vol) NaCl. The Baranyi and Roberts growth model was fitted to the experimental data and parameter estimates were statistically compared. For salted binary mixtures, a decrease in the population maximum cell density was observed with increasing gelatin concentration. In this series, for one type of microstructure, i.e., a gelatin matrix phase with a disperse dextran phase, the maximum cell density decreased with decreasing percentage of dextran phase. However, this relation no longer held when other types of microstructure were observed. Compared to singular systems, adding a second gelling agent in the presence of NaCl had an effect on population lag phases and maximum cell densities. For unsalted media, the growth parameters of singular and binary mixtures were comparable. Introducing this information into mathematical models leads to more reliable growth predictions and enhanced food safety.

Effect of cell immobilization on heat-induced sublethal injury of Escherichia coli, Salmonella Typhimurium and Listeria innocua

The occurrence of sublethally injured cells in foods poses major public health concerns and is an essential aspect when assessing the microbial response to food preservation strategies, yet there is limited research dealing with its specific implications for mild heating. All available studies so far have been performed in broths colonized by planktonic cells, although their susceptibility to lethal agents has often been reported to be markedly different to the stress tolerance of cell colonies developed in solid foods. In this work, the effect of planktonic and colony growth, as well as the influence of colony density on sublethal injury induced by mild heating of Escherichia coli, Salmonella Typhimurium and Listeria innocua were assessed in food model systems. Detection of injured survivors relied on their inability to form visible colonies on salt-based selective media, which do not affect the growth of healthy cells. Sublethal injury (SI) increased rapidly with shorter exposure times and afterwards, decreased progressively, suggesting a mechanism of cumulative damage triggering lethal instead of SI. Cell arrangement affected the degree of SI, higher values being generally found for gelified systems, although the effect of colony density depended on the target microorganism. This information is essential for optimizing the design of food safety assurance systems.