Flow cytometry applications in the food industry

Rapid and accurate quantification of viable Bifidobacterium cells in milk powder with a propidium monoazide-antibiotic fluorescence in situ hybridization-flow cytometry method

Due to its beneficial effects on human health, Bifidobacterium is commonly added to milk powder. Accurate quantification of viable Bifidobacterium is essential for assessing the therapeutic efficacy of milk powder. In this study, we introduced a novel propidium monoazide (PMA)-antibiotic fluorescence in situ hybridization (AFISH)-flow cytometry (FC) method to rapidly and accurately quantify viable Bifidobacterium cells in milk powder. Briefly, Bifidobacterium cells were treated with chloramphenicol (CM) to increase their rRNA content, followed by staining with RNA-binding oligonucleotide probes, based on the AFISH technique. Then, the DNA-binding dye PMA was used to differentiate between viable and nonviable cells. The PMA-AFISH-FC method, including sample pretreatment, CM treatment, dual staining, and FC analysis, required approximately 2 h and was found to be better than the current methods. This is the first study to implement FC combined with PMA and an oligonucleotide probe for detecting Bifidobacterium.

Comparison of plate counting with flow cytometry, using four different fluorescent dye techniques, for the enumeration of Bacillus cereus in milk

This study aimed to compare the performance of flow cytometry methods with plate counting for the enumeration of bacteria, using Bacillus cereus as a model organism. It was found that the cFDA-propidium iodide, CellROX™ Green-propidium iodide, and DiOC2 dye techniques had similar accuracy to plate counting, while the SYTO 24-propidium iodide dye technique was not as accurate. The four dye techniques had comparable precision to plate counting, with the CellROX™ Green-propidium iodide dye having the greatest precision. The consistency of the position and shape of the cell clusters on the flow cytometry plots, and the extent of separation of the cell from background clusters, was greatest with the DiOC2 and CellROX™ Green-propidium iodide dyes. Furthermore, the DiOC2 and CellROX™ Green-propidium iodide dyes performed well, even when a sample was measured containing reconstituted whole milk powder at a 10-1 dilution, without the use of sample preparation to specifically remove the milk constituents prior to measurement. Given gating of only one cell cluster was required to be managed with the DiOC2 dye, to determine the viable number of cells, it was found that the DiOC2 dye had the greatest ease-of-use. Overall, results indicated that the DiOC2 dye is an ideal candidate for the enumeration of viable bacteria in dairy samples on a high-throughput, routine basis.

Spectral analysis and sorting of microbial organisms using a spectral sorter

This chapter discusses the problems related to the application of conventional flow cytometers to microbiology. To address some of those limitations, the concept of spectral flow cytometry is introduced and the advantages over conventional flow cytometry for bacterial sorting are presented. We demonstrate by using ThermoFisher's Bigfoot spectral sorter where the spectral signatures of different stains for staining bacteria are demonstrated with an example of performing unmixing on spectral datasets. In addition to the Bigfoot's spectral analysis, the special biosafety features of this instrument are discussed. Utilizing these biosafety features, the sorting and patterning at the single cell level is optimized using non-pathogenic bacteria. Finally, the chapter is concluded by presenting a novel, label free, non-destructive, and rapid phenotypic method called Elastic Light Scattering (ELS) technology for identification of the patterned bacterial cells based on their unique colony scatter patterns.

Global transcriptome analysis of Pseudomonas aeruginosa NT06 response to potassium chloride, sodium lactate, sodium citrate, and microaerophilic conditions in a fish ecosystem

Pseudomonas aeruginosa is an opportunistic pathogen that recently has been increasingly isolated from foods, especially from minimally processed fish-based products. Those are preserved by the addition of sodium chloride (NaCl) and packaging in a modified atmosphere. However, the current trends of minimizing NaCl content may result in an increased occurrence of P. aeruginosa. NaCl can be replaced with potassium chloride (KCl) or sodium salts of organic acids. Herein, we examined the antimicrobial effects of KCl, sodium lactate (NaL), sodium citrate (NaC), and sodium acetate (NaA) against P. aeruginosa NT06 isolated from fish. Transcriptome response of cells grown in medium imitating a fish product supplemented with KCl and KCl/NaL/NaC and maintained under microaerophilic conditions was analysed. Flow cytometry analysis showed that treatment with KCl and KCl/NaL/NaC resulted in changed metabolic activity of cells. In response to KCl and KCl/NaL/NaC treatment, genes related to cell maintenance, stress response, quorum sensing, virulence, efflux pump, and metabolism were differentially expressed. Collectively, our results provide an improved understanding of the response of P. aeruginosa to NaCl alternative compounds that can be implemented in fish-based products and encourage further exploration of the development of effective methods to protect foods against the P. aeruginosa, underestimate foodborne bacteria.

Convergence of flow cytometry and bacteriology. Current and future applications: a focus on food and clinical microbiology

Since its development in the 1960s, flow cytometry (FCM) was quickly revealed a powerful tool to analyse cell populations in medical studies, yet, for many years, was almost exclusively used to analyse eukaryotic cells. Instrument and methodological limitations to distinguish genuine bacterial signals from the background, among other limitations, have hampered FCM applications in bacteriology. In recent years, thanks to the continuous development of FCM instruments and methods with a higher discriminatory capacity to detect low-size particles, FCM has emerged as an appealing technique to advance the study of microbes, with important applications in research, clinical and industrial settings. The capacity to rapidly enumerate and classify individual bacterial cells based on viability facilitates the monitoring of bacterial presence in foodstuffs or clinical samples, reducing the time needed to detect contamination or infectious processes. Besides, FCM has stood out as a valuable tool to advance the study of complex microbial communities, or microbiomes, that are very relevant in the context of human health, as well as to understand the interaction of bacterial and host cells. This review highlights current developments in, and future applications of, FCM in bacteriology, with a focus on those related to food and clinical microbiology.

Instinctive Recognition of Pathogens in Rice Using Reformed Fractional Differential Segmentation and Innovative Fuzzy Logic-Based Probabilistic Neural Network

Rice is an essential primary food crop in the world, and it plays a significant part in the country’s economy. It is the most often eaten stable food and is in great demand in the market as the world’s population continues to expand. Rice output should be boosted to fulfil the growing demand. As a result, the yield of plant crops diminishes, creating an environment conducive to the spread of infectious illnesses. To boost the production of agricultural fields, it is necessary to remove plant diseases from the environment. This study presents ways for recognising three types of rice plant diseases, as well as a healthy leaf, in rice plants. This includes image capture, image preprocessing, segmentation, feature extraction, and classification of three rice plant illnesses, as well as classification of a healthy leaf, among other techniques. Following the K-means segmentation, the features are extracted utilising three criteria, which are colour, shape, and texture, to generate a final product. Colour, shape, and texture are the parameters used in the extraction of the features. It is proposed that a novel intensity-based technique is used to retrieve colour features from the infected section, whereas the form parameters of the infected section, such as the area and diameter, and the texture characteristics of the infected section are extracted using a grey-level co-occurrence matrix. The colour features are retrieved depending on the characteristics of the features. All three previous techniques were surpassed by the proposed fuzzy logic-based probabilistic neural network on a range of performance metrics, with the new network obtaining greater accuracy. Finally, the result is validated using the fivefold cross-validation method, with the final accuracy for the diseases such as bacterial leaf blight, brown spot, healthy leaf, and rice blast being 95.20 percent, 97.60 percent, 99.20 percent, and 98.40 percent, respectively, and 95.40 percent for the disease brown spot.

Comparison of growth and survival of single strains of Lactococcus lactis and Lactococcus cremoris during Cheddar cheese manufacture

Traditionally, starter cultures for Cheddar cheese are combinations of Lactococcus lactis and Lactococcus cremoris. Our goal was to compare growth and survival of individual strains during cheesemaking, and after salting and pressing. Cultures used were 2 strains of L. lactis (SSM 7605, SSM 7436) and 2 strains of L. cremoris (SSM 7136, SSM 7661). A standardized Cheddar cheese make procedure was used that included a 38°C cook temperature and salting levels of 2.0, 2.4, 2.8, 3.2, and 3.6% from which were selected cheeses with salt-in-moisture levels of 3.5, 4.5, and 5.5%. Vats of cheese were made using each strain on its own as biological duplicates on different days. Starter culture numbers were enumerated by plate counting during cheesemaking and after 6 d storage at 6°C. Flow cytometry with fluorescent staining by SYBR Green and propidium iodide was used to determine the number of live and dead cells in cheese at the different salt levels. Differences in cheese make times were strain dependent rather than species dependent. Even with correction for average culture chain length, cheeses made using L. lactis strains contained ∼4 times (∼0.6 log) more bacterial cells than those made using L. cremoris strains. Growth of the strains used in this study was not influenced by the amount of salt added to the curd. The higher pH of cheeses with higher salting levels was attributed to those cheeses having a lower moisture content. Based on flow cytometry, ∼5% of the total starter culture cells in the cheese were dead after 6 d of storage. Another 3 to 19% of the cells were designated as being live, but semipermeable, with L. cremoris strains having the higher number of semipermeable cells.

Potential of Flow Cytometric Approaches for Rapid Microbial Detection and Characterization in the Food Industry-A Review

As microbial contamination is persistent within the food and bioindustries and foodborne infections are still a significant cause of death, the detection, monitoring, and characterization of pathogens and spoilage microorganisms are of great importance. However, the current methods do not meet all relevant criteria. They either show (i) inadequate sensitivity, rapidity, and effectiveness; (ii) a high workload and time requirement; or (iii) difficulties in differentiating between viable and non-viable cells. Flow cytometry (FCM) represents an approach to overcome such limitations. Thus, this comprehensive literature review focuses on the potential of FCM and fluorescence in situ hybridization (FISH) for food and bioindustry applications. First, the principles of FCM and FISH and basic staining methods are discussed, and critical areas for microbial contamination, including abiotic and biotic surfaces, water, and air, are characterized. State-of-the-art non-specific FCM and specific FISH approaches are described, and their limitations are highlighted. One such limitation is the use of toxic and mutagenic fluorochromes and probes. Alternative staining and hybridization approaches are presented, along with other strategies to overcome the current challenges. Further research needs are outlined in order to make FCM and FISH even more suitable monitoring and detection tools for food quality and safety and environmental and clinical approaches.

Imaging Flow Cytometry to Study Biofilm-Associated Microbial Aggregates

The aim of the research was to design an advanced analytical tool for the precise characterization of microbial aggregates from biofilms formed on food-processing surfaces. The approach combined imaging flow cytometry with a machine learning-based interpretation protocol. Biofilm samples were collected from three diagnostic points of the food-processing lines at two independent time points. The samples were investigated for the complexity of microbial aggregates and cellular metabolic activity. Thus, aggregates and singlets of biofilm-associated microbes were simultaneously examined for the percentages of active, mid-active, and nonactive (dead) cells to evaluate the physiology of the microbial cells forming the biofilm structures. The tested diagnostic points demonstrated significant differences in the complexity of microbial aggregates. The significant percentages of the bacterial aggregates were associated with the dominance of active microbial cells, e.g., 75.3% revealed for a mushroom crate. This confirmed the protective role of cellular aggregates for the survival of active microbial cells. Moreover, the approach enabled discriminating small and large aggregates of microbial cells. The developed tool provided more detailed characteristics of bacterial aggregates within a biofilm structure combined with high-throughput screening potential. The designed methodology showed the prospect of facilitating the detection of invasive biofilm forms in the food industry environment.

How to Evaluate Non-Growing Cells-Current Strategies for Determining Antimicrobial Resistance of VBNC Bacteria

Thanks to the achievements in sanitation, hygiene practices, and antibiotics, we have considerably improved in our ongoing battle against pathogenic bacteria. However, with our increasing knowledge about the complex bacterial lifestyles and cycles and their plethora of defense mechanisms, it is clear that the fight is far from over. One of these resistance mechanisms that has received increasing attention is the ability to enter a dormancy state termed viable but non-culturable (VBNC). Bacteria that enter the VBNC state, either through unfavorable environmental conditions or through potentially lethal stress, lose their ability to grow on standard enrichment media, but show a drastically increased tolerance against antimicrobials including antibiotics. The inability to utilize traditional culture-based methods represents a considerable experimental hurdle to investigate their increased antimicrobial resistance and impedes the development and evaluation of effective treatments or interventions against bacteria in the VBNC state. Although experimental approaches were developed to detect and quantify VBNCs, only a few have been utilized for antimicrobial resistance screening and this review aims to provide an overview of possible methodological approaches.

Survival and metabolism of hydroxycinnamic acids by Dekkera bruxellensis in monovarietal wines

Volatile phenols in wines are responsible for unpleasant aromas, which negatively affect the quality of the wine. These compounds are produced from the metabolism of hydroxycinnamic acids, mainly by the yeasts Brettanomyces/Dekkera. Relevant data, potentially useful to support decisions on how to manage the risk of contamination of wines by Brettanomyces/Dekkera, according to the grape varieties used in the vinification, is important to the wine industry. Therefore, the aim of this work was to evaluate the survival and the metabolism of hydroxycinnamic acids by Dekkera bruxellensis in monovarietal wines. Yeast growth and survival were monitored in fifteen wines, five from each of the grape varieties Touriga Nacional, Cabernet Sauvignon and Syrah, inoculated with a strain of D. bruxellensis. Yeast culturable populations of 10⁷ CFU mL^-1 were reduced to undetectable numbers in 24 h in all wines. Plate counts of 10⁴-10⁶ CFU mL^-1 were, however, detected after 48 h in most of Touriga Nacional and Cabernet Sauvignon wines and later in Syrah. Viability measurement by flow cytometry showed that a significant part of the populations was in a viable but non-culturable state (VBNC). The time required for the recovery of the culturable state was dependent on the wine, being longer on Syrah wines. Besides the production of ethylphenols, the metabolism of hydroxycinnamic acids by VBNC cells led to the accumulation of vinylphenols at relatively high levels, independently of the grape variety. The flow cytometry methodology showed a higher survival capacity of D. bruxellensis in Touriga Nacional wines, which corroborates with the higher amounts of volatile phenols found on this variety.

Separation, characterization and identification of microplastics and nanoplastics in the environment

Microplastics (MPs) have globally been detected in aquatic and marine environments, which has raised scientific interests and public health concerns during the past decade. MPs are those polymeric particles with at least one dimension <5 mm. MPs possess complex physicochemical properties that vary their mobility, bioavailability and toxicity toward organisms and interactions with their surrounding pollutants. Similar to nanomaterials and nanoparticles, accurate and reliable detection and measurement of MPs or nanoplastics and their characteristics are important to warrant a comprehensive understanding of their environmental and ecological impacts. This review elaborates the principles and applications of diverse analytical instruments or techniques for separation, characterization and quantification of MPs in the environment. The strength and weakness of different instrumental methods in separation, morphological, physical classification, chemical characterization and quantification for MPs are critically compared and analyzed. There is a demand for standardized experimental procedures and characterization analysis due to the complex transformation, cross-contamination and heterogeneous properties of MPs in size and chemical compositions. Moreover, this review highlights emerging and promising characterization techniques that may have been overlooked by research communities to study MPs. The future research efforts may need to develop and implement new analytical tools and combinations of hyphenated technologies to complement respective limitations of detection and yield reliable characterization information for MPs. The goal of this critical review is to facilitate the research of plastic particles and pollutants in the environment and understanding of their environmental and human health effects.

Rapid Flow Cytometric Detection of Single Viable Salmonella Cells in Milk Powder

Salmonella, a highly virulent food-borne pathogen transmitted through food, can cause severe infectious diseases in a large number of people through a single outbreak, due to its low infective doses. In this study, a flow cytometry (FCM)-based method was developed for the rapid detection of single viable Salmonella cells with dual staining of fluorescein isothiocyanate (FITC)-labeled anti-Salmonella antibody and propidium iodide (PI) dyes. The FCM-based method includes 6 h of pre-enrichment, 40 min of target cell isolation, and 20 min of dual staining and FCM analysis. The developed method demonstrated high specificity for the detection of 23 Salmonella strains and 22 food-borne pathogenic non-Salmonella strains. Furthermore, the analyses of 30 samples of milk powder artificially contaminated with single Salmonella cells, 123 samples of retail milk powder, and 6 samples of Salmonella-positive milk powder were performed by the FCM-based as well as traditional plate-based methods for testing the efficiency of the methods. The two methods yielded similar results for the detection of pathogens in all milk powder samples. In conclusion, the developed FCM-based method was found to be efficient in detecting single viable Salmonella cells in milk powder within 7 h. The proposed dual-color FITC assay combined with pre-enrichment offers a great potential for the rapid and sensitive detection of other pathogens in dairy products.

A novel pharmaceutical approach for the analytical validation of probiotic bacterial count by flow cytometry

INTRODUCTION

Flow cytometry is a powerful and sensitive technique able to characterize single cells within a heterogeneous population. Different fluorescent dyes can be combined and used together to analyze a great variety of parameters simultaneously. In particular, flow-cytometry allows to measure viability and vitality of probiotics measuring their metabolic activity, fermentation capacity, acidification potential or oxygen uptake ability (Hayouni et al., 2008). To now, plate counting is considered the gold standard in microbiological technique for probiotic enumeration. However, this approach is limited to the detection of only those viable cells which are able to proliferate and form colonies on a solid medium but is not able to recognize not cultivable bacteria and nonviable cells.

AIM

The aim of the present study was to apply The International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use (ICH) parameters for the validation of new analytical methods in microbiology. ICH requirements, which are commonly employed for the analysis of drugs and chemical analytes, have been here applied to live cells for the comparison between a flow-cytometric assay and the traditional plate count method for the quantification of viable probiotics bacteria.

METHODS AND RESULTS

Combining specific viability dyes such as thiazole orange (TO) and propidium iodide (PI), probiotic counts of Lactobacillus and Bifidobacterium species were carried out using a FACS Verse (BD Biosciences) cytometer. Analyses were conducted in parallel with the traditional plate count, on specific media. Raw data were analyzed using the FACSuite software (BD Biosciences) and then elaborated with the statistical software Neolicy (VWR International). Results indicated that flow cytometry provides very similar results in cell counting if compared to classical microbiology approaches, showing better performances (ICH parameters) than the traditional plate count method.

CONCLUSIONS

This work demonstrated the analytical ICH validation of probiotic counts in food supplement products using a robust flow cytometric approach able to enumerate and to assess bacteria viability with stronger results in comparison to the traditional plate count.

Applications of flow cytometry in molluscan immunology: Current status and trends

Flow cytometry (FCM) is routinely used in fundamental and applied research, clinical practice, and clinical trials. In the last three decades, this technique has also become a routine tool used in immunological studies of molluscs to analyse physical and chemical characteristics of haemocytes. Here, we briefly review the current implementation of FCM in the field of molluscan immunology. These applications cover a diverse range of practices from straightforward total cell counts and cell viability to characterize cell subpopulations, and further extend to analyses of DNA content, phagocytosis, oxidative stress and apoptosis. The challenges and prospects of FCM applications in immunological studies of molluscs are also discussed.

Cell Cytometry: Review and Perspective on Biotechnological Advances

Cell identification and enumeration are essential procedures within clinical and research laboratories. For over 150 years, quantitative investigation of body fluids such as counts of various blood cells has been an important tool for diagnostic analysis. With the current evolution of point-of-care diagnostics and precision medicine, cheap and precise cell counting technologies are in demand. This article reviews the timeline and recent notable advancements in cell counting that have occurred as a result of improvements in sensing including optical and electrical technology, enhancements in image processing capabilities, and contributions of micro and nanotechnologies. Cell enumeration methods have evolved from the use of manual counting using a hemocytometer to automated cell counters capable of providing reliable counts with high precision and throughput. These developments have been enabled by the use of precision engineering, micro and nanotechnology approaches, automation and multivariate data analysis. Commercially available automated cell counters can be broadly classified into three categories based on the principle of detection namely, electrical impedance, optical analysis and image analysis. These technologies have many common scientific uses, such as hematological analysis, urine analysis and bacterial enumeration. In addition to commercially available technologies, future technological trends using lab-on-a-chip devices have been discussed in detail. Lab-on-a-chip platforms utilize the existing three detection technologies with innovative design changes utilizing advanced nano/microfabrication to produce customized devices suited to specific applications.

Araújo DS, Quiroga MV, Amado AM, Neto ADD, Sarmento H, Metz SD, Unrein F. flowDiv: a new pipeline for analyzing flow cytometric diversity

BACKGROUND

Flow cytometry (FCM) is one of the most commonly used technologies for analysis of numerous biological systems at the cellular level, from cancer cells to microbial communities. Its high potential and wide applicability led to the development of various analytical protocols, which are often not interchangeable between fields of expertise. Environmental science in particular faces difficulty in adapting to non-specific protocols, mainly because of the highly heterogeneous nature of environmental samples. This variety, although it is intrinsic to environmental studies, makes it difficult to adjust analytical protocols to maintain both mathematical formalism and comprehensible biological interpretations, principally for questions that rely on the evaluation of differences between cytograms, an approach also termed cytometric diversity. Despite the availability of promising bioinformatic tools conceived for or adapted to cytometric diversity, most of them still cannot deal with common technical issues such as the integration of differently acquired datasets, the optimal number of bins, and the effective correlation of bins to previously known cytometric populations.

RESULTS

To address these and other questions, we have developed flowDiv, an R language pipeline for analysis of environmental flow cytometry data. Here, we present the rationale for flowDiv and apply the method to a real dataset from 31 freshwater lakes in Patagonia, Argentina, to reveal significant aspects of their cytometric diversities.

CONCLUSIONS

flowDiv provides a rather intuitive way of proceeding with FCM analysis, as it combines formal mathematical solutions and biological rationales in an intuitive framework specifically designed to explore cytometric diversity.

Mentha piperita L. essential oil inactivates spoilage yeasts in fruit juices through the perturbation of different physiological functions in yeast cells

This study evaluated the efficacy of the essential oil from Mentha piperita L. (MPEO) to inactivate cells of the potentially spoilage yeasts Candida albicans, Candida tropicalis, Pichia anomala and Saccharomyces cerevisiae in cashew, guava, mango and pineapple juices during 72 h of refrigerated storage. Damage in different physiological functions caused by MPEO in S. cerevisiae in cashew and guava juices were investigated using flow cytometry (FC). The effects of the incorporation of an effective anti-yeast MPEO dose on sensory characteristics of juices were also evaluated. MPEO displayed minimum inhibitory concentration of 1.875 μL/mL against all tested yeasts. A >5 log reduction in counts of C. albicans, P. anomala and S. cerevisiae was observed in cashew and guava juices with 7.5 and 3.75 μL/mL MPEO. Tested MPEO concentrations (1.875, 3.75 and 7.5 μL/mL) were not effective to cause >5 log reduction in counts of target yeasts in mango and pineapple juices during 72 h of exposure. Incorporation of 1.875 μL/mL MPEO in cashew and guava juices strongly compromised membrane permeability, membrane potential, enzymatic activity and efflux pump activity in S. cerevisiae cells. This same MPEO concentration did not affect appearance, odor and viscosity in fruit juices, but negatively affected their taste and aftertaste. These results show the efficacy of MPEO to inactivate potentially spoilage yeasts in fruit juices through disturbance of different physiological functions in yeast cells. However, the combined use of MPEO with other technologies should be necessary to decrease its effective anti-yeast dose in fruit juices and, consequently, the possible negative impacts on specific sensory properties of these products.

Hyper-Osmotic Stress Elicits Membrane Depolarization and Decreased Permeability in Halotolerant Marine Debaryomyces hansenii Strains and in Saccharomyces cerevisiae

The use of seawater and marine microorganisms can represent a sustainable alternative to avoid large consumption of freshwater performing industrial bioprocesses. Debaryomyces hansenii, which is a known halotolerant yeast, possess metabolic traits appealing for developing such processes. For this purpose, we studied salt stress exposure of two D. hansenii strains isolated from marine fauna. We found that the presence of sea salts during the cultivation results in a slight decrease of biomass yields. Nevertheless, higher concentration of NaCl (2 M) negatively affects other growth parameters, like growth rate and glucose consumption rate. To maintain an isosmotic condition, the cells accumulate glycerol as compatible solute. Flow cytometry analysis revealed that the osmotic adaptation causes a reduced cellular permeability to cell-permeant dye SYBR Green I. We demonstrate that this fast and reversible phenomenon is correlated to the induction of membrane depolarization, and occurred even in presence of high concentration of sorbitol. The decrease of membrane permeability induced by osmotic stress confers to D. hansenii resistance to cationic drugs like Hygromycin B. In addition, we describe that also in Saccharomyces cerevisiae the exposure to hyper-osmotic conditions induced membrane depolarization and reduced the membrane permeability. These aspects are very relevant for the optimization of industrial bioprocesses, as in the case of fermentations and bioconversions carried out by using media/buffers containing high nutrients/salts concentrations. Indeed, an efficient transport of molecules (nutrients, substrates, and products) is the prerequisite for an efficient cellular performance, and ultimately for the efficiency of the industrial process.

Railing cells along 3D microelectrode tracks for continuous-flow dielectrophoretic sorting

We demonstrate a unique microfluidic device for continuous-flow cell sorting by railing target cells along physical tracks (electrode sidewalls) based on the combined effect of dielectrophoresis and hydrodynamic drag. The tracks are the raised digits of comb-like structures made of conducting bulk silicon as the electrodes. Unlike other volumetric electrodes, the structures feature a segmented sidewall profile with linear and concave segments forming the tracks and supporting columns, respectively. The interdigitated bulk electrodes lead to a built-in flow chamber in which the digits (tracks) extend downstream at a characteristic angle with respect to the flow, which runs through the passages between the columns. Target cells leaving the passages are levitated and docked against the tracks under positive dielectrophoresis and railed under hydrodynamic drag. Railing efficiency, as high as >95%, is reported against the activation voltage and flow rate for the designs 7°, 16°, and 26° as the track angles. A collection efficiency of about 86% is noted for both target (HCT116) and non-target cells (K562) in the 16° design at a sample flow rate of 8.3 μL min-1 and an activation voltage of 12.5 Vp at 200 kHz. This performance is comparable if not better than those obtained with thin-film surface microelectrodes and yet achieved here at an order of magnitude higher sample flow rate. This enhancement mainly arises from a considerably low drag along the tracks in relation to the chamber top or bottom surface where the thin-film electrodes would be typically placed.

Sublethal Injury and Viable but Non-culturable (VBNC) State in Microorganisms During Preservation of Food and Biological Materials by Non-thermal Processes

The viable but non-culturable (VBNC) state, as well as sublethal injury of microorganisms pose a distinct threat to food safety, as the use of traditional, culture-based microbiological analyses might lead to an underestimation or a misinterpretation of the product's microbial status and recovery phenomena of microorganisms may occur. For thermal treatments, a large amount of data and experience is available and processes are designed accordingly. In case of innovative inactivation treatments, however, there are still several open points with relevance for the investigation of inactivation mechanisms as well as for the application and validation of the preservation processes. Thus, this paper presents a comprehensive compilation of non-thermal preservation technologies, i.e., high hydrostatic pressure (HHP), pulsed electric fields (PEFs), pulsed light (PL), and ultraviolet (UV) radiation, as well as cold plasma (CP) treatments. The basic technological principles and the cellular and molecular mechanisms of action are described. Based on this, appropriate analytical methods are outlined, i.e., direct viable count, staining, and molecular biological methods, in order to enable the differentiation between viable and dead cells, as well as the possible occurrence of an intermediate state. Finally, further research needs are outlined.

Zeta potential changes of Saccharomyces cerevisiae during fermentative and respiratory cycles

Saccharomyces cerevisiae is a type of yeast, widely used in diverse biotechnological food-beverage processes. Although the performance of an industrial fermentation process depends largely on the number of cells, it is necessary to consider the physiological state of the cultures. In this context, the aim of this study was to determine in a yeast culture how factors such as growth conditions affect surface properties at the different growth stages. Our results show that, S. cerevisiae spp. exhibits different zeta potential mean values along the exponential, post-diauxic and stationary growth phases. In addition, there were differences depending on whether they are in aerobic or anaerobic conditions. When the effect of pH on the media was studied, a different dependence of zeta potential at each stage reveals that in the living cells the surface potential depends on the interaction between secreted acids and the constituents of the surfaces, according to the growth conditions. In order to have a view at the cellular level, the zeta potential on individual cells by optical microscopy has been determined at different stages of culture in aerobic and anaerobic conditions. This single-cell method allows for the identification and following of the development of different cell subpopulations during each growth stage. Furthermore, the behavior of the dead cells provided evidence to relate the large negatively charged population with cell wall damage. Overall, the results obtained in the present work represent an important milestone for a novel application of zeta potential technique on yeast.

Use of flow cytometry and total viable count to determine the effects of orange juice composition on the physiology of Escherichia coli

Orange juice (OJ) contains numerous compounds some of which are known to play key roles in growth and survival of bacteria. This study aimed to investigate the effects of natural or processing-induced variations in OJ composition on the physiology of Escherichia coli. OJ and model OJ (MOJ) samples containing various sugars, organic acids, amino acids, or ascorbic acid were inoculated with E. coli K-12 MG1655 in different growth phases. The culturability, viability, and physiology of the cells were investigated during storage using plate counting and flow cytometry. Generally, stationary-phase cells displayed the greatest survival in both MOJ and OJ. Increase in incubation temperature from 4 to 22.5ºC caused a significant decrease in both healthy and culturable cell populations. Supplementation of MOJ with ascorbic acid and amino acids increased both the viability and culturability of the cells. Similar trends were observed in amino acid-supplemented OJ, albeit at a slower rate. In contrast, variations in sugar or organic acid composition had negligible effects on the physiological status of the cells. In summary, natural variation in ascorbic acid or amino acid concentrations could potentially have an adverse effect on the microbiological safety of orange juice.

Flow cytometry: a versatile technology for specific quantification and viability assessment of micro-organisms in multistrain probiotic products

AIMS

Classical microbiology techniques are the gold standard for probiotic enumeration. However, these techniques are limited by parameters of time, specificity and incapacity to detect viable but nonculturable (VBNC) micro-organisms and nonviable cells. The aim of the study was to evaluate flow cytometry as a novel method for the specific quantification of viable and nonviable probiotics in multistrain products.

METHODS AND RESULTS

Custom polyclonal antibodies were produced against five probiotic strains from different species (Bifidobacterium bifidum R0071, Bifidobacterium longum ssp. infantis R0033, Bifidobacterium longum ssp. longum R0175, Lactobacillus helveticus R0052 and Lactobacillus rhamnosus R0011). Evaluation of specificity confirmed that all antibodies were specific at least at the subspecies level. A flow cytometry method combining specific antibodies and viability assessment with SYTO^® 24 and propidium iodide was applied to quantify these strains in three commercial products. Analyses were conducted on two flow cytometry instruments by two operators and compared with classical microbiology using selective media. Results indicated that flow cytometry provides higher cell counts than classical microbiology (P < 0·05) in 73% of cases highlighting the possible presence of VBNC. Equivalent performances (repeatability and reproducibility) were obtained for both methods.

CONCLUSIONS

This study showed that flow cytometry methods can be applied to probiotic enumeration and viability assessment. Combination with polyclonal antibodies can achieve sufficient specificity to differentiate closely related strains.

SIGNIFICANCE AND IMPACT OF THE STUDY

Flow cytometry provides absolute and specific quantification of viable and nonviable probiotic strains in a very short time (<2 h) compared with classical techniques (>48 h), bringing efficient tools for research and development and quality control.

Detecting Lactococcus lactis Prophages by Mitomycin C-Mediated Induction Coupled to Flow Cytometry Analysis

Most analyzed Lactococcus lactis strains are predicted to harbor one or more prophage genomes within their chromosome; however, the true extent of the inducibility and functionality of such prophages cannot easily be deduced from sequence analysis alone. Chemical treatment of lysogenic strains with Mitomycin C is known to cause induction of temperate phages, though it is not always easy to clearly identify a lysogenic strain or to measure the number of released phage particles. Here, we report the application of flow cytometry as a reliable tool for the detection and enumeration of released lactococcal prophages using the green dye SYTO-9.

Flow Cytometry Sorting to Separate Viable Giant Viruses from Amoeba Co-culture Supernatants

Flow cytometry has contributed to virology but has faced many drawbacks concerning detection limits, due to the small size of viral particles. Nonetheless, giant viruses changed many concepts in the world of viruses, as a result of their size and hence opened up the possibility of using flow cytometry to study them. Recently, we developed a high throughput isolation of viruses using flow cytometry and protozoa co-culture. Consequently, isolating a viral mixture in the same sample became more common. Nevertheless, when one virus multiplies faster than others in the mixture, it is impossible to obtain a pure culture of the minority population. Here, we describe a robust sorting system, which can separate viable giant virus mixtures from supernatants. We tested three flow cytometry sorters by sorting artificial mixtures. Purity control was assessed by electron microscopy and molecular biology. As proof of concept, we applied the sorting system to a co-culture supernatant taken from a sample containing a viral mixture that we couldn't separate using end point dilution. In addition to isolating the quick-growing Mimivirus, we sorted and re-cultured a new, slow-growing virus, which we named “Cedratvirus.” The sorting assay presented in this paper is a powerful and versatile tool for separating viral populations from amoeba co-cultures and adding value to the new field of flow virometry.

Flow Cytometry as a Tool for Quality Control of Fluorescent Conjugates Used in Immunoassays

The use of antibodies in immunodiagnostic kits generally implies the conjugation of these proteins with other molecules such as chromophores or fluorochromes. The development of more sensitive quality control procedures than spectrophotometry is essential to assure the use of better fluorescent conjugates since the fluorescent conjugates are critical reagents for a variety of immunodiagnostic kits. In this article, we demonstrate a new flow cytometric protocol to evaluate conjugates by molecules of equivalent soluble fluorochromes (MESF) and by traditional flow cytometric analysis. We have coupled microspheres with anti-IgG-PE and anti-HBSAg-PE conjugates from distinct manufactures and/or different lots and evaluated by flow cytometry. Their fluorescence intensities were followed for a period of 18 months. Our results showed that there was a great difference in the fluorescence intensities between the conjugates studied. The differences were observed between manufactures and lots from both anti-IgG-PE and anti-HBSAg-PE conjugates. Coefficients of variation (CVs) showed that this parameter can be used to determine better coupling conditions, such as homogenous coupling. The MESF analysis, as well as geometric mean evaluation by traditional flow cytometry, showed a decrease in the values for all conjugates during the study and were indispensable tools to validate the results of stability tests. Our data demonstrated the feasibility of the flow cytometric method as a standard quality control of immunoassay kits.

Use of flow cytometry for rapid and accurate enumeration of live pathogenic Leptospira strains

Enumeration of Leptospira, the causative agent of leptospirosis, is arduous mainly because of its slow growth rate. Rapid and reliable tools for numbering leptospires are still lacking. The current standard for Leptospira cultures is the count on Petroff-Hausser chamber under dark-field microscopy, but this method remains time-consuming, requires well-trained operators and lacks reproducibility. Here we present the development of a flow-cytometry technique for counting leptospires. We showed that upon addition of fluorescent dyes, necessary to discriminate the bacterial population from debris, several live Leptospira strains could be enumerated at different physiologic states. Flow cytometry titers were highly correlated to counts with Petroff-Hausser chambers (R²>0.99). Advantages of flow cytometry lie in its rapidity, its reproducibility significantly higher than Petroff-Hausser method and its wide linearity range, from 10⁴ to 10⁸leptospires/ml. Therefore, flow cytometry is a fast, reproducible and sensitive tool representing a promising technology to replace current enumeration techniques of Leptospira in culture. We were also able to enumerate Leptospira in artificially infected urine and blood with a sensitivity limit of 10⁵leptospires/ml and 10⁶leptospires/ml, respectively, demonstrating the feasibility to use flow cytometry as first-line tool for diagnosis or bacterial dissemination studies.

A hydrogel based rapid test method for detection of Escherichia coli (E. coli) in contaminated water samples

We have formulated a new chemical composition for rapid detection ofEscherichia coli(E. coli) with currently available enzymatic substrates.

The effects of orange juice clarification on the physiology of Escherichia coli; growth-based and flow cytometric analysis

Orange juice (OJ) is a food product available in various forms which can be processed to a greater or lesser extent. Minimally-processed OJ has a high consumer perception but presents a potential microbiological risk due to acid-tolerant bacteria. Clarification of OJ (such as removal of cloud) is a common processing step in many OJ products. However, many of the antimicrobial components of OJ such as essential oils are present in the cloud fraction. Here, the effect of clarification by filtration on the viability and physiology of Escherichia coli K-12 was tested using total viable count (TVC) and flow cytometric (FCM) analysis. The latter technique was also used to monitor intracellular pH during incubation in OJ. Removal of the OJ cloud fraction was shown to have dramatic effects on bacterial viability and physiology during storage at a range of incubation temperatures. For instance, at 4 °C, a significantly lower number of healthy cells and a significantly higher number of injured cells were observed in 0.22 μm-filtered OJ at 24h post-inoculation, compared to filtered OJ samples containing particles between 0.22 μm and 11 μm in size. Similarly, there was a significant difference between the number of healthy bacteria in the 0.7 μm-filtered OJ and both 0.22 μm-filtered and 1.2 μm-filtered OJ after 24 hour incubation at 22.5 °C. This indicated that OJ cloud between 0.7 μm and 0.22 μm in size might have an adverse effect on the viability of E. coli K-12. Furthermore, FCM allowed the rapid analysis of bacterial physiology without the requirement for growth on agar plates, and revealed the extent of the viable but non-culturable (VBNC) population. For example, at 4 °C, while the FCM viable count did not substantially decrease until 48 h, decreases in TVC were observed between 0 and 48 hour incubation, due to a subset of injured bacteria entering the VBNC state, hence being unable to grow on agar plates. This study highlights the application of FCM in monitoring bacterial physiology in foods, and potential effects of OJ clarification on bacterial physiology.

Fluorescence-based tools for single-cell approaches in food microbiology

The better understanding of the functioning of microbial communities is a challenging and crucial issue in the field of food microbiology, as it constitutes a prerequisite to the optimization of positive and technological microbial population functioning, as well as for the better control of pathogen contamination of food. Heterogeneity appears now as an intrinsic and multi-origin feature of microbial populations and is a major determinant of their beneficial or detrimental functional properties. The understanding of the molecular and cellular mechanisms behind the behavior of bacteria in microbial communities requires therefore observations at the single-cell level in order to overcome "averaging" effects inherent to traditional global approaches. Recent advances in the development of fluorescence-based approaches dedicated to single-cell analysis provide the opportunity to study microbial communities with an unprecedented level of resolution and to obtain detailed insights on the cell structure, metabolism activity, multicellular behavior and bacterial interactions in complex communities. These methods are now increasingly applied in the field of food microbiology in different areas ranging from research laboratories to industry. In this perspective, we reviewed the main fluorescence-based tools used for single-cell approaches and their concrete applications with specific focus on food microbiology.

Impact of high hydrostatic pressure processing on individual cellular resuscitation times and protein aggregates in Escherichia coli

Live cell biology approaches can contribute to a more comprehensive understanding of heterogeneous injury and resuscitation phenomena in stressed populations of foodborne pathogens and spoilage microorganisms, and in turn lead to better insights in the mechanisms and dynamics of inactivation that can improve food safety and preservation measures. Especially in the context of designing minimal processing strategies, which depend on a synergistic combination of different mild stresses to ensure sufficient microbial reduction, a more profound understanding of the impact of each such stress or hurdle is mandatory. High hydrostatic pressure (HHP) stress is an interesting hurdle in this concept since cells that manage to survive this stress nevertheless tend to be injured and sensitized to subsequent stresses. In this study, populations of Escherichia coli were subjected to different HHP intensities and studied at the single-cell level with time-lapse fluorescence microscopy while monitoring resuscitation times and protein aggregate integrity at the single-cell level. This approach revealed that higher pressure intensities lead to longer and more variable resuscitation times of surviving cells as well as an increased dispersal of intracellular protein aggregates. Interestingly, at mild HHP exposure, cells within the population incurring less dispersion of protein aggregates appeared to have a higher probability of survival.

Investigation of the effectiveness of disinfectants against planktonic and biofilm forms of P. aeruginosa and E. faecalis cells using a compilation of cultivation, microscopic and flow cytometric techniques

This study evaluated the effectiveness of selected disinfectants against bacterial cells within a biofilm using flow cytometry, the conventional total viable count test and scanning electron microscopy (SEM). A flow cytometric procedure based on measurement of the cellular redox potential (CRP) was demonstrated to have potential for the rapid evaluation of activity against biofilm and planktonic forms of microbes. Quaternary ammonium compound-based disinfectant (QACB) demonstrated a higher level of anti-microbial activity than a performic acid preparation (PAP), with mean CRP values against P. aeruginosa cells of 2 and 1.33 relative fluorescence units (RFU) vs 63.33 and 61.33 RFU for 8 and 24 h cultures respectively. Flow cytometric evaluation of the anti-biofilm activity demonstrated a higher efficacy of QACB compared to PAP for P. aeruginosa cells of 1 and 0.66 RFU vs 18.33 and 22.66 RFU for 8 and 24 h cultures respectively. SEM images of treated P. aeruginosa cells demonstrated disinfectant-specific effects on cell morphology.

Phenolic composition, antioxidant capacity of Salvia verticcilata and effect on multidrug resistant bacteria by flow-cytometry

BACKGROUND

Antioxidants are of great importance for preventing oxidative stress that may cause several degenerative diseases. Studies have indicated phytochemicals have high free-radical scavenging activity, which helps to reduce the risk of chronic diseases. The aim of the present study is the determination of antioxidant properties, polyphenolic content and multidrug resistant bacteria of Salvia verticcilata L.

MATERIALS AND METHODS

Methanol was used as the extraction solvent. The total phenolic content was calculated using Folin-Ciocalteau method and phenolic composition was determined by HPLC. The radical scavenging activity of plant was evaluated in vitro based on the reduction of the stable DPPH free radical. The reducing capacity was identified by using the FRAP method. The ability of Salvia verticcilata L. to increase the permeability of multidrug resistant bacterial cells was conducted by flow cytometric assay on Listeria innocua and E-coli.

RESULTS

The amount of total phenolics was found to be 347.5 mg GA/g extract. The IC50 value and FRAP assay are 0.61, and 0.944 respectively, Free radical scavenging effect and FRAP values are less than synthetic antioxidant compounds (BHA and BHT). Eight phenolic compounds were found in Salvia verticcilata L. Intense concentration of S. verticcilata L. has destroyed 97 % of living cells for Listeria innocua and 94.86% for E-coli.

CONCLUSION

This study shows that methanolic extracts of Salvia verticcilata L. is a potential source of natural antioxidants and antimicrobial agent and can form the basis for pharmacological studies.

Evolution of microbiological analytical methods for dairy industry needs

Traditionally, culture-based methods have been used to enumerate microbial populations in dairy products. Recent developments in molecular methods now enable faster and more sensitive analyses than classical microbiology procedures. These molecular tools allow a detailed characterization of cell physiological states and bacterial fitness and thus, offer new perspectives to integration of microbial physiology monitoring to improve industrial processes. This review summarizes the methods described to enumerate and characterize physiological states of technological microbiota in dairy products, and discusses the current deficiencies in relation to the industry’s needs. Recent studies show that Polymerase chain reaction-based methods can successfully be applied to quantify fermenting microbes and probiotics in dairy products. Flow cytometry and omics technologies also show interesting analytical potentialities. However, they still suffer from a lack of validation and standardization for quality control analyses, as reflected by the absence of performance studies and official international standards.

Denitrification of industrial wastewater: Influence of glycerol addition on metabolic activity and community shifts in a microbial consortium

The wastewater originating from explosives manufacturing plants are characterized by a high concentration of nitrates (3200mgNL(-1)), sulfates (1470mgL(-1)) and low pH (1.5) as well as the presence of organic compounds, such as nitroglycerin (1.9mgL(-1)) and nitroglycol (4.8mgL(-1)). The application of glycerol (C/N=3) at such a high concentration enabled complete removal of nitrates and did not cause the anaerobic glycerol metabolic pathway of the DNC4 consortium to activate, as confirmed by the low concentrations of 1,3-propanediol (0.16gL(-1)) and acetic acid (0.11gL(-1)) in the wastewater. Increasing the glycerol content (C/N=5) contributed to a notable increase in the concentration of both compounds: 1.12gL(-1) for acetic acid and 1.82 for 1,3-PD (1,3-propanediol). The nitrate reduction rate was at 44mgNg(-1) biomass d(-1). In order to assess the metabolic activity of the microorganisms, a method to determine the redox potential was employed. It was established, that the microorganisms can be divided into four groups, based on the determined denitrification efficiency and zero-order nitrate removal constants. The first group, involving Pseudomonas putida and Pseudomonas stutzeri, accounts for microorganisms capable of the most rapid denitrification, the second involves rapid denitrifying microbes (Citrobacter freundi and Pseudomonas alcaligenes), the third group are microorganisms exhibiting moderate denitrification ability: Achrobactrum xylosoxidans, Ochrobactrum intermedium and Stenotrophomonas maltophila, while the last group consists of slow denitrifying bacteria: Rodococcus rubber and Sphignobacterium multivorum.

Effect of acidic pH on flow cytometric detection of bacteria stained with SYBR Green I and their distinction from background

Unspecific background caused by biotic or abiotic particles, cellular debris, or autofluorescence is a well-known interfering parameter when applying flow cytometry to the detection of microorganisms in combination with fluorescent dyes. We present here an attempt to suppress the background signal intensity and thus to improve the detection of microorganisms using the nucleic acid stain SYBR^® Green I. It has been observed that the fluorescent signals from SYBR Green I are greatly reduced at acidic pH. When lowering the pH of pre-stained samples directly prior to flow cytometric analysis, we hypothesized that the signals from particles and cells with membrane damage might therefore be reduced. Signals from intact cells, temporarily maintaining a neutral cytosolic pH, should not be affected. We show here that this principle holds true for lowering background interference, whereas the signals of membrane-compromised dead cells are only affected weakly. Signals from intact live cells at low pH were mostly comparable to signals without acidification. Although this study was solely performed with SYBR^® Green I, the principle of low pH flow cytometry (low pH-FCM) might hold promise when analyzing complex matrices with an abundance of non-cellular matter, especially when expanded to non-DNA binding dyes with a stronger pH dependence of fluorescence than SYBR Green I and a higher pK_a value.