Flow cytometry applications in physiological study and detection of foodborne microorganisms

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.

Copper-Based Compounds against Erwinia amylovora: Response Parameter Analysis and Suppression of Fire Blight in Apple

Fire blight, caused by Erwinia amylovora, is one of the major bacterial disease of apple and pear, causing enormous economic losses worldwide. Several control measures against E. amylovora have been reported till date, however, none of them have proved to be effective significantly against the pathogen. In this study, mechanisms of the copper-based control agents (CBCAs): copper oxychloride (COCHL), copper oxide (COX), copper hydroxide (CHY), copper sulfate basic (CSB), and tribasic copper sulfate (TCS) and their disease severity reduction efficacy against E. amylovora were analyzed. Bis-1,3-dibutylbarbituric acid trimethine oxonol, carboxyl fluorescein diacetate succinimidyl ester, and 5-cyano-2,3-ditolyl tetrazolium chloride staining were used to check the damage of membrane potential, cytoplasmic pHin, and respiration of CBCAs-treated E. amylovora, respectively. High disturbance in the membrane potential of E. amylovora was found under COX and COCHL treatments. Similarly, higher significant changes in the inner cytoplasmic pHin were observed under COX, COCHL, and TCS treatment. CHY and COCHL-treated E. amylovora showed a significant reduction in respiration. In vitro bioassay results revealed that CHY, CSB, and TCS at 2,000 ppm reduced the severity of fire blight both in pre- and post-treatment of CBCAs in immature apple fruits and seedlings. Overall, the most effective CBCAs against E. amylovora could be CHY at 2,000 ppm as its showed inhibition mechanisms and disease severity reduction.

Effect of curcumin on the quality properties of millet fresh noodle and its inhibitory mechanism against the isolated spoilage bacteria

Fresh noodle product has attracted increasing attention due to its nutritive value and convenience. However, the relative short shelf life of fresh noodle is still a concern that needs to resolve. The objective of this study was to evaluate the preservative effect of curcumin (CUR) on millet fresh noodle during storage and its inhibitory mechanism against two isolated spoilage bacteria (Bacillus cereus and Escherichia coli). The effects of CUR were evaluated with regard to the quality and sensory evaluation of millet fresh noodle, the changes of bacterial growth curve, cell intracellular substances, cell viability, and bacterial morphology. The results showed that CUR could decrease the total colony number and prolong the shelf life of millet fresh noodle stored at 25°C from 20 to 30 hr. Quality and sensory evaluations showed that addition of CUR caused no negative effect on noodle quality and was determined to be sensory acceptable. The minimum inhibitory concentration of CUR against B. cereus and E. coli was 0.125 and 0.5 mg/ml, respectively. The growth curve revealed that CUR presented good antibacterial effect against both bacteria. The leakage of intracellular substances, cell viability, and bacterial morphology change after CUR treatment confirmed the destructive effects of CUR on plasma membrane integrity. These results indicated that CUR had the potential to be applied as a natural preservative for controlling the growth of spoilage microorganisms and extending the shelf life of millet fresh noodle.

Flow Cytometry Combined With Single Cell Sorting to Study Heterogeneous Germination of Bacillus Spores Under High Pressure

Isostatic high pressure (HP) of 150 MPa can trigger the germination of bacterial spores, making them lose their extreme resistance to stress factors, and increasing their susceptibility to milder inactivation strategies. However, germination response of spores within a population is very heterogeneous, and tools are needed to study this heterogeneity. Here, classical methods were combined with more recent and powerful techniques such as flow cytometry (FCM) and fluorescence activated cell sorting (FACS) to investigate spore germination behavior under HP. Bacillus subtilis spores were treated with HP at 150 MPa and 37°C, stained with SYTO16 and PI, and analyzed via FCM. Four sub-populations were detected. These sub-populations were for the first time isolated on single cell level using FACS and characterized in terms of their heat resistance (80°C, 10 min) and cultivability in a nutrient-rich environment. The four isolated sub-populations were found to include (1) heat-resistant and mostly cultivable superdormant spores, i.e., spores that remained dormant after this specific HP treatment, (2) heat-sensitive and cultivable germinated spores, (3) heat-sensitive and partially-cultivable germinated spores, and (4) membrane-compromised cells with barely detectable cultivability. Of particular interest was the physiological state of the third sub-population, which was previously referred to as "unknown". Moreover, the kinetic transitions between different physiological states were characterized. After less than 10 min of HP treatment, the majority of spores germinated and ended up in a sublethally damaged stage. HP treatment at 150 MPa and 37°C did not cause inactivation of all geminated spores, suggesting that subsequent inactivation strategies such as mild heat inactivation or other inactivation techniques are necessary to control spores in food. This study validated FCM as a powerful technique to investigate the heterogeneous behavior of spores under HP, and provided a pipeline using FACS for isolation of different sub-populations and subsequent characterization to understand their physiological states.

Botrytis cinerea response to pulsed light: Cultivability, physiological state, ultrastructure and growth ability on strawberry fruit

Botrytis cinerea causes postharvest spoilage in important crops such as strawberry and other berries. Pulsed light (PL) treatment could be an environmentally friendly postharvest alternative to synthetic fungicides in berries. Cultivability, physiological state, ultrastructure of Botrytis cinerea suspended in peptone water and irradiated with PL (fluence = 1.2 to 47.8 J/cm²) were investigated by using conventional plate count technique, flow cytometry analysis (FCM) and transmission electron microscopy. In addition, PL effect on B. cinerea development in artificially contaminated strawberries throughout storage at (5 ± 1) °C was evaluated. PL reduced fungus' ability to form colonies on agarized culture media. Survival curve fitted with the Weibullian model evidenced a wide distribution of conidia sensitivity to PL. FCM showed that most of irradiated conidia entered in a viable non-culturable state, although a subpopulation without esterase activity and compromised membranes and a subpopulation with active esterase and intact membranes were also detected. PL attacked multiple targets in B. cinerea. Ultrastructural changes varied with the dose and within the conidia population, supporting FCM results. Damage included plasmalemma detachment from cell wall, cytoplasm collapse, and vacuolization of cytoplasm, disruption of cell wall and plasmalemma with massive loss of cytoplasm and/or disruption of organelles. In strawberries artificially contaminated with B. cinerea, a 2-day delay on the onset of the infection and a lower incidence in PL-treated strawberries (11.9 and 23.9 J/cm²) compared to control (16-20%) up to 10 days of cold storage was observed. Results indicated that PL significantly reduces B. cinerea growth in peptone water and in inoculated strawberries. However, other preservation factor(s) in combination would be needed to increase PL action for a better control of this fungus.

Heat and Chemical Treatments Affect the Viability, Morphology, and Physiology of Staphylococcus aureus and Its Subsequent Antibody Labeling for Flow Cytometric Analysis

The effects of heat and chemical treatments on Staphylococcus aureus viability and physiology and their subsequent effects on antibody binding ability and cell morphology were measured. Treatments included lethal and sublethal heat; exposure to organic acids, salt, and sodium hydroxide; and freeze-thawing. Strain-related differences in viability were noted depending on treatment and were reflected in changes in physiology as monitored by flow cytometry (FCM) using three different staining protocols: SYTO 9/propidium iodide (PI), DiOC₂(3), or calcein acetoxymethyl ester (calcein-AM)/PI. Treatments that resulted in significant losses in viability as measured by plate counting were reflected better by the first two staining combinations, as intracellular calcein-AM uptake may have been impaired by certain treatments. FCM analysis using labeling by commercial anti-S. aureus antibodies indicated that differences in cell physiology as a result of treatments influenced immunofluorescence detection. The ratio of the mean fluorescence intensities of stained cells to those of unstained cells [MFI/MFI_(us)] varied with treatment, five of these treatments, including freeze-thaw, citric acid, oxalic acid, NaCl, and NaOH treatments, resulted in significantly lower fluorescence values compared to controls.IMPORTANCE FCM data indicated that cells conventionally considered to be dead and which would not give rise to CFU in a plate count assay, e.g., cells heated to 80°C, were labeled by antibody staining. This finding suggests that without the inclusion of a live/dead discriminating dye, these cells would be erroneously detected as viable within an FCM assay. Reductions in antibody staining due to physicochemical treatment were strain related, reflecting the complexity of the phenomenon under study and illustrating that substantial validation of any new antibody detection-based method, including physiological staining and cell sorting, should be undertaken. Researchers should be aware of physicochemical treatments causing false-negative results: in this study, freeze-thawing severely reduced antibody binding without affecting the viability of a substantial percentage of cells. Scanning electron microscopy carried out on treated cells revealed a range of morphological changes resulting from physicochemical treatments which may have hindered antibody binding.

Forecasting cell fate during antibiotic exposure using stochastic gene expression

Antibiotic killing does not occur at a single, precise time for all cells within a population. Variability in time to death can be caused by stochastic expression of genes, resulting in differences in endogenous stress-resistance levels between individual cells in a population. Here we investigate whether single-cell differences in gene expression prior to antibiotic exposure are related to cell survival times after antibiotic exposure for a range of genes of diverse function. We quantified the time to death of single cells under antibiotic exposure in combination with expression of reporters. For some reporters, including genes involved in stress response and cellular processes like metabolism, the time to cell death had a strong relationship with the initial expression level of the genes. Our results highlight the single-cell level non-uniformity of antibiotic killing and also provide examples of key genes where cell-to-cell variation in expression is strongly linked to extended durations of antibiotic survival.

Antibacterial Effects of Phytic Acid against Foodborne Pathogens and Investigation of Its Mode of Action

This study investigated the antimicrobial mechanism of phytic acid (PA) and its antibacterial effects in combination with ethanol. The MIC of PA on Escherichia coli ATCC 11229, Staphylococcus aureus ATCC 6538P, Bacillus subtilis ATCC 6633, and Salmonella Typhimurium CICC 27483 were 0.24, 0.20, 0.26, and 0.28% (w/w), respectively. E. coli ATCC 11229 and S. aureus ATCC 6538P were selected to investigate the mechanism of PA by analyzing its effects at 1/2MIC and at MIC on the cell morphology, intracellular ATP, and cell membrane integrity. Environmental scanning electron microscope images revealed that PA was able to change the cell morphology and disrupt the intercellular adhesion. PA retarded bacterial growth and caused cell membrane dysfunction, which was accompanied by decreased intracellular ATP concentrations. Flow cytometry analysis further revealed that almost all the bacterial cells were damaged after treatment with PA at its MIC for 2 h. Moreover, PA has a synergistic antimicrobial ability when used in combination with ethanol. These results suggested that PA is effective in inhibiting growth of foodborne pathogens mainly by the mechanism of cell membrane damage and to provide a theoretical basis for the development of natural antimicrobial agents in the food industry.

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.

Efficacy of blue LED in microbial inactivation: Effect of photosensitization and process parameters

Efficacy of blue (462 ± 3 nm) Light emitting diode (LED) illumination to inactivate the foodborne pathogens like Escherichia coli and Staphylococcus aureus in the presence of exogenous photosensitizer (curcumin) was studied in vitro. The effect of temperature, concentration of photosensitizer and incubation time with photosensitizer for microbial inactivation was investigated and sublethal injury of cells was determined. Mechanism of inactivation by the combination of photosensitizer and blue light was also examined. A maximum reduction of 5.94 ± 0.22 and 5.91 ± 0.20 log CFU/ml was obtained for E. coli and S. aureus, respectively, when treated with photosensitizer (20 μM) at 13 J/cm² of blue light. There was no significant change in the inactivation of these pathogens both at 9 °C and 27 °C in the presence of photosensitizer. Even, the incubation with the photosensitizer didn't show any significant difference on the inactivation of these food-borne pathogens. Sublethal injury (>90% injury) was also observed for the cells treated with photosensitizer and blue light simultaneously. Confocal laser scanning microscopy analysis revealed that membrane integrity was disturbed due to photodynamic activity of curcumin in both the bacteria. Further, both cells produced intracellular reactive oxygen species by the action of photosensitizer and blue light. Scanning electron microscopy of E. coli and S. aureus cells treated with photosensitizer and blue light showed morphological changes in the cell wall compared to untreated group. The study indicated that photodynamic inactivation of foodborne pathogens using LED-based photosensitization can be explored as a potential technique for food safety.

A novel synthetic flavonoid with potent antibacterial properties: In vitro activity and proposed mode of action

The emergence of pathogenic multidrug-resistant bacteria demands new approaches in finding effective antibacterial agents. Synthetic flavonoids could be a reliable solution due to their important antimicrobial activity. We report here the potent in vitro antibacterial activity of ClCl-flav—a novel synthetic tricyclic flavonoid. The antimicrobial effects were tested using the minimum inhibitory concentration (MIC), time kill and biofilm formation assays. Fluorescence microscopy and scanning electron microscopy were employed to study the mechanism of action. MTT test was used to assess the cytotoxicity of ClCl-flav. Our results showed that Gram positive bacteria were more sensitive (MIC = 0.24 μg/mL) to ClCl-flav compared to the Gram negative ones (MIC = 3.9 μg/mL). We found that our compound showed significantly enhanced antibacterial activities, 32 to 72-fold more active than other synthetic flavonoids. ClCl-flav showed bactericidal activity at concentrations ranging from 0.48 to 15.62 μg/mL. At twice the MIC, all Escherichia coli and Klebsiella pneumoniae cells were killed within 1 h. Also ClCl-flav presented good anti-biofilm activity. The mechanism of action is related to the impairment of the cell membrane integrity. No or very low cytotoxicity was evidenced at effective concentrations against Vero cells. Based on the strong antibacterial activity and cytotoxicity assessment, ClCl-flav has a good potential for the design of new antimicrobial agents.

Flow Cytometry Is a Powerful Tool for Assessment of the Viability of Fungal Conidia in Metalworking Fluids

Fungal contamination of metalworking fluids (MWF) is a dual problem in automated processing plants because resulting fungal biofilms obstruct cutting, drilling, and polishing machines. Moreover, some fungal species of MWF comprise pathogens such as Fusarium solani Therefore, the development of an accurate analytical tool to evaluate conidial viability in MWF is important. We developed a flow cytometric method to measure fungal viability in MWF using F. solani as the model organism. To validate this method, viable and dead conidia were mixed in several proportions and flow was cytometrically analyzed. Subsequently, we assessed the fungicidal activity of two commercial MWF using flow cytometry (FCM) and compared it with microscopic analyses and plating experiments. We evaluated the fungal growth in both MWF after 7 days using quantitative PCR (qPCR) to assess the predictive value of FCM. Our results showed that FCM distinguishes live from dead conidia as early as 5 h after exposure to MWF, whereas the microscopic germination approach detected conidial viability much later and less accurately. At 24 h, microscopic analyses of germinating conidia and live/dead analyses by FCM correlated well, although the former consistently underestimated the proportion of viable conidia. In addition, the reproducibility and sensitivity of the flow cytometric method were high and allowed assessment of the fungicidal properties of two commercial MWF. Importantly, the obtained flow cytometric results on viability of F. solani conidia at both early time points (5 h and 24 h) correlated well with fungal biomass measurements assessed via a qPCR methodology 7 days after the start of the experiment.IMPORTANCE This result shows the predictive power of flow cytometry (FCM) in assessing the fungicidal capacity of MWF formulations. It also implies that FCM can be implemented as a rapid detection tool to estimate the viable fungal load in an industrial processing matrix (MWF).

Online flow cytometry, an interesting investigation process for monitoring lipid accumulation, dimorphism, and cells' growth in the oleaginous yeast Yarrowia lipolytica JMY 775

This study aims to understand and better control the main biological mechanisms and parameters modulating the various phenomena affecting Yarrowia lipolytica JMY775 and its lipids accumulation. The results obtained in this study stress forward that the use of an original tool, consisting of coupling bioreactors to online flow cytometry, is highly efficient. Throughout 48 h of culturing, this emerging process allowed an online continuous observation of the effects of pH and/or aeration on the cell growth and dimorphism and lipid accumulation by Y. lipolytica. This present study showed clearly that online flow cytometry is an advantageous tool for the real-time monitoring of microbial culture at a single-cell level. Indeed, the present investigation showed for the first time that profiling of the various phenomena and their monitoring upon culture time is now possible by coupling online cytometry with culture bioreactors.

Stochastic expression of a multiple antibiotic resistance activator confers transient resistance in single cells

Transient resistance can allow microorganisms to temporarily survive lethal concentrations of antibiotics. This can be accomplished through stochastic mechanisms, where individual cells within a population display diverse phenotypes to hedge against the appearance of an antibiotic. To date, research on transient stochastic resistance has focused primarily on mechanisms where a subpopulation of cells enters a dormant, drug-tolerant state. However, a fundamental question is whether stochastic gene expression can also generate variable resistance levels among growing cells in a population. We hypothesized that stochastic expression of antibiotic-inducible resistance mechanisms might play such a role. To investigate this, we focused on a prototypical example of such a system: the multiple antibiotic resistance activator MarA. Previous studies have shown that induction of MarA can lead to a multidrug resistant phenotype at the population level. We asked whether MarA expression also has a stochastic component, even when uninduced. Time lapse microscopy showed that isogenic cells express heterogeneous, dynamic levels of MarA, which were correlated with transient antibiotic survival. This finding has important clinical implications, as stochastic expression of resistance genes may be widespread, allowing populations to hedge against the sudden appearance of an antibiotic.

Using UVC Light-Emitting Diodes at Wavelengths of 266 to 279 Nanometers To Inactivate Foodborne Pathogens and Pasteurize Sliced Cheese

UVC light is a widely used sterilization technology. However, UV lamps have several limitations, including low activity at refrigeration temperatures, a long warm-up time, and risk of mercury exposure. UV-type lamps only emit light at 254 nm, so as an alternative, UV light-emitting diodes (UV-LEDs) which can produce the desired wavelengths have been developed. In this study, we validated the inactivation efficacy of UV-LEDs by wavelength and compared the results to those of conventional UV lamps. Selective media inoculated with Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, and Listeria monocytogenes were irradiated using UV-LEDs at 266, 270, 275, and 279 nm in the UVC spectrum at 0.1, 0.2, 0.5, and 0.7 mJ/cm(2), respectively. The radiation intensity of the UV-LEDs was about 4 μW/cm(2), and UV lamps were covered with polypropylene films to adjust the light intensity similar to those of UV-LEDs. In addition, we applied UV-LED to sliced cheese at doses of 1, 2, and 3 mJ/cm(2). Our results showed that inactivation rates after UV-LED treatment were significantly different (P < 0.05) from those of UV lamps at a similar intensity. On microbiological media, UV-LED treatments at 266 and 270 nm showed significantly different (P < 0.05) inactivation effects than other wavelength modules. For sliced cheeses, 4- to 5-log reductions occurred after treatment at 3 mJ/cm(2) for all three pathogens, with negligible generation of injured cells.

Responses of Escherichia coli, Listeria monocytogenes, and Staphylococcus aureus to simulated food processing treatments, determined using fluorescence-activated cell sorting and plate counting

Three common food pathogenic microorganisms were exposed to treatments simulating those used in food processing. Treated cell suspensions were then analyzed for reduction in growth by plate counting. Flow cytometry (FCM) and fluorescence-activated cell sorting (FACS) were carried out on treated cells stained for membrane integrity (Syto 9/propidium iodide) or the presence of membrane potential [DiOC2(3)]. For each microbial species, representative cells from various subpopulations detected by FCM were sorted onto selective and nonselective agar and evaluated for growth and recovery rates. In general, treatments giving rise to the highest reductions in counts also had the greatest effects on cell membrane integrity and membrane potential. Overall, treatments that impacted cell membrane permeability did not necessarily have a comparable effect on membrane potential. In addition, some bacterial species with extensively damaged membranes, as detected by FCM, appeared to be able to replicate and grow after sorting. Growth of sorted cells from various subpopulations was not always reflected in plate counts, and in some cases the staining protocol may have rendered cells unculturable. Optimized FCM protocols generated a greater insight into the extent of the heterogeneous bacterial population responses to food control measures than did plate counts. This study underlined the requirement to use FACS to relate various cytometric profiles generated by various staining protocols with the ability of cells to grow on microbial agar plates. Such information is a prerequisite for more-widespread adoption of FCM as a routine microbiological analytical technique.

Membrane permeabilization and cellular death of Escherichia coli, Listeria monocytogenes and Saccharomyces cerevisiae as induced by high pressure carbon dioxide treatment

In this study, the relationship between (irreversible) membrane permeabilization and loss of viability in Escherichia coli, Listeria monocytogenes and Saccharomyces cerevisiae cells subjected to high pressure carbon dioxide (HPCD) treatment at different process conditions including temperature (35-45 degrees C), pressure (10.5-21.0 MPa) and treatment time (0-60 min) was examined. Loss of membrane integrity was measured as increased uptake of the fluorescent dye propidium iodide (PI) with spectrofluorometry, while cell inactivation was determined by viable cell count. Uptake of PI by all three strains indicated that membrane damage is involved in the mechanism of HPCD inactivation of vegetative cells. The extent of membrane permeabilization and cellular death increased with the severity of the HPCD treatment. The resistance of the three tested organisms to HPCD treatment changed as a function of treatment time, leading to significant tailing in the survival curves, and was dependent on pressure and temperature. The results in this study also indicated a HPCD-induced damage on nucleic acids during cell inactivation. Transmission electron microscopy showed that HPCD treatment had a profound effect on the intracellular organization of the micro-organisms and influenced the permeability of the bacterial cells by introducing pores in the cell wall.

Toxicity of triclosan, penconazole and metalaxyl on Caulobacter crescentus and a freshwater microbial community as assessed by flow cytometry

Biocides are widely used for domestic hygiene, agricultural and industrial applications. Their widespread use has resulted in their introduction into the environment and raised concerns about potential deleterious effects on aquatic ecosystems. In this study, the toxicity of the biocides triclosan, penconazole and metalaxyl were evaluated with the freshwater bacterium Caulobacter crescentus and with a freshwater microbial community using a combination of single- and double-stain flow cytometric assays. Growth of C.  crescentus and the freshwater community were repressed by triclosan but not by penconazole or metalaxyl at concentrations up to 250 μM. The repressive effect of triclosan was dependent on culture conditions. Caulobacter crescentus was more sensitive to triclosan when grown with high glucose at high cell density than when grown directly in sterilized lake water at low cell density. This suggests that the use of conventional growth conditions may overestimate biocide toxicity. Additional experiments showed that the freshwater community was more sensitive to triclosan than C.  crescentus, with 10 nM of triclosan being sufficient to repress growth and change the phylogenetic composition of the community. These results demonstrate that isolate-based assays may underestimate biocide toxicity and highlight the importance of assessing toxicity directly on natural microbial communities. Because 10 nM of triclosan is within the range of concentrations observed in freshwater systems, these results also raise concerns about the risk of introducing triclosan into the environment.

Estimation of microbial viability using flow cytometry

For microorganisms in particular, viability is a term that is difficult to define and a state consequently difficult to measure. The traditional (and gold-standard) usage equates viability and culturability (i.e., the ability to multiply), but the process of determining culturability is often too slow. Flow cytometry provides the opportunity to make rapid and quantitative measurements of dye uptake in large numbers of cells, and we can therefore exploit the flow cytometric approach to evaluate so-called viability stains and to develop protocols for more routine assessments of microbial viability. This unit is primarily commentary, but several basic protocols have been included to ensure that users have a firm basis for attempting these reasonably difficult assays on traditional flow cytometer instruments. What is clear is that each assay must be carefully validated with the particular microorganism of interest before being applied in any research, clinical, or service form.

Acid tolerance of Streptococcus macedonicus as assessed by flow cytometry and single-cell sorting

An in situ flow cytometric viability assay employing carboxyfluorescein diacetate and propidium iodide was used to identify Streptococcus macedonicus acid tolerance phenotypes. The logarithmic-phase acid tolerance response (L-ATR) was evident when cells were (i) left to autoacidify unbuffered medium, (ii) transiently exposed to nonlethal acidic pH, or (iii) systematically grown under suboptimal acidic conditions (acid habituation). Stationary-phase ATR was also detected; this phenotype was gradually degenerated while cells resided at this phase. Single-cell analysis of S. macedonicus during induction of L-ATR revealed heterogeneity in both the ability and the rate of tolerance acquisition within clonal populations. L-ATR was found to be partially dependent on de novo protein synthesis and compositional changes of the cell envelope. Interestingly, acid-habituated cells were interlaced in lengthier chains and exhibited an irregular pattern of active peptidoglycan biosynthesis sites when probed with BODIPY FL vancomycin. L-ATR caused cells to retain their membrane potential after lethal challenge, as judged by ratiometric analysis with oxonol [DiBAC(4)(3)]. Furthermore, F-ATPase was important during the induction of L-ATR, but in the case of a fully launched response, inhibition of F-ATPase affected acid resistance only partially. Activities of both F-ATPase and the glucose-specific phosphoenolpyruvate-dependent phosphotransferase system were increased after L-ATR induction, distinguishing S. macedonicus from oral streptococci. Finally, the in situ viability assessment was compared to medium-based recovery after single-cell sorting, revealing that the culturability of subpopulations with identical fluorescence characteristics is dependent on the treatments imposed to the cells prior to acid challenge.

Viability and physiological state transitions of Rhizopus oligosporus sporangiospores in tempe starter culture

The viability and various physiological characteristics of individual sporangiospores of Rhizopus oligosporus in tempe starter cultures that had been stored for 8, 10, 16 and 30 months were examined by flow cytometry in combination with fluorescent dyes. Besides live, dead, and dormant spores we distinguished a category of sublethally damaged spores. Results indicated that the shelf-life of tempe starters was not limited by the death of spores, but by sublethal damage to spores as well as by dormancy which can be overcome by resuscitation, respiratory activation. During storage, the number of dormant and sublethally damaged spores increased: the longer the starter cultures were stored, the less dormant spores could still be activated. In contrast, the transition from sublethally damaged (spores that are not able to transform cFDA and emit green fluorescence except by activation treatment) to activated spores did not decrease with longer storage. However, after very long (30 months) storage, sublethally damaged spores could still be activated but could not germinate anymore. The shelf-life of spores in tempe starter is related to the physiological state of spores being sublethally damaged; a mechanism of physiological state transitions of R. oligosporus sporangiospores is proposed.

Rapid assessment of the physiological status of Streptococcus macedonicus by flow cytometry and fluorescence probes

Flow cytometry in combination with fluorescence probes was applied to rapidly assess the physiological status of Streptococcus macedonicus ACA-DC 198, a newly described member of the lactic acid bacteria group with technologically important features (e.g. lantibiotic production). A sonication procedure was developed for disaggregating typical streptococci chains in order to optimize cell preparations for single cell analysis. Single stained live and dead populations of S. macedonicus cells were clearly resolved based on membrane potential by bis-oxonol [DiBAC(4)(3)], membrane integrity by Propidium Iodide (PI) and enzymatic activity as well as membrane integrity by Carboxyfluorescein Diacetate (cFDA). Further, estimation of both live and dead cells by a cFDA/PI two-colour flow cytometric assay showed excellent correlation with the dead cells in the samples (dead(FCM)=0.9945 dead(S)-0.806, R(2)=0.9986 and live(FCM)=-0.978 dead(S)+98.895, R(2)=0.9992). Finally, the assay was applied to study the physiology of S. macedonicus after acid stress. Interestingly, in situ assessment of the physiological status of stressed S. macedonicus cells by flow cytometry and single cell sorting revealed the coexistence of three distinct subpopulations according to their fluorescence labelling behaviour and culturability, representing intact/culturable, permeabilized/dead and potentially injured cells with the latter exhibiting both metabolic activity and membrane permeabilization as well as decreased culturability.

Bioaerosol characterization by flow cytometry with fluorochrome

Traditional culture and microscopy methods for evaluation of bioaerosols are slow, tedious, and rather imprecise. In this study, the application of flow cytometry that was combined with a fluorescent technique (FCM/FL) was evaluated as a technique to quickly and accurately determine and quantify the total concentration and viability of bioaerosols. The optimal conditions of five fluorescent dyes [acridine orange (AO), SYTO-13, propidium iodide (PI), YOPRO-1, and 5-cyano-2,3-ditolytetrazolium chloride (CTC)] used in FCM/FL were determined for laboratory samples of bacterial aerosols (Escherichia coli, and endospores of Bacillus subtilis) and fungal aerosols (Candida famata and Penicillium citrinum spores). Based on the measured cell concentration, fluorescence intensity, and staining efficiency as indicators for dye performance evaluation, SYTO-13 was found to be the most suitable fluorescent dye for determining the total concentration of the bioaerosols, as well as YOPRO-1 was the most suitable for determining viability. Moreover, the established optimal FCM/FL with dyes was validated for characterizing microorganism profiles from both air and water samples from the aeration tank of hospital wastewater treatment plant. In conclusion, the FCM/FL successfully assessed the total concentration and viability for bacterial and fungal microorganisms in environmental field samples.

Cellular injuries upon exposure of Escherichia coli and Lactobacillus rhamnosus to high-intensity ultrasound

AIMS

To examine cellular injuries occurring in cells of Escherichia coli (Gram-negative bacteria) and Lactobacillus rhamnosus (Gram-positive bacteria) in response to a high-intensity ultrasound treatment using classical plate count technique and flow cytometry.

METHOD AND RESULTS

According to plate count results, E. coli (D-value 8.3 min) was far more sensitive than L. rhamnosus (D-value 18.1 min) in their response to the ultrasound intensity applied (20 kHz, 17.6 W). The dye precursor carboxyfluorescein diacetate (cFDA) could freely diffuse across the cytoplasmic membrane of intact cells of Gram-positive bacteria L. rhamnosus, resulting in its intracellular enzymatic conversion and emission of green fluorescence. In contrast, the presence of an outer membrane on E. coli, which represents the class of Gram-negative bacteria, apparently disabled the penetration of viability marker cFDA. Ultrasound application on E. coli yielded in an increasing population with disintegrated outer membrane, which allowed penetration of cFDA and its intracellular enzymatic conversion as well as accumulation. In both organisms evaluated only a small population was labelled by propidium iodide upon exposure to ultrasound for up to 20 min. Within the experimental conditions investigated ultrasound did not considerably affect the cytoplasmic membrane, although according to plate count results viability loss occurred.

CONCLUSIONS

The results compiled suggest, that ultrasound induced cell death, which may not be related to membrane damage.

SIGNIFICANCE AND IMPACT OF THE STUDY

Limitation on the use of bacteriocins, which are aimed on destabilization of cytoplasmic membrane but inhibited by the outer membrane, could be overcome by ultrasound-assisted physical disruption of the outer membrane.

Application of a microplate scale fluorochrome staining assay for the assessment of viability of probiotic preparations

Cell viability in probiotic preparations is traditionally assessed by the plate count technique. Additionally, fluorescent staining combined with epifluorescence microscopy or flow cytometry has been developed for the viability assessment, but the currently available assays are either laborious or require highly sophisticated equipment. The aim of this study was to investigate the applicability of a microplate scale fluorochrome assay for predicting the cell state of freeze-dried Lactobacillus rhamnosus and Bifidobacterium animalis subsp. lactis preparations. In addition to viability assessment with LIVE/DEAD BacLight Bacterial Viability Kit, DiBAC(4)3 stain was used for the kinetic measurement of changes in bifidobacterial cell membrane functions during exposure to low pH. The microplate scale fluorochrome assay results on the viability and cell numbers of probiotic preparations correlated well with the results obtained with the culture-based technique and (with few exceptions) with epifluorescence microscopy. The assay was applicable also for the viability assessment of stressed (acid-treated) cells provided that the cell density in treatments was adjusted to the optimal measurement level of the fluorometer. The microplate scale fluorochrome assay offers a rapid and robust tool for the viability assessment of probiotic preparations, and enables also kinetic measurements.

Real-time quantitative PCR with gene probe, fluorochrome and flow cytometry for microorganism analysis

Microorganism concentrations and viability can be better understood and clarified by using both culture and non-culture methods. Here, using pure suspensions of E. coli, three non-culture methods, namely, flow cytometry (FCM), epifluorescence microscopy (EFM), and real-time quantitative polymerase chain reaction (real-time qPCR), were compared with a traditional culture-based method. Using fluorocome-labeling methods with FCM and EFM applications, acridine orange (AO) and propidium iodide (PI) dyes were used to determine the total cell concentration and microorganism viability, respectively. The results indicated that total cell concentrations determined using FCM were statistically higher (2.62-4.94 times) than those determined using EFM. The difference might be due to cell losses induced by extensive preparations needed for EFM. In addition, EFM and FCM were highly associated for both the total cell concentration and viability. FCM-measured viability was the highest, whereas the culture-measured viability was the lowest. Furthermore, DNA concentrations measured by real-time qPCR with gene probe were highly associated with the total number concentrations measured by either the EFM or FCM. In summary, the three non-culture methods compared here could provide rapid and accurate information about microorganism concentrations and viabilities.

Use of fluorescence ratio imaging microscopy and flow cytometry for estimation of cell vitality for Bacillus licheniformis

For Bacillus licheniformis SJ4628, an organism widely used in the enzyme industry, methods for determination of cell vitality at a single cell level using 5(6)-carboxyfluorescein diacetate succinimidyl ester in combination with fluorescence ratio imaging microscopy and flow cytometry were developed. Immediately after inoculation and during growth, changes in intracellular pH values determined by fluorescence ratio imaging microscopy and in green fluorescence intensities determined by flow cytometry were observed. Correlations between the capacity to multiply and intracellular pH or green fluorescence intensity were demonstrated. Populations of cells not having a pH gradient or exhibiting low fluorescence intensities had significantly longer lag phases than populations of cells with a pH gradient and high fluorescence intensities.

Weakening and Delayed Mortality of Fusarium oxysporum by Heat Treatment: Flow Cytometry and Growth Studies

ABSTRACT Survival of Fusarium oxysporum f. sp. niveum following heat treatments was studied using flow cytometric, physiological, and microscopic assays. We exposed germinating conidia to sublethal temperatures from 36 to 42 degrees C for 60 min, followed by rhodamine 123 staining and flow cytometry, and found increasing levels of fluorescence that reflect a change in mitochondrial membrane potential, indicating a weakening induced by stress. Viability of conidia or germinating conidia of the fungus exposed to heat decreased with increasing temperature, as assessed by fluorescent staining. However, viability was higher than that assessed with the 5-day-long plate count method and was further reduced 13 and 24 h after treatment, suggesting delayed mortality of the heat-treated germinating conidia. Delayed mortality was substantiated by observing these conidia with light and fluorescent scanning electron microscopy and by subculturing single germinating conidia that had been previously heated. Programmed cell death was not observed in heat-treated conidia or germinating conidia of F. oxysporum based on the detection of plasma membrane phosphatidylserine translocation, cell-cycle measurements, detection of DNA fragmentation, or microscopic observation of apoptotic bodies. We hypothesize that propagules, which survived the heating and apparently are alive, may undergo further irreversible detrimental processes, eventually leading to their death by yet unidentified mechanisms. These findings suggest that pathogen propagules also might be affected under lower temperatures, possibly facilitating pathogen control by heating.

Development of a flow cytometric method to analyze subpopulations of bacteria in probiotic products and dairy starters

Flow cytometry (FCM) is a rapid and sensitive technique that can determine cell numbers and measure various physiological characteristics of individual cells by using appropriate fluorescent probes. Previously, we developed an FCM assay with the viability probes carboxyfluorescein diacetate (cFDA) and TOTO-1 [1'-(4,4,7,7-tetramethyl-4,7-diazaundecamethylene)-bis-4-[3-methyl-2,3dihydro(benzo-1,3-oxazole)-2-methylidene]-1-(3'-trimethylammoniumpropyl)-pyridinium tetraiodide] for (stressed) lactic acid bacteria (C. J. Bunthof, K. Bloemen, P. Breeuwer, F. M. Rombouts, and T. Abee, Appl. Environ. Microbiol. 67:2326-2335, 2001). cFDA stains intact cells with enzymatic activity, and TOTO-1 stains membrane-permeabilized cells. Here we used this assay to study the viability of bacterial suspensions in milk, dairy fermentation starters, and probiotic products. To facilitate FCM analysis of bacteria in milk, a commercially available milk-clearing solution was used. The procedure was optimized to increase the signal-to-noise ratio. FCM enumerations were accurate down to a concentration of 10(5) cells ml(-1). The level of retrieval of Lactobacillus plantarum WCFS 1 suspended in milk was high, and viability was not affected by the procedure. The plate counts for cleared samples of untreated cell suspensions were nearly as high as the total FCM counts, and the correlation was strong (r > 0.99). In dairy fermentation starters and in probiotic products the FCM total cell counts were substantially higher than the numbers of CFU. Three functional populations could be distinguished: culturable cells, cells that are intact and metabolically active but not culturable, and permeabilized cells. The proportions of the populations differed in the products tested. This FCM method provides tools to assess the functionality of different populations in fermentation starters and probiotic products.

Rapid assessment of the physiological status of the polychlorinated biphenyl degrader Comamonas testosteroni TK102 by flow cytometry

The viability of the polychlorinated biphenyl-degrading bacterium Comamonas testosteroni TK102 was assessed by flow cytometry (FCM) with the fluorogenic ester Calcein-AM (CAM) and the nucleic acid dye propidium iodide (PI). CAM stained live cells, whereas PI stained dead cells. When double staining with CAM and PI was performed, three physiological states, i.e., live (calcein positive, PI negative), dead (calcein negative, PI positive), and permeabilized (calcein positive, PI positive), were detected. To evaluate the reliability of this double-staining method, suspensions of live and dead cells were mixed in various proportions and analyzed by FCM. The proportion of dead cells measured by FCM directly correlated with the proportion of dead cells in the sample (y = 0.9872 x + 0.18; R(2) = 0.9971). In addition, the proportion of live cells measured by FCM inversely correlated with the proportion of dead cells in the sample (y = -0.9776 x + 98.36; R(2) = 0.9962). The proportion of permeabilized cells was consistently less than 2%. These results indicate that FCM in combination with CAM and PI staining is rapid (<or=1 h) and distinguishes correctly among live, dead, and permeabilized cells.

Debating the biological reality of modelling preservation

Predictive food microbiology is a rapidly developing science and has made great advances. The aim is to debate a number of issues in modelling preservation: (1) inoculum and prehistory effects on lag times and process susceptibility; (2) mechanistic vs. empirical modelling; and (3) concluding remarks (the Species concept, methodology and biovariability). Increasing the awareness in these issues may bridge the gap between the complex reality in food microbial physiology and the application potential of predictive models. The challenge of bringing integrated preservation or risk analysis further and developing ways to truly model and link biological susceptibility distributions from raw ingredients via process survival to outgrowth probabilities in the final product remains.

A study of the minimum inhibitory concentration and mode of action of oregano essential oil, thymol and carvacrol

AIMS

The minimum inhibitory concentration (MIC) of oregano essential oil (OEO) and two of its principle components, i.e. thymol and carvacrol, against Pseudomonas aeruginosa and Staphylococcus aureus was assessed by using an innovative technique. The mechanism of action of the above substances was also investigated.

METHODS AND RESULTS

The applied technique uses 100-well microtitre plate and collects turbidimetric growth data. To produce the inhibition profiles, a wide range of concentrations were tested for each of the three compounds, as well as for carvacrol-thymol mixtures. Following a specific mathematical analysis of the observed inhibition profiles from all compounds, it was suggested that mixtures of carvacrol and thymol gave an additive effect and that the overall inhibition by OEO can be attributed mainly to the additive antimicrobial action of these two compounds. Addition of low amounts of each additive: (a) increased permeability of cells to the nuclear stain EB, (b) dissipated pH gradients as indicated by the CFDA-SE fluorescent probe irrespective of glucose availability and (c) caused leakage of inorganic ions.

CONCLUSION

Mixing carvacrol and thymol at proper amounts may exert the total inhibition that is evident by oregano essential oil. Such inhibition is due to damage in membrane integrity, which further affects pH homeostasis and equilibrium of inorganic ions.

SIGNIFICANCE AND IMPACT OF THE STUDY

The knowledge of extent and mode of inhibition of specific compounds, which are present in plant extracts, may contribute to the successful application of such natural preservatives in foods, since certain combinations of carvacrol-thymol provide as high inhibition as oregano essential oil with a smaller flavour impact.

Membrane permeabilization in relation to inactivation kinetics of Lactobacillus species due to pulsed electric fields

Membrane permeabilization due to pulsed electric field (PEF) treatment of gram-positive Lactobacillus cells was investigated by using propidium iodide uptake and single-cell analysis with flow cytometry. Electric field strength, energy input, treatment time, and growth phase affected membrane permeabilization of Lactobacillus plantarum during PEF treatment. A correlation between PEF inactivation and membrane permeabilization of L. plantarum cells was demonstrated, whereas no relationship was observed between membrane permeabilization and heat inactivation. The same results were obtained with a Lactobacillus fermentum strain, but the latter organism was more PEF resistant and exhibited less membrane permeabilization, indicating that various bacteria have different responses to PEF treatment. While membrane permeabilization was the main factor involved in the mechanism of inactivation, the growth phase and the acidity of the environment also influenced inactivation. By using flow cytometry it was possible to sort cells in the L. plantarum population based on different cell sizes and shapes, and the results were confirmed by image analysis. An apparent effect of morphology on membrane permeabilization was observed, and larger cells were more easily permeabilized than smaller cells. In conclusion, our results indicate that the ability of PEF treatment to cause membrane permeabilization is an important factor in determining inactivation. This finding should have an effect on the final choice of the processing parameters used so that all microorganisms can be inactivated and, consequently, on the use of PEF treatment as an alternative method for preserving food products.