Rapid detection of viable microbes with 5-cyano-2,3-di-(p-tolyl)tetrazolium chloride and 5(6)-carboxyfluorescein diacetate using a fibre fluorescence spectroscopy system

AIMS

To assess the efficacy of two commercially available viability dyes, 5-cyano-2,3-di-(p-tolyl)tetrazolium chloride (CTC) and 5(6)-carboxyfluorescein diacetate (CFDA), in reporting on viable cell concentration and species using an all-fibre fluorometer.

METHODS AND RESULTS

Four bacterial species (two Gram-positive and two Gram-negative) commonly associated with food poisoning or food spoilage (Escherichia coli, Salmonella enterica, Staphylococcus aureus, and Bacillus cereus) were stained with CTC or CFDA and the fibre fluorometer was used to collect full fluorescence emission spectra. A good correlation between concentration and fluorescence intensity was found for Gram-negative bacteria between 107 and 108 colony-forming units (CFU) ml-1. There was no correlation with concentration for Gram-positive bacteria; however, the information in the CTC and CFDA spectra shows the potential to distinguish Gram-negative cells from Gram-positive cells, although it may simply reflect the overall bacterial metabolic activity under staining conditions from this study.

CONCLUSIONS

The limit of detection (LoD) is too high in the dip-probe approach for analysis; however, the development of an approach measuring the fluorescence of single cells may improve this limitation. The development of new bacteria-specific fluorogenic dyes may also address this limitation. The ability to differentiate bacteria using these dyes may add value to measurements made to enumerate bacteria using CTC and CFDA.

A preliminary investigation into bacterial viability using scanning electron microscopy–energy-dispersive X-ray analysis: The case of antibiotics

The metabolic stages of bacterial development and viability under different stress conditions induced by disinfection, chemical treatments, temperature, or atmospheric changes have been thoroughly investigated. Here, we aim to evaluate early metabolic modifications in bacteria following induced stress, resulting in alterations to bacterial metabolism. A protocol was optimized for bacterial preparation using energy-dispersive X-ray (EDX) microanalysis coupled with scanning electron microscopy (SEM), followed by optimizing EDX data acquisition and analysis. We investigated different preparation methods aiming to detect modifications in the bacterial chemical composition at different states. We first investigated Escherichia coli, acquiring data from fresh bacteria, after heat shock, and after contact with 70% ethanol, in order to prove the feasibility of this new strategy. We then applied the new method to different bacterial species following 1 h of incubation with increasing doses of antibiotics used as a stress-inducing agent. Among the different materials tested aiming to avoiding interaction with bacterial metabolites, phosphorous-doped silicon wafers were selected for the slide preparation. The 15 kV acceleration voltage ensured all the chemical elements of interest were excited. A thick layer of bacterial culture was deposited on the silicon wafer providing information from multiple cells and intra-cellular composition. The EDX spectra of fresh, heat-killed, and alcohol-killed E. coli revealed important modifications in magnesium, potassium, and sodium. Those same alterations were detected when applying this strategy to bacteria exposed to antibiotics. Tests based on SEM–EDX acquisition systems would provide early predictions of the bacterial viability state in different conditions, yielding earlier results than culture.

Methods to monitor bacterial growth and replicative rates at the single-cell level

The heterogeneity of bacterial growth and replicative rates within a population was proposed a century ago notably to explain the presence of bacterial persisters. The term "growth rate" at the single-cell level corresponds to the increase in size or mass of an individual bacterium while the "replicative rate" refers to its division capacity within a defined temporality. After a decades long hiatus, recent technical innovative approaches allow population growth and replicative rates heterogeneity monitoring at the single-cell level resuming in earnest. Among these techniques, the oldest and widely used is time-lapse microscopy, most recently combined with microfluidics. We also discuss recent fluorescence dilution methods informing only on replicative rates and best suited. Some new elegant single cell methods so far only sporadically used such as buoyant mass measurement and stable isotope probing have emerged. Overall, such tools are widely used to investigate and compare the growth and replicative rates of bacteria displaying drug-persistent behaviors to that of bacteria growing in specific ecological niches or collected from patients. In this review, we describe the current methods available, discussing both the type of queries these have been used to answer and the specific strengths and limitations of each method.

A Possible Flow Cytometry-Based Viability and Vitality Assessment Protocol for Pathogenic Vibrio cholerae O1 and O139 Postexposure to Simulated Gastric Fluid

During the intake of contaminated water, for diarrheal disease to occur, Vibrio cholerae must survive through the bactericidal digestive secretion of gastric fluid during passage through the stomach. Determining the viability of these bacteria is challenging, with the standard cultivation methods for viability being time-consuming and unable to culture cells that may still function accordingly. This study assessed the use of enzyme action and membrane integrity as alternatives for determining vitality and viability, respectively, in gastric acid-stressed pathogenic Vibrio cholerae O1 and O139, using fluorescent probes thiazole orange (TO) for viability based on membrane integrity, carboxyfluorescein diacetate (CFDA) with acetoxymethyl ester (AM) for vitality based on metabolic activity, and propidium iodide (PI) for cell death/damage due to loss of membrane integrity, with flow cytometry. Simulated gastric fluid-treated bacterial cells were labelled with blends of TO+PI and CFDA-AM+PI, and these stained cells were separated into heterologous populations based on their fluorescence signal. The gastric acid exposed cells presented with high green fluorescence signals after staining with the metabolic probe CFDA-AM, which indicated intact (live) cells due to being metabolically active, whereas when the same cells were stained with the DNA probe (TO), these appeared to be in a “stressed state” due to loss of membrane integrity. Damaged cells (dead cells) showed high red fluorescence levels after staining with PI probe. The use of flow cytometry with fluorescent probes is a favorable method for evaluating the vitality and viability of bacteria when cells are labelled with a combination of CFDA-AM+PI.

Optimisation of Mycobacterium bovis BCG Fermentation and Storage Survival

Mycobacterium bovis Bacillus Calmette–Guérin (M. bovis BCG) was generated over a century ago for protection against Mycobacterium tuberculosis (Mtb) and is one the oldest vaccines still in use. The BCG vaccine is currently produced using a pellicle growth method, which is a complex and lengthy process that has been challenging to standardise. Fermentation for BCG vaccine production would reduce the complexity associated with pellicle growth and increase batch to batch reproducibility. This more standardised growth lends itself to quantification of the total number of bacilli in the BCG vaccine by alternative approaches, such as flow cytometry, which can also provide information about the metabolic status of the bacterial population. The aim of the work reported here was to determine which batch fermentation conditions and storage conditions give the most favourable outcomes in terms of the yield and stability of live M. bovis BCG Danish bacilli. We compared different media and assessed growth over time in culture, using total viable counts, total bacterial counts, and turbidity throughout culture. We applied fluorescent viability dyes and flow cytometry to measure real-time within-culture viability. Culture samples were stored in different cryoprotectants at different temperatures to assess the effect of these combined conditions on bacterial titres. Roisin’s minimal medium and Middlebrook 7H9 medium gave comparable, high titres in fermenters. Flow cytometry proved to be a useful tool for enumeration of total bacterial counts and in the assessment of within-culture cell viability and cell death. Of the cryoprotectants evaluated, 5% (v/v) DMSO showed the most significant positive effect on survival and reduced the negative effects of low temperature storage on M. bovis BCG Danish viability. In conclusion, we have shown a reproducible, more standardised approach for the production, evaluation, and storage of high titre, viable, BCG vaccine.

Rapid detection of Mycobacterium tuberculosis in sputum with a solvatochromic trehalose probe

Tuberculosis (TB) is the leading cause of death from an infectious bacterial disease. Poor diagnostic tools to detect active disease plague TB control programs and affect patient care. Accurate detection of live Mycobacterium tuberculosis (Mtb), the causative agent of TB, could improve TB diagnosis and patient treatment. We report that mycobacteria and other corynebacteria can be specifically detected with a fluorogenic trehalose analog. We designed a 4-N,N-dimethylamino-1,8-naphthalimide-conjugated trehalose (DMN-Tre) probe that undergoes >700-fold increase in fluorescence intensity when transitioned from aqueous to hydrophobic environments. This enhancement occurs upon metabolic conversion of DMN-Tre to trehalose monomycolate and incorporation into the mycomembrane of Actinobacteria. DMN-Tre labeling enabled the rapid, no-wash visualization of mycobacterial and corynebacterial species without nonspecific labeling of Gram-positive or Gram-negative bacteria. DMN-Tre labeling was detected within minutes and was inhibited by heat killing of mycobacteria. Furthermore, DMN-Tre labeling was reduced by treatment with TB drugs, unlike the clinically used auramine stain. Lastly, DMN-Tre labeled Mtb in TB-positive human sputum samples comparably to auramine staining, suggesting that this operationally simple method may be deployable for TB diagnosis.

Methods of Rapid Microbiological Assay and Their Application to Pharmaceutical and Medical Device Fabrication

　There are several rapid microbiological methods becoming available that have useful applications in pharmaceutical and medical devices. They are ATP bioluminescence, fluorescent labeling, electrical resistance, and nucleic acid probes. In choosing to employ rapid methods, the microbiologist should examine their prospective performances against the specific requirements for that sector. Some methods may require expensive equipment and offer full automation, and others represent only a small investment. The regulatory view of these methods is changing and they still officially have not been approved in medical and pharmaceutical area, but it will still be up to the microbiologist to demonstrate that the method chosen is fit for the purpose intended.

Application of flow cytometry to wine microorganisms

Flow cytometry (FCM) is a powerful technique allowing detection and enumeration of microbial populations in food and during food process. Thanks to the fluorescent dyes used and specific probes, FCM provides information about cell physiological state and allows enumeration of a microorganism in a mixed culture. Thus, this technique is increasingly used to quantify pathogen, spoilage microorganisms and microorganisms of interest. Since one decade, FCM applications to the wine field increase greatly to determine population and physiological state of microorganisms performing alcoholic and malolactic fermentations. Wine spoilage microorganisms were also studied. In this review we briefly describe FCM principles. Next, a deep revision concerning enumeration of wine microorganisms by FCM is presented including the fluorescent dyes used and techniques allowing a yeast and bacteria species specific enumeration. Then, the last chapter is dedicated to fluorescent dyes which are used to date in fluorescent microscopy but applicable in FCM. This chapter also describes other interesting "future" techniques which could be applied to study the wine microorganisms. Thus, this review seeks to highlight the main advantages of the flow cytometry applied to wine microbiology.

Extreme cellular adaptations and cell differentiation required by a cyanobacterium for carbonate excavation

Some cyanobacteria, known as euendoliths, excavate and grow into calcium carbonates, with their activity leading to significant marine and terrestrial carbonate erosion and to deleterious effects on coral reef and bivalve ecology. Despite their environmental relevance, the mechanisms by which they can bore have remained elusive and paradoxical, in that, as oxygenic phototrophs, cyanobacteria tend to alkalinize their surroundings, which will encourage carbonate precipitation, not dissolution. Therefore, cyanobacteria must rely on unique adaptations to bore. Studies with the filamentous euendolith, Mastigocoleus testarum, indicated that excavation requires both cellular energy and transcellular calcium transport, mediated by P-type ATPases, but the cellular basis for this phenomenon remains obscure. We present evidence that excavation in M. testarum involves two unique cellular adaptations. Long-range calcium transport is based on active pumping at multiple cells along boring filaments, orchestrated by the preferential localization of calcium ATPases at one cell pole, in a ring pattern, facing the cross-walls, and by repeating this placement and polarity, a pattern that breaks at branching and apical cells. In addition, M. testarum differentiates specialized cells we call calcicytes, that which accumulate calcium at concentrations more than 500-fold those found in other cyanobacteria, concomitantly and drastically lowering photosynthetic pigments and enduring severe cytoplasmatic alkalinization. Calcicytes occur commonly, but not exclusively, in apical parts of the filaments distal to the excavation front. We suggest that calcicytes allow for fast calcium flow at low, nontoxic concentrations through undifferentiated cells by providing buffering storage for excess calcium before final excretion to the outside medium.

Non-Invasive Microbial Metabolic Activity Sensing at Single Cell Level by Perfusion of Calcein Acetoxymethyl Ester

Phase contrast microscopy cannot give sufficient information on bacterial metabolic activity, or if a cell is dead, it has the fate to die or it is in a viable but non-growing state. Thus, a reliable sensing of the metabolic activity helps to distinguish different categories of viability. We present a non-invasive instantaneous sensing method using a fluorogenic substrate for online monitoring of esterase activity and calcein efflux changes in growing wild type bacteria. The fluorescent conversion product of calcein acetoxymethyl ester (CAM) and its efflux indicates the metabolic activity of cells grown under different conditions at real-time. The dynamic conversion of CAM and the active efflux of fluorescent calcein were analyzed by combining microfluidic single cell cultivation technology and fluorescence time lapse microscopy. Thus, an instantaneous and non-invasive sensing method for apparent esterase activity was created without the requirement of genetic modification or harmful procedures. The metabolic activity sensing method consisting of esterase activity and calcein secretion was demonstrated in two applications. Firstly, growing colonies of our model organism Corynebacterium glutamicum were confronted with intermittent nutrient starvation by interrupting the supply of iron and carbon, respectively. Secondly, bacteria were exposed for one hour to fatal concentrations of antibiotics. Bacteria could be distinguished in growing and non-growing cells with metabolic activity as well as non-growing and non-fluorescent cells with no detectable esterase activity. Microfluidic single cell cultivation combined with high temporal resolution time-lapse microscopy facilitated monitoring metabolic activity of stressed cells and analyzing their descendants in the subsequent recovery phase. Results clearly show that the combination of CAM with a sampling free microfluidic approach is a powerful tool to gain insights in the metabolic activity of growing and non-growing bacteria.

Flow cytometry as an auxiliary tool for the selection of probiotic bacteria

Selection of appropriate bacterial strains is crucial for development of new probiotic preparations. The fundamental prerequisite for potential efficacy of a probiotic preparation for oral application is the selection of appropriate bacterial strains with good gastrointestinal colonisation abilities, antimicrobial activity, and tolerance of conditions in the gastrointestinal tract, resistance to different antimicrobial agents, survival during processing and storage. The strain should be genetically stable, it should have good growth properties, to maintain its high viability at processing and when in storage. Mostly, the properties of promising strains are tested in the first phase in vitro, and only the best ones undergo subsequent in vivo testing. in vitro tests are often performed by classical microbiological cultivation methods which are material and time consuming, and they are not able to distinguish between ‘viable but nonculturable’ and dead bacteria. Flow cytometry is usually used for counting, phenotyping or functional characterisation of immune cells. Nowadays, flow cytometry is increasingly used in microbiology for counting bacteria, determining their viability and metabolic activity, detecting specific strains or testing their adherence abilities. The utilisation of flow cytometry in combination with an appropriate fluorescent labelling represents an effective and rapid method for the selection of probiotic bacteria.

Novel cyanine dyes and homodimeric styryl dyes as fluorescent probes for assessment of lactic acid bacteria cell viability

Innovations in labeling techniques and in the design and synthesis of dye structures are closely related to the development of service equipment such as light sources and detection methods. Novel styryl homodimers and monomethine cyanine dyes were synthesized and their staining abilities for discrimination between live and dead lactic acid bacterial cells were investigated. The dyes were combined in pairs based on their excitation and emission maxima and the capacity to penetrate through cell membranes of viable bacterial cells. The absorption maxima in the same region and the large Stocks shifts of the styryl derivatives allowed viability analysis to be done with epifluorescent microscope with a very basic configuration - one light source about 480nm and one filter for the fluorescent emissions. A staining protocol was developed and applied for live/dead analysis of Bulgarian yoghurt starters. The live cells quantification by the fluorescence dyes coincided well with the results of the much more time-consuming tests by plate counting. Thus, the proposed dye combinations are appropriate for rapid viability estimation in small laboratories that may have conventional equipment.

Cytofluorometric detection of wine lactic acid bacteria: application of malolactic fermentation to the monitoring

In this study we report for the first time a rapid, efficient and cost-effective method for the enumeration of lactic acid bacteria (LAB) in wine. Indeed, up to now, detection of LAB in wine, especially red wine, was not possible. Wines contain debris that cannot be separated from bacteria using flow cytometry (FCM). Furthermore, the dyes tested in previous reports did not allow an efficient staining of bacteria. Using FCM and a combination of BOX/PI dyes, we were able to count bacteria in wines. The study was performed in wine inoculated with Oenococcus oeni (106 CFU ml−1) stained with either FDA or BOX/PI and analyzed by FCM during the malolactic fermentation (MLF). The analysis show a strong correlation between the numbers of BOX/PI-stained cells determined by FCM and the cell numbers determined by plate counts (red wine: R2 ≥ 0.97, white wine R2 ≥ 0.965). On the other hand, we found that the enumeration of O. oeni labeled with FDA was only possible in white wine (R2 ≥ 0.97). Viable yeast and LAB populations can be rapidly discriminated and quantified in simultaneous malolactic-alcoholic wine fermentations using BOX/PI and scatter parameters in a one single measurement. This rapid procedure is therefore a suitable method for monitoring O. oeni populations during winemaking, offers a detection limit of <104 CFU ml−1 and can be considered a useful method for investigating the dynamics of microbial growth in wine and applied for microbiological quality control in wineries.

Comparative effects of the herbicides chlortoluron and mesotrione on freshwater microalgae

Extensive use of herbicides in agriculture is accompanied by the risk of environmental contamination of aquatic ecosystems. The present study shows the effects of the herbicides chlortoluron and mesotrione on three microalgae species: two chlorophyceae (Pediastrum tetras, Ankistrodesmus fusiformis) and one diatom (Amphora coffeaeformis). The authors calculated the IC50 for one chlorophyceae and the diatom. The order of toxicity (median inhibitory concentration [IC50]) for mesotrione was A. coffeaeformis (13.1 mg/L) > A. fusiformis (56.1 mg/L) and A. fusiformis (0.05 mg/L) > A. coffeaeformis (0.08 mg/L) for chlortoluron. The impact of herbicides applied at 0.2 mg/L was then examined in Erlenmeyer flasks by monitoring for growth, pigment content, and metabolic activity. Algal responses varied widely according to species and herbicide. For example, chlortoluron showed a significant inhibitory effect on the growth of A. coffeaeformis, whereas mesotrione induced an increase in cellular density in A. fusiformis. Other cellular parameters, such as pigment content in P. tetras, were stimulated by both herbicides. The results obtained confirmed that microalgae cultures are clearly affected by acute and chronic exposition to herbicides. Further monitoring should be carried out in the field to assess the impact of sublethal levels of toxicity and the growth-enhancing effects of mesotrione and chlortoluron on natural algae communities.

Assessment of the dynamics of the physiological states of Lactococcus lactis ssp. cremoris SK11 during growth by flow cytometry

AIMS

The aim of this study was to improve knowledge about the dynamics of the physiological states of Lactococcus lactis ssp. cremoris SK11, a chain-forming bacterium, during growth, and to evaluate whether flow cytometry (FCM) combined with fluorescent probes can assess these different physiological states.

METHODS AND RESULTS

Cellular viability was assessed using double labelling with carboxyfluorescein diacetate and propidium iodide. FCM makes it possible to discriminate between three cell populations: viable cells, dead cells and cells in an intermediate physiological state. During exponential and stationary phases, the cells in the intermediate physiological state were culturable, whereas this population was no longer culturable at the end of the stationary phase.

CONCLUSIONS, AND IMPACT OF THE STUDY

We introduced a new parameter, the ratio of the means of the fluorescence cytometric index to discriminate between viable culturable and viable nonculturable cells. Finally, this work confirms the relevance of FCM combined with two fluorescent stains to evaluate the physiological states of L. lactis SK11 cells during their growth and to distinguish viable cells from viable but not culturable cells.

Development of a nondestructive fluorescence-based enzymatic staining of microcolonies for enumerating bacterial contamination in filterable products

AIMS

Develop a nondestructive fluorescence-based staining procedure to rapidly detect and enumerate bacteria in filterable samples.

METHODS AND RESULTS

The study consists in the development of a staining solution and a protocol to fluorescently detect microcolonies on cellulose membranes. After detection, membranes can be re-incubated on media to yield colonies. Carboxyfluorescein diacetate was selected among other carboxyfluorescein derivatives for its staining efficiency and the absence of background. Several permeabilizers were evaluated for their ability to promote dye uptake into cells without affecting viability. We demonstrated that a combination of n-Octyl β-D-glucopyranoside, sodium hexametaphosphate, lithium chloride and rubidium chloride significantly increased the staining efficiency of bacteria without affecting their viability. The method developed allowed the detection in <9 h of all tested aerobic bacteria and in 48 h of the anaerobic slow grower Propionibacterium acnes.

CONCLUSIONS

This method allows the rapid detection of bacteria in filterable samples in at least three to five times faster than traditional microbiological method.

SIGNIFICANCE AND IMPACT OF THE STUDY

The advantage of this nondestructive procedure is to allow contaminants identification after membrane re-incubation. This method could be easily applied in routine in pharmaceutical, clinical and food and beverage industries to monitor contaminations.

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.

Cultivation-independent examination of horizontal transfer and host range of an IncP-1 plasmid among gram-positive and gram-negative bacteria indigenous to the barley rhizosphere

The host range and transfer frequency of an IncP-1 plasmid (pKJK10) among indigenous bacteria in the barley rhizosphere was investigated. A new flow cytometry-based cultivation-independent method for enumeration and sorting of transconjugants for subsequent 16S rRNA gene classification was used. Indigenous transconjugant rhizosphere bacteria were collected by fluorescence-activated cell sorting and identified by cloning and sequencing of 16S rRNA genes from the sorted cells. The host range of the pKJK10 plasmid was exceptionally broad, as it included not only bacteria belonging to the alpha, beta, and gamma subclasses of the Proteobacteria, but also Arthrobacter sp., a gram-positive member of the Actinobacteria. The transfer frequency (transconjugants per donor) from the Pseudomonas putida donor to the indigenous bacteria was 7.03 x 10(-2) +/- 3.84 x 10(-2). This is the first direct documentation of conjugal transfer between gram-negative donor and gram-positive recipient bacteria in situ.

Rapid method for enumeration of viable Legionella pneumophila and other Legionella spp. in water

A sensitive and specific method has been developed to enumerate viable L. pneumophila and other Legionella spp. in water by epifluorescence microscopy in a short period of time (a few hours). This method allows the quantification of L. pneumophila or other Legionella spp. as well as the discrimination between viable and nonviable Legionella. It simultaneously combines the specific detection of Legionella cells using antibodies and a bacterial viability marker (ChemChrome V6), the enumeration being achieved by epifluorescence microscopy. The performance of this immunological double-staining (IDS) method was investigated in 38 natural filterable water samples from different aquatic sources, and the viable Legionella counts were compared with those obtained by the standard culture method. The recovery rate of the IDS method is similar to, or higher than, that of the conventional culture method. Under our experimental conditions, the limit of detection of the IDS method was <176 Legionella cells per liter. The examination of several samples in duplicates for the presence of L. pneumophila and other Legionella spp. indicated that the IDS method exhibits an excellent intralaboratory reproducibility, better than that of the standard culture method. This immunological approach allows rapid measurements in emergency situations, such as monitoring the efficacy of disinfection shock treatments. Although its field of application is as yet limited to filterable waters, the double-staining method may be an interesting alternative (not equivalent) to the conventional standard culture methods for enumerating viable Legionella when rapid detection is required.

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.

Profiling bacterial survival through a water treatment process and subsequent distribution system

AIMS

To profile fractions of active bacteria and of bacteria culturable with routine heterotrophic plate count (HPC) methods through a typical water treatment process and subsequent distribution system. In doing so, investigate how water treatment affects both bacterial abundance and diversity, and reveal the identities of active bacteria not detected by traditional HPC culture.

METHODS AND RESULTS

Profiling active fractions was performed by flow cytometric cell sorting of either membrane-intact (BacLight kit) or enzymatically active (carboxyfluorescein diacetate, CFDA) bacteria, followed by eubacterial 16S rDNA-directed PCR and denaturing gradient gel electrophoresis (DGGE). Water treatment significantly reduced active bacterial numbers detected by the BacLight kit and CFDA assay by 2.89 and 2.81 log respectively. Bacterial diversity was also reduced from > 20 DGGE bands in the active fractions of reservoir water to only two bands in the active fractions of finished water. These two bands represented Stenotrophomonas maltophila, initially culturable by HPC, and a Burkholderia-related species. Both species maintained measurable traits of physiological activity in distribution system bulk water but were undetected by HPC.

CONCLUSIONS

Flow cytometric cell sorting with PCR-DGGE, to assess water treatment efficacy, identified active bacteria from a variety of major phylogenetic groups undetected by routine HPC. Following treatment S. maltophila and a Burkholderia-related species retained activity and entered distribution undetected by HPC.

SIGNIFICANCE AND IMPACT OF THE STUDY

Methods used here demonstrate how water treatment operators can better monitor water treatment plant efficacy and assess distribution system instability by the detection and identification of active bacteria recalcitrant to routine HPC culture.

Rapid and automated detection of fluorescent total bacteria in water samples

Traditional methods for the detection and enumeration of bacteria in water samples are growth-based and require several days to obtain the result. New techniques which reduce the time of analysis have been developed. The objective of this work was to test a rapid method for the detection and enumeration of total viable bacteria using direct fluorescent labelling and detection by laser scanning. This method (referred to as TVC for Total Viable Count) was compared to the R2A culture method and the cyano-ditolyl-tetrazolium chloride (CTC) staining method for the analysis of samples before the final chlorination (after GAC filtration) and drinking water samples. For the comparison of TVC and CTC, the outcome depends on the water type: for samples after GAC filtration, TVC counts were significantly lower than CTC counts by up to 2 log10 orders of magnitude. For chlorinated water samples, TVC counts were not significantly different from CTC counts. The comparison of TVC and R2A showed that TVC counts could be lower than R2A counts or equivalent depending on the type of water. For drinking water, the TVC method proved to yield results equivalent to those of the R2A method. The TVC method requires much shorter time frame than others. It is also simple to use and allows the analysis of large volumes (100 ml) of drinking water.

Five essential oils from aromatic plants of Cameroon: their antibacterial activity and ability to permeabilize the cytoplasmic membrane of Listeria innocua examined by flow cytometry

AIMS

To investigate the antibacterial effect of five essential oils (EO) extracted from aromatic plants (Cymbopogon citratus, Ocimumbasilicum, Ocimum gratissimum, Thymus vulgaris and Zingiber officinale) of Cameroon against strains of Listeria monocytogenes, L. innocua and Staphylococcus aureus. The ability of selected EO to permeabilize the cytoplasmic membrane of L. innocua was also examined.

METHODS AND RESULTS

The antibacterial activity of the EO determined by the agar diffusion method showed that T. vulgaris had the highest activity followed by O. gratissimum and C. citratus. Lowest activity was recorded from Z. officinale and O. basilicum. Significant differences in sensitivity between strains of Listeria and S. aureus were observed. Flow cytometry of L. innocua stained with carboxy-fluorescein diacetate showed that the fluorescence intensity of cells exposed to EO decreased faster than nonexposed cells, indicating that EO permeabilized the cytoplasmic membrane with the leakage of carboxy-fluorescein.

CONCLUSIONS

Almost all the EO tested showed antibacterial activity to a different extent. The antibacterial effect was due to permeabilization of the cytoplasmic membrane.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study has identified the preservative potential of the EO examined. The use of sensitive method, such as flow cytometry, is advantageous for quick generation of data on the antibacterial effect of EO.

Rains, drains and active strains: towards online assessment of wastewater bacterial communities

Wastewater treatment is one of the largest scale and arguably the most commercially important biotechnological process in the world. Bacterial breakdown of waste materials facilitates the safe disposal of effluents into receiving water bodies. Given this significance, research has focused on identifying the keystone species on which efficient treatment is based. However, unravelling the microbial diversity within such systems has proven difficult. This is highlighted by our lack of detailed knowledge of the microbial interactions within these complex populations, limiting our ability to fully exploit bacterial degradative abilities. Even with the incorporation of new emerging molecular techniques, there have been no investigations linking genetic sequence to microbial function and successful treatment operation. To reach this goal, researchers need the ability to identify, enumerate and monitor the metabolic functions of subpopulations within these complex bacterial communities. Flow cytometry (FCM) combined with fluorescence-based molecular identification techniques provides a method for such studies. Moreover, single-cell sorting provides a unique opportunity to identify and remove individual cells of interest. Laboratory culture of sorted cells is often possible and permits the use of more traditional microbiological techniques to backup molecular investigations. Utilising this approach will advance our understanding of wastewater treatment processes and help maintain and enhance plant operation to improve efficiency.

A comparative study of carboxyfluorescein diacetate and carboxyfluorescein diacetate succinimidyl ester as indicators of bacterial activity

Staining bacteria with esterified fluorogenic substrates followed by flow cytometric analysis offers a means for rapid detection of metabolically active bacteria. Flow cytometry (FCM) was used to assess carboxyfluorescein diacetate (CFDA) and carboxyfluorescein diacetate succinimidyl ester (CFDA/SE) as indicators of bacterial activity for cultured bacteria, including Aeromonas hydrophila, Bacillus subtilis, Escherichia coli, Pseudomonas aeruginosa, Staphylococcus epidermidis and bacteria from environmental waters. In theory, CFDA/SE should be a better indicator of metabolic bacterial activity compared to CFDA due to greater intracellular retention of the fluorescent product. Qualitative and quantitative analysis of exponential phase cultures, mixtures of active and inactive cells and bacteria from environmental waters revealed CFDA was successful in detecting active bacteria, whereas CFDA/SE was not. CFDA/SE labelled inactive cells with intensities equal to that of the active population and could not even discriminate between bacteria in exponential phase growth and a fixed cell preparation. We propose that the specific mode of action of the succinimidyl ester (SE) group in combination with the nonenzymatic aqueous hydrolysis of the CFDA moiety results in the nonspecific labelling of all cells, irrespective of their metabolic state. This study shows that CFDA/SE is a poor marker of bacterial activity.

Multiparametric flow cytometry and cell sorting for the assessment of viable, injured, and dead bifidobacterium cells during bile salt stress

Using a flow cytometry-based approach, we assessed the viability of Bifidobacterium lactis DSM 10140 and Bifidobacterium adolescentis DSM 20083 during exposure to bile salt stress. Carboxyfluorescein diacetate (cFDA), propidium iodide (PI), and oxonol [DiBAC4(3)] were used to monitor esterase activity, membrane integrity, and membrane potential, respectively, as indicators of bacterial viability. Single staining with these probes rapidly and noticeably reflected the behavior of the two strains during stress exposure. However, the flow cytometry results tended to overestimate the viability of the two strains compared to plate counts, which appeared to be related to the nonculturability of a fraction of the population as a result of sublethal injury caused by bile salts. When the cells were simultaneously stained with cFDA and PI, flow cytometry and cell sorting revealed a striking physiological heterogeneity within the stressed bifidobacterium population. Three subpopulations could be identified based on their differential uptake of the probes: cF-stained, cF and PI double-stained, and PI-stained subpopulations, representing viable, injured, and dead cells, respectively. Following sorting and recovery, a significant fraction of the double-stained subpopulation (40%) could resume growth on agar plates. Our results show that in situ assessment of the physiological activity of stressed bifidobacteria using multiparameter flow cytometry and cell sorting may provide a powerful and sensitive tool for assessment of the viability and stability of probiotics.

Simultaneous fluorescent gram staining and activity assessment of activated sludge bacteria

Wastewater treatment is one of the most important commercial biotechnological processes, and yet the component bacterial populations and their associated metabolic activities are poorly understood. The novel fluorescent dye hexidium iodide allows assessment of Gram status by differential absorption through bacterial cell walls. Differentiation between gram-positive and gram-negative wastewater bacteria was achieved after flow cytometric analysis. This study shows that the relative proportions of gram-positive and gram-negative bacterial cells identified by traditional microscopy and hexidium iodide staining were not significantly different. Dual staining of cells for Gram status and activity proved effective in analyzing mixtures of cultured bacteria and wastewater populations. Levels of highly active organisms at two wastewater treatment plants, both gram positive and gram negative, ranged from 1.5% in activated sludge flocs to 16% in the activated sludge fluid. Gram-positive organisms comprised <5% of the total bacterial numbers but accounted for 19 and 55% of the highly active organisms within flocs at the two plants. Assessment of Gram status and activity within activated sludge samples over a 4-day period showed significant differences over time. This method provides a rapid, quantitative measure of Gram status linked with in situ activity within wastewater systems.

Cultivation of Mycobacterium bovis BCG in bioreactors

The Mycobacterium bovis BCG vaccine for commercial use is classically produced as surface pellicles by culture on synthetic medium. Under these conditions, reproducibility of the cultures and quality assessment are hampered by slow growth of the bacilli, the formation of bacterial aggregates and a high proportion of dead bacilli after processing and final formulation of the vaccine. Here, we established dispersed cultures of M. bovis BCG in synthetic media in small-scale bioreactors. These cultures allow recording and adjusting of culture parameters and give rise to single bacilli with a high degree of live bacteria. In the murine model, bioreactor-grown M. bovis BCG exhibited slightly stronger replication and persistence than the vaccine produced under the classical conditions. The protective efficacy against challenge with M. tuberculosis was identical for both vaccine preparations.

Adaptation of a neutrophilic dairy-associated Bacillus cereus isolate to alkaline pH

AIMS

This study identified and studied the response of five Bacillus strains, isolated from alkaline cleaning in place (CIP) solutions, to alkaline conditions.

METHODS AND RESULTS

Isolates were identified as B. cereus by 16S rDNA sequencing. External and internal cell pH and buffering capacity data of a representative strain, Bacillus DL5, were compared to B. cereus ATCC 10702. Results indicated that a buffering system was induced when the pH of the growth medium increased to above pH 10, which was effective up to pH 12 and presumably cell wall associated. Volume measurements and confocal scanning laser microscope images of Bacillus DL5 cells showed that cells exhibited more pronounced stress symptoms when exposed to pH 10 than at pHs above 10. Long-term exposure of Bacillus DL5 to pH 10 or 10.5 indicated that cells grew in planktonic form and formed biofilms at both pHs.

CONCLUSIONS

Bacillus DL5 was a neutrophile with a growth pH range similar to B. cereus ATCC 10702, but tolerated alkaline pH. This may be a general trait of the B. cereus species rather than a specific phenomenon of isolates from alkaline ecosystems.

SIGNIFICANCE AND IMPACT OF THE STUDY

Other neutrophilic B. cereus isolates may exhibit similar responses to alkaline conditions as the isolates studied here. These results may have important implications for dairy manufacturers.

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.

Assessment of activated sludge viability with flow cytometry

The aim of the study was to evaluate the applicability of fluorescent dyes and multiparameter flow cytometry for the rapid and direct viability/activity assessment of activated sludge samples taken from wastewater treatment plants. Viability and activity of the biomass were estimated respectively through cellular membrane integrity, staining with SYBR Green I and Propidium Iodide, and through fluorogenic dyes capable of detecting enzymatic activity, as FDA and BCECF-AM. A procedure has been developed to disaggregate sludge flocs before dyes staining and cytometric analysis. The developed procedure allows a high recovery of bacteria with good accuracy and repeatability, and minimize the damage of the cells suspension obtained from the disaggregation of the flocs. These measurements were applied to estimate the two main parameters required to define the biological activated sludge process: the endogenous decay rate and the specific growth rate in exponential phase with high F/M ratio. Oxygen utilization rate measurements (OUR) were conducted to conventionally monitor the activity of the biomass. The preliminary data are encouraging and support the possibility to investigate bacteria dynamics on wastewater treatment plants.

Confocal microscopy and microbial viability detection for food research

Confocal microscopy offers several advantages over other conventional microscopic techniques as a tool for studying the interaction of bacteria with food and the role of food microstructure in product quality and safety. When using confocal microscopy, samples can be observed without extensive preparation processes, which allows for the evaluation of food without introducing artifacts. In addition, observations can be made in three dimensions without physically sectioning the specimen. The confocal microscope can be used to follow changes over a period of time, such as the development of the food structure or changes in microbial population during a process. Microbial attachment to and detachment from food and food contact surfaces with complex three-dimensional (3-D) structures can be observed in situ. The fate of microbial populations in food system depends on processing, distribution, and storage conditions as well as decontamination procedures that are applied to inactivate and remove them. The ability to determine the physiological status of microorganisms without disrupting their physical relationship with a food system can be useful for determining the means by which microorganisms survive decontamination treatments. Conventional culturing techniques can detect viable cells; however, these techniques lack the ability to locate viable cells in respect to the microscopic structures of food. Various microscopic methods take advantage of physiological changes in bacterial cells that are associated with the viability to assess the physiologic status of individual cells while retaining the ability to locate the cell within a food tissue system. This paper reviews the application of confocal microscopy in food research and direct observation of viable bacteria with emphasis on their use in food microbiology.

Rapid detection of viable yeasts and bacteria in wine by flow cytometry

The potential of using flow cytometry (FCM) in combination with fluorescent dyes for rapidly estimating counts of yeasts and malolactic bacteria in laboratory media and wines was examined. In general, there was a good correlation (regression coefficient, 0.94) between viable counts of yeasts determined by FCM and by standard plate assay. The FCM detection limit of yeasts in YPDE medium and in Pinot noir must was 10(3) cells/ml. The lowest bacterial concentration detected by FCM was 10(4) cells/ml. When yeast and malolactic bacteria populations were simultaneously analysed in wine by FCM without any previous sample treatment, difficulties were encountered in the count of bacterial cells due to their size, which is similar to natural debries present in wine. However, after the optimisation of the sample preparation, the technique appeared promising in determining the presence of such microorganisms in wine with one single measurement. Because it is rapid and easy to use, flow cytometry can be considered a useful method for microbiological quality control in wineries and for the investigation of the growth dynamics of microorganisms in wine.

Flow cytometric assessment of viability of lactic acid bacteria

The viability of lactic acid bacteria is crucial for their applications as dairy starters and as probiotics. We investigated the usefulness of flow cytometry (FCM) for viability assessment of lactic acid bacteria. The esterase substrate carboxyfluorescein diacetate (cFDA) and the dye exclusion DNA binding probes propidium iodide (PI) and TOTO-1 were tested for live/dead discrimination using a Lactococcus, a Streptococcus, three Lactobacillus, two Leuconostoc, an Enterococcus, and a Pediococcus species. Plate count experiments were performed to validate the results of the FCM assays. The results showed that cFDA was an accurate stain for live cells; in exponential-phase cultures almost all cells were labeled, while 70 degrees C heat-killed cultures were left unstained. PI did not give clear live/dead discrimination for some of the species. TOTO-1, on the other hand, gave clear discrimination between live and dead cells. The combination of cFDA and TOTO-1 gave the best results. Well-separated subpopulations of live and dead cells could be detected with FCM. Cell sorting of the subpopulations and subsequent plating on agar medium provided direct evidence that cFDA labels the culturable subpopulation and that TOTO-1 labels the nonculturable subpopulation. Applied to cultures exposed to deconjugated bile salts or to acid, cFDA and TOTO-1 proved to be accurate indicators of culturability. Our experiments with lactic acid bacteria demonstrated that the combination of cFDA and TOTO-1 makes an excellent live/dead assay with versatile applications.