Quantifying the combined effect of salt and temperature on the growth of <i>Listeria</i> strains isolated from salmon and salmon processing environments

The effects of temperature and salt concentration on the growth of relevant strains of L. monocytogenes and Listeria innocua, were quantified based on growth in a liquid broth medium, monitored by measuring turbidity. Absorbance curves were used to estimate maximum growth rate and lag time. The growth rates increase with increasing temperatures. The levels were quite similar for both species, although L. monocytogenes showed higher growth rate than L. innocua at 4 °C. Lag time was more affected both by temperature and salt concentration, and these effects were most prominent for L. innocua.</i

Role of growth morphology in the formulation of NaCl-based selective media for injury detection of Escherichia coli, Salmonella Typhimurium and Listeria innocua

Sublethal injury (SI) poses major public health concerns since injured cells are responsible for serious limitations in food diagnostics and are susceptible to recovery, often developing adaptive stress responses. Detection of SI is based on the difference in plate counts between non-selective media, which represent the total cell population, and selective media, to which injured cells become sensitive. Selective media for detection of sublethal membrane damage are often based on NaCl supplement, although there is a lack of consensus in the literature about appropriate levels. Planktonic cells are generally used to investigate SI mechanisms, although they often exhibit different stress tolerance than cell colonies in/on solid food (model) systems. In this work, the effect of growth morphology, colony size and concentration of the gelling agent in the growth media, on the maximum non-inhibitory NaCl concentration in the plating medium was assessed for Escherichia coli, Salmonella Typhimurium and Listeria innocua. Stationary phase cultures of planktonic cells and large and small colonies grown in either 1.5% (w/v) xanthan gum-based system or 2.5% (w/v) xanthan gum-based system exhibited significantly different viable counts and osmotolerance. The effect of cell arrangement and xanthan gum percentage in the growth media depended on the microorganism under investigation. Additionally, differences in the maximum non-inhibitory concentration were evident, with 5.0% (w/v) NaCl for the Gram-negative bacteria and 6.5% (w/v), for L. innocua. Different extent of colony shrinkage and morphological damage was observed as NaCl concentration in the plating medium increased. This information will contribute to define NaCl-based selective media for accurate SI detection under realistic scenarios. © 2014

Role of growth morphology in the formulation of NaCl-based selective media for injury detection of Escherichia coli, Salmonella Typhimurium and Listeria innocua

Sublethal injury (SI) poses major public health concerns since injured cells are responsible for serious limitations in food diagnostics and are susceptible to recovery, often developing adaptive stress responses. Detection of SI is based on the difference in plate counts between non-selective media, which represent the total cell population, and selective media, to which injured cells become sensitive. Selective media for detection of sublethal membrane damage are often based on NaCl supplement, although there is a lack of consensus in the literature about appropriate levels. Planktonic cells are generally used to investigate SI mechanisms, although they often exhibit different stress tolerance than cell colonies in/on solid food (model) systems. In this work, the effect of growth morphology, colony size and concentration of the gelling agent in the growth media, on the maximum non-inhibitory NaCl concentration in the plating medium was assessed for Escherichia coli, Salmonella Typhimurium and Listeria innocua. Stationary phase cultures of planktonic cells and large and small colonies grown in either 1.5% (w/v) xanthan gum-based system or 2.5% (w/v) xanthan gum-based system exhibited significantly different viable counts and osmotolerance. The effect of cell arrangement and xanthan gum percentage in the growth media depended on the microorganism under investigation. Additionally, differences in the maximum non-inhibitory concentration were evident, with 5.0% (w/v) NaCl for the Gram-negative bacteria and 6.5% (w/v), for L. innocua. Different extent of colony shrinkage and morphological damage was observed as NaCl concentration in the plating medium increased. This information will contribute to define NaCl-based selective media for accurate SI detection under realistic scenarios. © 2014

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

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

Protective role of indigenous Leuconostoc carnosum against Listeria monocytogenes on vacuum packed Frankfurter sausages at suboptimal temperatures

© 2014. Listeria monocytogenes is one of the main target pathogens in the food industry. Novel strategies are continuously being investigated to ensure its absence from food products, such as the use of Lactic Acid Bacteria (LAB). They are known for their inhibitory action against pathogenic species due to the production of antimicrobial compounds and the competition for nutrients. In this work, the combined effect of storage temperature and LAB inoculum level on L. monocytogenes growth and the interaction between both microorganisms is examined on heat-treated Frankfurters. The indigenous LAB isolate from Frankfurter sausages was further identified as Leuconostoc carnosum and characterised as neither a bacteriocinogenic nor H2O2-producing species. However, it produces weak organic acids that acidify the food product and, overall, competes with L. monocytogenes for nutrients. Experiments were performed with vacuum packed, surface inoculated sausages, at different static temperatures (4, 8, 12 and 25°C) and inoculum levels of L. carnosum (102, 103 and 104CFU/g) and 102CFU/g L. monocytogenes. Results showed that at low temperature and high L. carnosum inoculum level, L. monocytogenes stops growing earlier than L. carnosum and the lowest maximum population is reached. The Dens et al. model described species interactions in a mechanistic way, revealing a predominant effect of L. carnosum on L. monocytogenes, and describing the decrease phase of Listeria population. The Baranyi and Roberts model, a special case of the Dens et al. model for monoculture, estimated apparent maximum population levels and brought into question the validity of the Jameson effect at low temperature. Results illustrated that indigenous species of meat products can be protective against foodborne pathogens.publisher: Elsevier articletitle: Protective role of indigenous Leuconostoc carnosum against Listeria monocytogenes on vacuum packed Frankfurter sausages at suboptimal temperatures journaltitle: Food Research International articlelink: http://dx.doi.org/10.1016/j.foodres.2014.08.011 content_type: article copyright: Copyright © 2014 Published by Elsevier Ltd.status: publishe

Optimal experimenal design for microbial inactivation

31 pagesN

Heat inactivation of Escherichia coli K12 MG1655: effect of microbial metabolites and acids in spent medium

Aim: The effect of spent medium, obtained after different time-temperature pre-histories, on the heat inactivation of Escherichia coli K12 MG1655 is studied. Methods and results: Stationary E. coli cells were heated in BHI broth (initial pH 7.5) at different time-temperature scenarios, i.e., (1) 30 °C to 55 °C at 0.14 °C/min, (2) 30 °C to 42 °C at 0.14 °C/min and (3) 30 °C to 42 °C at 0.8 °C/min. After the heat treatment, spent medium was filter-sterilized, non-stressed cells were added and inactivation experiments took place at 54 °C and 58 °C. In all scenarios, increased resistance was observed. The main characteristics of the spent medium - compared to the unmodified BHI broth - are (1) the presence of proteins (proven via SDS-PAGE) and (2) a lower pH of approximately 6. Possibly, the increased resistance is due to these proteins and/or the lower pH. Further experiments revealed that each factor separately may lead to an increased heat resistance. Conclusions: It can be concluded that this increased heat resistance resulted from both the presence of the heat shock proteins in the spent medium and the lowered pH. Experiments, which separate both effects, showed that mainly the lower pH resulted in the increased thermotolerance. Significance and impact of study: This study may lead to a better understanding and control of the heat stress adaptation phenomenon as displayed by E. coli at lethal temperatures. Therefore, it contributes to an improved assessment of the effect of temperature during thermal processes in the food industry. © 2011

Optimal experimental design for discriminating between microbial growth models as function of suboptimal temperature.

International audienceIn the field of predictive microbiology, mathematical models play an important role for describing microbial growth, survival and inactivation. Often different models are available for describing the microbial dynamics in a similar way. However, the model that describes the system in the best way is desired. Optimal experimental design for model discrimination (OED-MD) is an efficient tool for discriminating among rival models. In this work the T12-criterion proposed by Atkinson and Fedorov (1975) [1] and applied efficiently by Ucinski and Bogacka (2005) [2] and the Schwaab-approach proposed by Schwaab et al. (2008) [3] and Donckels et al. (2009) [4] will be applied for discriminating among rival models for the microbial growth rate as a function of temperature. The two methods will be tested in silico and their performances will be compared. Results from a simulation study indicate that it is possible to validate the case that one of the proposed models is more accurate for describing the temperature effect on the microbial growth rate. Both methods are able to design inputs with a sufficient discrimination potential. However, it has been observed that the Schwaab-approach provides inputs with a higher discrimination potential in combination with more accurate parameter estimates

Heat inactivation of Escherichia coli K12 MG1655: Effect of microbial metabolites and acids in spent medium

Aim: The effect of spent medium, obtained after different time-temperature pre-histories, on the heat inactivation of Escherichia coli K12 MG1655 is studied. Methods and results: Stationary E. coli cells were heated in BHI broth (initial pH 7.5) at different time-temperature scenarios, i.e., (1) 30 °C to 55 °C at 0.14 °C/min, (2) 30 °C to 42 °C at 0.14 °C/min and (3) 30 °C to 42 °C at 0.8 °C/min. After the heat treatment, spent medium was filter-sterilized, non-stressed cells were added and inactivation experiments took place at 54 °C and 58 °C. In all scenarios, increased resistance was observed. The main characteristics of the spent medium - compared to the unmodified BHI broth - are (1) the presence of proteins (proven via SDS-PAGE) and (2) a lower pH of approximately 6. Possibly, the increased resistance is due to these proteins and/or the lower pH. Further experiments revealed that each factor separately may lead to an increased heat resistance. Conclusions: It can be concluded that this increased heat resistance resulted from both the presence of the heat shock proteins in the spent medium and the lowered pH. Experiments, which separate both effects, showed that mainly the lower pH resulted in the increased thermotolerance. Significance and impact of study: This study may lead to a better understanding and control of the heat stress adaptation phenomenon as displayed by E. coli at lethal temperatures. Therefore, it contributes to an improved assessment of the effect of temperature during thermal processes in the food industry. © 2011