Quantitative analysis of population heterogeneity of the adaptive salt stress response and growth capacity of Bacillus cereus ATCC 14579

Thuricin17 Production and Proteome Differences in Bacillus thuringiensis NEB17 Cell-Free Supernatant Under NaCl Stress

Bacillus thuringiensis strain NEB17, produces a bacteriocin, thuricin17 (Th17) and is known to promote the growth more effectively under salt stress conditions. In this study, bacterial salt stress tolerance screening and the possible changes in its secretome under two levels of NaCl stress was evaluated. The salt tolerance screening suggested that the bacterium is able to grow and survive in up to 900 mM NaCl. Thuricin17 production at salt levels from 100 to 500 mM NaCl was quantified using High Performance Liquid Chromatography (HPLC). Salt stress adversely affected the production of Th17 at levels as low as 100 mM NaCl; and the production stopped at 500 mM NaCl, despite the bacterium thriving at these salt levels. Hence, a comparative proteomic study was conducted on the supernatant of the bacterium after 42 h of growth, when Th17 production peaked in the control culture, as determined by Liquid Chromatography - Tandem Mass Spectrometry (LC-MS/MS). Optimal (salt free) bacterial culture served as a control and 200 and 500 mM NaCl as stress conditions. As salt levels increased, the major enzyme classes, transferases, hydrolases, lyases, and ligases showed increased abundance as compared to the control, mostly related to molecular function mechanisms. Some of the notable up-regulated proteins in 500 mM NaCl stress conditions included an S-layer protein, chitin binding domain 3 protein, enterotoxins, phosphopentomutase, glucose 6-phosphate isomerase and bacterial translation initiation factor; while notable down-regulated proteins included hemolytic enterotoxin, phospholipase, sphingomyelinase C, cold shock DNA-binding protein family and alcohol dehydrogenase. These results indicate that, as the salt stress levels increase, the bacterium probably shuts down the production of Th17 and regulates its molecular functional mechanisms to overcome stress. This study indicates that end users have the option of using Th17 as a biostimulant or the live bacterial inoculum depending on the soil salt characteristics, for crop production. The mass spectrometry proteomics data have been deposited to Mass Spectrometry Interactive Virtual Environment (MassIVE) with the dataset identifier PXD024069, and doi: 10.25345/C5RB8T.

Recovery of Heat Treated Bacillus cereus Spores Is Affected by Matrix Composition and Factors with Putative Functions in Damage Repair

The ability of spores to recover and grow out after food processing is affected by cellular factors and by the outgrowth conditions. In the current communication we studied the recovery and outgrowth of individually sorted spores in BHI and rice broth media and on agar plates using flow cytometry. We show that recovery of wet heat treated Bacillus cereus ATCC 14579 spores is affected by matrix composition with highest recovery in BHI broth or on rice agar plates, compared to BHI agar plates and rice broth. Data show that not only media composition but also its liquid or solid state affect the recovery of heat treated spores. To determine the impact of factors with putative roles in recovery of heat treated spores, specific genes previously shown to be highly expressed in outgrowing heat-treated spores were selected for mutant construction. Spores of nine B. cereus ATCC 14579 deletion mutants were obtained and their recovery from wet heat treatment was evaluated using BHI and rice broth and agar plates. Deletion mutant spores showed different capacity to recover from heat treatment compared to wild type with the most pronounced effect for a mutant lacking BC5242, a gene encoding a membrane protein with C2C2 zinc finger which resulted in over 95% reduction in recovery compared to the wild type in BHI broth. Notably, similar relative performance of wild type and mutants was observed using the other recovery conditions. We obtained insights on the impact of matrix composition and state on recovery of individually sorted heat treated spores and identified cellular factors with putative roles in this process. These results may provide leads for future developments in design of more efficient combined preservation treatments.

Bacterial Colonies in Solid Media and Foods: A Review on Their Growth and Interactions with the Micro-Environment

Bacteria, either indigenous or added, are immobilized in solid foods where they grow as colonies. Since the 80's, relatively few research groups have explored the implications of bacteria growing as colonies and mostly focused on pathogens in large colonies on agar/gelatine media. It is only recently that high resolution imaging techniques and biophysical characterization techniques increased the understanding of the growth of bacterial colonies, for different sizes of colonies, at the microscopic level and even down to the molecular level. This review covers the studies on bacterial colony growth in agar or gelatine media mimicking the food environment and in model cheese. The following conclusions have been brought to light. Firstly, under unfavorable conditions, mimicking food conditions, the immobilization of bacteria always constrains their growth in comparison with planktonic growth and increases the sensibility of bacteria to environmental stresses. Secondly, the spatial distribution describes both the distance between colonies and the size of the colonies as a function of the initial level of population. By studying the literature, we concluded that there systematically exists a threshold that distinguishes micro-colonies (radius < 100-200 μm) from macro-colonies (radius >200 μm). Micro-colonies growth resembles planktonic growth and no pH microgradients could be observed. Macro-colonies growth is slower than planktonic growth and pH microgradients could be observed in and around them due to diffusion limitations which occur around, but also inside the macro-colonies. Diffusion limitations of milk proteins have been demonstrated in a model cheese around and in the bacterial colonies. In conclusion, the impact of immobilization is predominant for macro-colonies in comparison with micro-colonies. However, the interaction between the colonies and the food matrix itself remains to be further investigated at the microscopic scale.

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.

Porous Alumina as a Promising Biomaterial for Public Health

Porous aluminum is a nanostructured material characterized by unique properties, such as chemical stability, regular uniformity, dense hexagonal porous lattice with high aspect ratio nanopores, excellent mechanical strength, and biocompatibility. This overview examines how the structure and properties of porous alumina can be exploited in the field of public health. Porous alumina can be employed for fabricating membranes and filters for bioremediation, water ultrafiltration, and microfiltration/nanofiltration, being a promising technique for having clean and fresh water, which is essential for human health. Porous alumina-based nanobiosensor coated with specific antibodies or peptides seem to be a useful tool to detect and remove pathogens both in food and in water, as well as for environmental monitoring. Further, these applications, being low-energy demanding and cost-effective, are particularly valuable in resource-limited settings and contexts, and can be employed as point of use devices in developing countries, where there is an urgent need of hygiene and safety assurance.

Influence of food matrix on outgrowth heterogeneity of heat damaged Bacillus cereus spores

Spoilage of heat treated foods can be caused by the presence of surviving spore-formers. It is virtually impossible to prevent contamination at the primary production level as spores are ubiquitous present in the environment and can contaminate raw products. As a result spore inactivation treatments are widely used by food producing industries to reduce the microbial spore loads. However consumers prefer mildly processed products that have less impact on its quality and this trend steers industry towards milder preservation treatments. Such treatments may result in damaged instead of inactivated spores, and these spores may germinate, repair, and grow out, possibly leading to quality and safety issues. The ability to repair and grow out is influenced by the properties of the food matrix. In the current communication we studied the outgrowth from heat damaged Bacillus cereus ATCC 14579 spores on Anopore membrane, which allowed following outgrowth heterogeneity of individual spores on broccoli and rice-based media as well as standard and mildly acidified (pH 5.5) meat-based BHI. Rice, broccoli and BHI pH 5.5 media resulted in delayed outgrowth from untreated spores, and increased heterogeneity compared to BHI pH 7.4, with the most pronounced effect in rice media. Exposure to wet heat for 1 min at 95 °C caused 2 log inactivation and approximately 95% of the spores in the surviving fraction were damaged resulting in substantial delay in outgrowth based on the time required to reach a maximum microcolony size of 256 cells. The delay was most pronounced for heat-treated spores on broccoli medium followed by spores on rice media (both untreated and treated). Interestingly, the increase in outgrowth heterogeneity of heat treated spores on BHI pH 7.4 was more pronounced than on rice, broccoli and BHI pH 5.5 conceivably reflecting that conditions in BHI pH 7.4 better support spore damage repair. This study compares the effects of three main factors, namely heat treatment, pH of BHI and the effect of food matrix highlighting the impact of different (model) food recovery media on outgrowth efficiency and heterogeneity of non-heat-treated and heat-damaged B. cereus spores.

Quantification of the impact of single and multiple mild stresses on outgrowth heterogeneity of Bacillus cereus spores

Outgrowth heterogeneity of bacterial spore populations complicates both prediction and efficient control of spore outgrowth. In this study, the impact of mild preservation stresses on outgrowth of Bacillus cereus ATCC 14579 spores was quantified during the first stages of outgrowth. Heterogeneity in outgrowth of heat-treated (90°C for 10 min) and non-heat-treated germinated single spores to the maximum micro-colony stage of 256 cells was assessed by direct imaging on Anopore strips, placed on BHI plates at pH7 and pH5.5, without and with added NaCl or sorbic acid (HSA). At pH7 non-heated and heat-treated germinated spores required 6h to reach the maximum microcolony stage with limited heterogeneity, and these parameters were only slightly affected with both types of spores when incubated at pH7 with added NaCl. Notably, the most pronounced effects were observed during outgrowth of spores at pH5.5 without and with added NaCl or HSA. Non-heat-treated germinated spores showed again efficient outgrowth with limited heterogeneity reaching the maximum microcolony stage after 6h at pH5.5, which increased to 12h and 16 h with added NaCl and HSA, respectively. In contrast, heat-treated spores displayed a strong delay between initial germination and swelling and further outgrowth at pH5.5, resulting in large heterogeneity and low numbers of fastest growers reaching the maximum microcolony stage after 10, 12 and 24h, without and with added NaCl or HSA, respectively. This work shows that Anopore technology provides quantitative information on the impact of combined preservation stresses on outgrowth of single spores, showing that outgrowth of germinated heat-treated spores is significantly affected at pH5.5 with a large fraction of spores arrested in the early outgrowth stage, and with outgrowing cells showing large heterogeneity with only a small fraction committed to relatively fast outgrowth.

Thermal inactivation kinetics of Shiga toxin-producing Escherichia coli in buffalo Mozzarella curd

The use of raw milk in the processing of buffalo Mozzarella cheese is permitted, but the heat treatment used for stretching the curd must ensure that the final product does not contain pathogens such as Shiga toxin-producing Escherichia coli (STEC) that may be present on buffalo dairy farms. This study carried out challenge tests at temperatures between 68 °C and 80 °C for 2 to 10 min to simulate curd temperatures during the stretching phase. Curd samples were inoculated with 2 STEC strains (serotypes O157 and O26), and their inactivation rates were assessed in the different challenge tests. The curd samples were digested with papain to ensure a homogeneous dispersion of bacteria. The STEC cells were counted after inoculation (range 7.1-8.7 log cfu/g) and after heat treatments using the most probable number (MPN) technique. A plot of log MPN/g versus time was created for each separate experiment. The log linear model with tail was used to provide a reasonable fit to observed data. Maximum inactivation rate (k(max), min(-1)), residual population (log MPN/g), decimal reduction time (min), and time for a 4D (4-log10) reduction (min) were estimated at each temperature tested. A 4D reduction of the O26 STEC strain was achieved when curd was heated at 68 °C for 2.6 to 6.3 min or at 80 °C for 2.1 to 2.3 min. Greater resistance was observed for the O157 strain at 68 °C because k(max) was 1.48 min(-1). The model estimates can support cheesemakers in defining appropriate process criteria needed to control possible STEC contamination in raw milk intended for the production of Mozzarella.

Bacillus cereus cell response upon exposure to acid environment: toward the identification of potential biomarkers

Microorganisms are able to adapt to different environments and evolve rapidly, allowing them to cope with their new environments. Such adaptive response and associated protections toward other lethal stresses, is a crucial survival strategy for a wide spectrum of microorganisms, including food spoilage bacteria, pathogens, and organisms used in functional food applications. The growing demand for minimal processed food yields to an increasing use of combination of hurdles or mild preservation factors in the food industry. A commonly used hurdle is low pH which allows the decrease in bacterial growth rate but also the inactivation of pathogens or spoilage microorganisms. Bacillus cereus is a well-known food-borne pathogen leading to economical and safety issues in food industry. Because survival mechanisms implemented will allow bacteria to cope with environmental changes, it is important to provide understanding of B. cereus stress response. Thus this review deals with the adaptive traits of B. cereus cells facing to acid stress conditions. The acid stress response of B. cereus could be divided into four groups (i) general stress response (ii) pH homeostasis, (iii) metabolic modifications and alkali production and (iv) secondary oxidative stress response. This current knowledge may be useful to understand how B. cereus cells may cope to acid environment such as encountered in food products and thus to find some molecular biomarkers of the bacterial behavior. These biomarkers could be furthermore used to develop new microbial behavior prediction tools which can provide insights into underlying molecular physiological states which govern the behavior of microorganisms and thus opening the avenue toward the detection of stress adaptive behavior at an early stage and the control of stress-induced resistance throughout the food chain.

Oxygen relieves the CO2 and acetate dependency of Lactobacillus johnsonii NCC 533

Oxygen relieves the CO₂ and acetate dependency of Lactobacillus johnsonii NCC 533. The probiotic Lactobacillus johnsonii NCC 533 is relatively sensitive to oxidative stress; the presence of oxygen causes a lower biomass yield due to early growth stagnation. We show however that oxygen can also be beneficial to this organism as it relieves the requirement for acetate and CO₂ during growth. Both on agar- and liquid-media, anaerobic growth of L. johnsonii NCC 533 requires CO₂ supplementation of the gas phase. Switching off the CO₂ supply induces growth arrest and cell death. The presence of molecular oxygen overcomes the CO₂ dependency. Analogously, L. johnsonii NCC 533 strictly requires media with acetate to sustain anaerobic growth, although supplementation at a level that is 100-fold lower (120 microM) than the concentration in regular growth medium for lactobacilli already suffices for normal growth. Analogous to the CO₂ requirement, oxygen supply relieves this acetate-dependency for growth. The L. johnsonii NCC 533 genome indicates that this organism lacks genes coding for pyruvate formate lyase (PFL) and pyruvate dehydrogenase (PDH), both CO₂ and acetyl-CoA producing systems. Therefore, C1- and C2- compound production is predicted to largely depend on pyruvate oxidase activity (POX). This proposed role of POX in C2/C1-generation is corroborated by the observation that in a POX deficient mutant of L. johnsonii NCC 533, oxygen is not able to overcome acetate dependency nor does it relieve the CO₂ dependency.

A microfluidic platform for rapid, stress-induced antibiotic susceptibility testing of Staphylococcus aureus

The emergence and spread of bacterial resistance to ever increasing classes of antibiotics intensifies the need for fast phenotype-based clinical tests for determining antibiotic susceptibility. Standard susceptibility testing relies on the passive observation of bacterial growth inhibition in the presence of antibiotics. In this paper, we present a novel microfluidic platform for antibiotic susceptibility testing based on stress-activation of biosynthetic pathways that are the primary targets of antibiotics. We chose Staphylococcus aureus (S. aureus) as a model system due to its clinical importance, and we selected bacterial cell wall biosynthesis as the primary target of both stress and antibiotic. Enzymatic and mechanical stresses were used to damage the bacterial cell wall, and a β-lactam antibiotic interfered with the repair process, resulting in rapid cell death of strains that harbor no resistance mechanism. In contrast, resistant bacteria remained viable under the assay conditions. Bacteria, covalently-bound to the bottom of the microfluidic channel, were subjected to mechanical shear stress created by flowing culture media through the microfluidic channel and to enzymatic stress with sub-inhibitory concentrations of the bactericidal agent lysostaphin. Bacterial cell death was monitored via fluorescence using the Sytox Green dead cell stain, and rates of killing were measured for the bacterial samples in the presence and absence of oxacillin. Using model susceptible (Sanger 476) and resistant (MW2) S. aureus strains, a metric was established to separate susceptible and resistant staphylococci based on normalized fluorescence values after 60 min of exposure to stress and antibiotic. Because this ground-breaking approach is not based on standard methodology, it circumvents the need for minimum inhibitory concentration (MIC) measurements and long wait times. We demonstrate the successful development of a rapid microfluidic-based and stress-activated antibiotic susceptibility test by correctly designating the phenotypes of 16 additional clinically relevant S. aureus strains in a blinded study. In addition to future clinical utility, this method has great potential for studying the effects of various stresses on bacteria and their antibiotic susceptibility.

Impact of sorbic acid on germination and outgrowth heterogeneity of Bacillus cereus ATCC 14579 spores

Population heterogeneity complicates the predictability of the outgrowth kinetics of individual spores. Flow cytometry sorting and monitoring of the germination and outgrowth of single dormant spores allowed the quantification of acid-induced spore population heterogeneity at pH 5.5 and in the presence of sorbic acid. This showed that germination efficiency was not a good predictor for heterogeneity in final outgrowth.

High-salt stress conditions increase the pAW63 transfer frequency in Bacillus thuringiensis

Conjugation experiments with Bacillus thuringiensis and transfer kinetics demonstrated that salt stress has a positive impact on plasmid transfer efficiency. Compared to standard osmotic conditions (0.5% NaCl), plasmid transfer occurred more rapidly, and at higher frequencies (>100-fold), when bacteria were exposed to a high-salt stress (5% NaCl) in liquid brain heart infusion (BHI). Under milder salt conditions (2.5% NaCl), only a 10-fold effect was observed in Luria-Bertani broth and no difference was detected in BHI. These observations are particularly relevant in the scope of potential gene exchanges among members of the Bacillus cereus group, which includes food-borne contaminants and pathogens.

Microcolony imaging of Aspergillus fumigatus treated with echinocandins reveals both fungistatic and fungicidal activities

Background

The echinocandins are lipopeptides that can be employed as antifungal drugs that inhibit the synthesis of 1,3-β-glucans within the fungal cell wall. Anidulafungin and caspofungin are echinocandins used in the treatment of Candida infections and have activity against other fungi including Aspergillus fumigatus. The echinocandins are generally considered fungistatic against Aspergillus species.

Methods

Culture of A. fumigatus from conidia to microcolonies on a support of porous aluminium oxide (PAO), combined with fluorescence microscopy and scanning electron microscopy, was used to investigate the effects of anidulafungin and caspofungin. The PAO was an effective matrix for conidial germination and microcolony growth. Additionally, PAO supports could be moved between agar plates containing different concentrations of echinocandins to change dosage and to investigate the recovery of fungal microcolonies from these drugs. Culture on PAO combined with microscopy and image analysis permits quantitative studies on microcolony growth with the flexibility of adding or removing antifungal agents, dyes, fixatives or osmotic stresses during growth with minimal disturbance of fungal microcolonies.

Significance

Anidulafungin and caspofungin reduced but did not halt growth at the microcony level; additionally both drugs killed individual cells, particularly at concentrations around the MIC. Intact but not lysed cells showed rapid recovery when the drugs were removed. The classification of these drugs as either fungistatic or fungicidal is simplistic. Microcolony analysis on PAO appears to be a valuable tool to investigate the action of antifungal agents.

Rapid susceptibility testing and microcolony analysis of Candida spp. cultured and imaged on porous aluminum oxide

BACKGROUND

Acquired resistance to antifungal agents now supports the introduction of susceptibility testing for species-drug combinations for which this was previously thought unnecessary. For pathogenic yeasts, conventional phenotypic testing needs at least 24 h. Culture on a porous aluminum oxide (PAO) support combined with microscopy offers a route to more rapid results.

METHODS

Microcolonies of Candida species grown on PAO were stained with the fluorogenic dyes Fun-1 and Calcofluor White and then imaged by fluorescence microscopy. Images were captured by a charge-coupled device camera and processed by publicly available software. By this method, the growth of yeasts could be detected and quantified within 2 h. Microcolony imaging was then used to assess the susceptibility of the yeasts to amphotericin B, anidulafungin and caspofungin (3.5 h culture), and voriconazole and itraconazole (7 h culture).

SIGNIFICANCE

Overall, the results showed good agreement with EUCAST (86.5% agreement; n = 170) and E-test (85.9% agreement; n = 170). The closest agreement to standard tests was found when testing susceptibility to amphotericin B and echinocandins (88.2 to 91.2%) and the least good for the triazoles (79.4 to 82.4%). Furthermore, large datasets on population variation could be rapidly obtained. An analysis of microcolonies revealed subtle effects of antimycotics on resistant strains and below the MIC of sensitive strains, particularly an increase in population heterogeneity and cell density-dependent effects of triazoles. Additionally, the method could be adapted to strain identification via germ tube extension. We suggest PAO culture is a rapid and versatile method that may be usefully adapted to clinical mycology and has research applications.

Use of sequential-batch fermentations to characterize the impact of mild hypothermic temperatures on the anaerobic stoichiometry and kinetics of Saccharomyces cerevisiae

This work presents a characterization of the stoichiometry and kinetics of anaerobic batch growth of Saccharomyces cerevisiae at cultivation temperatures between 12 and 30°C. To minimize the influence of the inoculum condition and ensure full adaptation to the cultivation temperature, the experiments were carried out in sequencing batch reactors. It was observed that the growth rate obtained in the first batch performed after each temperature shift was 10-30% different compared with the subsequent batches at the same temperature, which were much more reproducible. This indicates that the sequencing batch approach provides accurate and reproducible growth rate data. Data reconciliation was applied to the measured time patterns of substrate, biomass, carbon dioxide and byproducts with the constraint that the elemental conservation relations were satisfied, allowing to obtain consistent best estimates of all uptake and secretion rates. Subsequently, it was attempted to obtain an appropriate model description of the temperature dependency of these rates. It was found that the Ratkowsky model provided a better description of the temperature dependency of growth, uptake and secretion rates than the Arrhenius law. Most interesting was to find that most of the biomass-specific rates have the same temperature dependency, leading to a near temperature independent batch stoichiometry.

Where bio meets nano: the many uses for nanoporous aluminum oxide in biotechnology

Porous aluminum oxide (PAO) is a ceramic formed by an anodization process of pure aluminum that enables the controllable assembly of exceptionally dense and regular nanopores in a planar membrane. As a consequence, PAO has a high porosity, nanopores with high aspect ratio, biocompatibility and the potential for high sensitivity imaging and diverse surface modifications. These properties have made this unusual material attractive to a disparate set of applications. This review examines how the structure and properties of PAO connect with its present and potential uses within research and biotechnology. The role of PAO is covered in areas including microbiology, mammalian cell culture, sensitive detection methods, microarrays and other molecular assays, and in creating new nanostructures with further uses within biology.

The role of physiological heterogeneity in microbial population behavior

As the ability to analyze individual cells in microbial populations expands, it is becoming apparent that isogenic microbial populations contain substantial cell-to-cell differences in physiological parameters such as growth rate, resistance to stress and regulatory circuit output. Subpopulations exist that are manyfold different in these parameters from the population average, and these differences arise by stochastic processes. Such differences can dramatically affect the response of cells to perturbations, especially stress, which in turn dictates overall population response. Defining the role of cell-to-cell heterogeneity in population behavior is important for understanding population-based research problems, including those involving infecting populations, normal flora and bacterial populations in water and soils. Emerging technological breakthroughs are poised to transform single-cell analysis and are critical for the next phase of insights into physiological heterogeneity in the near future. These include technologies for multiparameter analysis of live cells, with downstream processing and analysis.

Escherichia coli population heterogeneity: subpopulation dynamics at super-optimal temperatures

In the past years, we explored the dynamics of Escherichia coli K12 at super-optimal temperatures under static and dynamic temperature conditions (Van Derlinden et al. (2008b, 2009, 2010). Disturbed sigmoid growth curves, i.e., a sequence of growth, inactivation and re-growth, were observed, especially close to the maximum growth temperature. Based on the limited set of experiments (i.e., 2 static temperatures and 2 dynamic temperature profiles), the irregular growth curves were explained by postulating the co-existence of two subpopulations: a more resistant, growing population and a temperature sensitive, inactivating population. In this study, the dynamics of the two subpopulations are studied rigorously at 11 constant temperature levels in the region between 45°C and 46°C, with at least five repetitions per temperature. At all temperatures, the total population follows a sequence of growth, inactivation and re-growth. The sequence of different stages in the growth curves can be explained by the two subpopulations. The first growth phase and the inactivation phase reflect the presence of the sensitive subpopulation. Hereafter, the population's dynamics are dominated by the growth of the resistant subpopulation. Generally, cell counts are characterized by a large variability. The dynamics of the two subpopulations are carefully analyzed using a heterogeneous subpopulation type model to study the relation between the kinetic parameters of the two subpopulations and temperature, and to evaluate if the fraction d of resistant cells varies with temperature. Results indicate that the growth rate of the sensitive subpopulation decreases with increasing temperature within the range of 45-46°C. Furthermore, results point in the direction that the duration of this initial growth phase is approximately constant, i.e., around 2h. Possibly, the stress resistance of the cells decreases after a certain period because the metabolism is fully adapted to exponential growth. Also, the growth rate of the resistant subpopulation decreases with increasing temperature. Due to the extreme variability in the cell density data, derivation of accurate relations was not possible. From the heterogeneous model implementations, given the experimental set-up, both a constant d value and a temperature dependent d value seem plausible.

Direct-imaging-based quantification of Bacillus cereus ATCC 14579 population heterogeneity at a low incubation temperature

Bacillus cereus ATCC 14579 was cultured in microcolonies on Anopore strips near its minimum growth temperature to directly image and quantify its population heterogeneity at an abusive refrigeration temperature. Eleven percent of the microcolonies failed to grow during low-temperature incubation, and this cold-induced population heterogeneity could be partly attributed to the loss of membrane integrity of individual cells.

Population heterogeneity of Lactobacillus plantarum WCFS1 microcolonies in response to and recovery from acid stress

Within an isogenic microbial population in a homogenous environment, individual bacteria can still exhibit differences in phenotype. Phenotypic heterogeneity can facilitate the survival of subpopulations under stress. As the gram-positive bacterium Lactobacillus plantarum grows, it acidifies the growth medium to a low pH. We have examined the growth of L. plantarum microcolonies after rapid pH downshift (pH 2 to 4), which prevents growth in liquid culture. This acidification was achieved by transferring cells from liquid broth onto a porous ceramic support, placed on a base of low-pH MRS medium solidified using Gelrite. We found a subpopulation of cells that displayed phenotypic heterogeneity and continued to grow at pH 3, which resulted in microcolonies dominated by viable but elongated (filamentous) cells lacking septation, as determined by scanning electron microscopy and staining cell membranes with the lipophilic dye FM4-64. Recovery of pH-stressed cells from these colonies was studied by inoculation onto MRS-Gelrite-covered slides at pH 6.5, and outgrowth was monitored by microscopy. The heterogeneity of the population, calculated from the microcolony areas, decreased with recovery from pH 3 over a period of a few hours. Filamentous cells did not have an advantage in outgrowth during recovery. Specific regions within single filamentous cells were more able to form rapidly dividing cells, i.e., there was heterogeneity even within single recovering cells.

The growth limits of a large number of Listeria monocytogenes strains at combinations of stresses show serotype--and niche-specific traits

AIMS

The aim of this study was to associate the growth limits of Listeria monocytogenes during exposure to combined stresses with specific serotypes or origins of isolation, and identify potential genetic markers.

METHODS AND RESULTS

The growth of 138 strains was assessed at different temperatures using combinations of low pH, sodium lactate, and high salt concentrations in brain heart infusion broth. None of the strains was able to grow at pH < or = 4.4, a(w) < or = 0.92, or pH < or = 5.0 combined with a(w) < or = 0.94. In addition, none of the strains grew at pH < or = 5.2 and NaLac > or = 2%. At 30 degrees C, the serotype 4b strains showed the highest tolerance to low pH and high NaCl concentrations at both pH neutral (pH 7.4) and mild acidic conditions (pH 5.5). At 7 degrees C, the serotype 1/2b strains showed the highest tolerance to high NaCl concentrations at both pH 7.4 and 5.5. Serotype 1/2b meat isolates showed the highest tolerance to low pH in the presence of 2% sodium lactate at 7 degrees C. ORF2110 and gadD1T1 were identified as potential biomarkers for phenotypic differences.

CONCLUSIONS

Differences in growth limits were identified between specific L. monocytogenes strains and serotypes, which could in some cases be associated with specific genetic markers.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our data confirm the growth limits of L. monocytogenes as set out by the European Union for ready-to-eat foods and provides an additional criterion. The association of L. monocytogenes serotypes with certain stress responses might explain the abundance of certain serotypes in retail foods while others are common in clinical cases.