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

Rapid In Vitro Assessment of Antimicrobial Drug Effect Bridging Clinically Relevant Pharmacokinetics: A Comprehensive Methodology

Rapid in vitro assessment of antimicrobial drug efficacy under clinically relevant pharmacokinetic conditions is an essential element of both drug development and clinical use. Here, we present a comprehensive overview of a recently developed novel integrated methodology for rapid assessment of such efficacy, particularly against the emergence of resistant bacterial strains, as jointly researched by the authors in recent years. This methodology enables rapid in vitro assessment of the antimicrobial efficacy of single or multiple drugs in combination, following clinically relevant pharmacokinetics. The proposed methodology entails (a) the automated collection of longitudinal time-kill data in an optical-density instrument; (b) the processing of collected time-kill data with the aid of a mathematical model to determine optimal dosing regimens under clinically relevant pharmacokinetics for single or multiple drugs; and (c) in vitro validation of promising dosing regimens in a hollow fiber system. Proof-of-concept of this methodology through a number of in vitro studies is discussed. Future directions for the refinement of optimal data collection and processing are discussed.

Antimicrobial Activity of Sertraline on Listeria monocytogenes

We explored the antimicrobial activity of sertraline on Listeria monocytogenes and further investigated the effects of sertraline on biofilm formation and the virulence gene expression of L. monocytogenes. The minimum inhibitory concentration and minimum bactericidal concentration for sertraline against L. monocytogenes were in the range of 16-32 μg/mL and 64 μg/mL, respectively. Sertraline-dependent damage of the cell membrane and a decrease in intracellular ATP and pH_in in L. monocytogenes were observed. In addition, sertraline reduced the biofilm formation efficiency of the L. monocytogenes strains. Importantly, low concentrations (0.1 μg/mL and 1 μg/mL) of sertraline significantly down-regulated the expression levels of various L. monocytogens virulence genes (prfA, actA, degU, flaA, sigB, ltrC and sufS). These results collectively suggest a role of sertraline for the control of L. monocytogenes in the food industry.

Bacterial composition of a competitive exclusion product and its correlation with product efficacy at reducing Salmonella in poultry

The mature intestinal microbiome is a formidable barrier to pathogen colonization. Day-old chicks seeded with cecal contents of adult hens are resistant to colonization with Salmonella, the basis of competitive exclusion. Competitive exclusion products can include individual microbes but are commonly undefined intestinal communities taken from adult animals and in commercial production is amplified in fermentator and sold commercially in freeze dried lots. While superior to single and multiple species probiotics, reducing Salmonella colonization by multiple logs, undefined products have limited acceptance because of their uncharacterized status. In this study, the bacterial composition of the master stock, preproduction seed stocks and commercial lots of a poultry competitive exclusion product, was defined by 16S rRNA sequence analysis, targeting the 16S rRNA variable region (V1-V3). The samples contained a diversity of genera (22-52 distinct genera) however, the commercial lots displayed less diversity compared to the seeds and the master stock. Community composition varied between seeds and the master stock and was not a good predictor of potency, in terms of log₁₀ reduction in Salmonella abundance. While there was significant correlation in composition between seeds and their commercial lots, this too was a not a good predictor of potency. There was linear correlation between unclassified Actinobacteria, Peptococcus, and unclassified Erysipelotrichaceae, and Salmonella abundance (r ² > .75) for commercial seeds. However, upon review of the literature, these three genera were not consistently observed across studies or between trials that examined the correlation between intestinal community composition and Salmonella prevalence or abundance.

Antimicrobial activity of cyanidin-3-O-glucoside-lauric acid ester against Staphylococcus aureus and Escherichia coli

Enzymatic acylation of anthocyanin with fatty acid improves its lipophilic solubility and application potential. Nevertheless, evaluation of functional properties of product is premise for application. This study investigated the antimicrobial potential and the underlying mechanisms of an acylated anthocyanin, namely, cyanidin-3-O-glucoside-lauric acid ester (C3G-LA), to provide guidelines for its application. C3G-LA exhibited outstanding antibacterial activity against Staphylococcus aureus [minimum inhibitory concentration (MIC) = 0.3125 mg/mL] and modest activity against Escherichia coli (MIC = 5 mg/mL). Moreover, C3G-LA manifested bactericide ability against S. aureus at 0.625 mg/mL. Decreases in membrane integrity (by 96% and 92% at MIC in S. aureus and E. coli, respectively), intracellular ATP concentration (by 96% and 92%) and intracellular pH (by 11% and 9%) and changes in cellular morphology altogether indicated the dysfunction of cell membrane under C3G-LA treatment. These findings demonstrated that C3G-LA could be adopted as an alternative food preservative against foodborne pathogens.

Discerning in vitro pharmacodynamics from OD measurements: A model-based approach

Time-kill experiments can discern the pharmacodynamics of infectious bacteria exposed to antibiotics in vitro, and thus help guide the design of effective therapies for challenging clinical infections. This task is resource-limited, therefore typically bypassed in favor of empirical shortcuts. The resource limitation could be addressed by continuously assessing the size of a bacterial population under antibiotic exposure using optical density measurements. However, such measurements count both live and dead cells and are therefore unsuitable for declining populations of live cells. To fill this void, we develop here a model-based method that infers the count of live cells in a bacterial population exposed to antibiotics from continuous optical-density measurements of both live and dead cells combined. The method makes no assumptions about the underlying mechanisms that confer resistance and is widely applicable. Use of the method is demonstrated by an experimental study on Acinetobacter baumannii exposed to levofloxacin.

Physicochemical and transcriptomic responses of Lactobacillus brevis JLD715 to sodium selenite

BACKGROUND

Elemental selenium, as a new type of selenium supplement, can be prepared by microorganisms reducing inorganic selenium. In this study, Lactobacillus brevis JLD715 was incubated in broth containing different concentrations of sodium selenite (Na₂ SeO₃ ).

RESULTS

The results showed that the bacterial biomass of L. brevis JLD715 decreased due to the inhibition of Na₂ SeO₃ . The cell membrane of L. brevis JLD715 treated with Na₂ SeO₃ was damaged, as evidenced by the reduction of intracellular ATP concentration, depolarization of cell membrane, reduction of intracellular pH and impairment of membrane integrity. In addition, we investigated the metabolism mechanism of Na₂ SeO₃ by L. brevis JLD715 based on transcriptome sequencing. A total of 461 genes were significantly differentially expressed under Na₂ SeO₃ treatment, of which 231 genes were up-regulated and 230 genes were down-regulated. These genes were involved in pathways such as pyruvate metabolism, fatty acid biosynthesis, selenocompound metabolism and nucleotide-binding oligomerization domain-like (NOD-like) receptor signaling. Meanwhile, the genes related to sulfhydryl oxidoreductase, electron carrier proteins and transmembrane transport proteins synthesis were significantly up-regulated.

CONCLUSION

To sum up, the findings of this research will contribute to providing support for the application of L. brevis JLD715 in selenium-enriched functional foods. © 2021 Society of Chemical Industry.

Role of Coptis chinensis in antibiotic susceptibility of carbapenem-resistant Klebsiella pneumoniae

BACKGROUND

The incidence of carbapenem-resistant Klebsiella pneumoniae (CRKP) has rapidly increased. This study aimed to assess the effect of Coptis chinensis and its compounds on the minimal inhibitory concentrations (MICs) of eight antibiotics against CRKP.

METHODS

Cell cultures were used to investigate the effects of C. chinensis and its compounds on the MICs of eight antibiotics against CRKP. The MICs for antibiotics alone and antibiotics with C. chinensis or compounds were measured and compared. Furthermore, the effects of C. chinensis on cell membrane injury and intracellular adenosine triphosphate (ATP) CRKP concentration were also measured. The Mann-Whitney rank-sum test was used to analyze the differences between means.

RESULTS

C. chinensis exhibits a notable MIC bacteriostatic effect at 5 mg/mL on CRKP. A significant MIC reduction against CRKP exists when C. chinensis was added to colistin and colistin-containing two-antibiotic combinations. Moreover, C. chinensis could damage cell membrane integrity and decrease intracellular ATP concentration in CRKP. Thus, C. chinensis exhibits antimicrobial activity superiority with colistin against CRKP. Furthermore, the effects of identified compounds in C. chinensis on the MICs of colistin, four-to eight-, two-to four-, and one-to two-fold reductions were found in ferulic acid, magnoflorine, and jatrorrhizine hydrochloride, respectively. Among these compounds, ferulic acid destroys membrane integrity and decreases intracellular ATP concentration.

CONCLUSION

C. chinensis and ferulic acid can potentiate the antimicrobial activity of colistin and may represent a promising component of combination therapy against CRKP infections in a clinical setting.

Antimicrobial Activity and Antibiofilm Potential of Coenzyme Q0 against Salmonella Typhimurium

Coenzyme Q₀ (CoQ₀) has anti-inflammatory and anti-tumor effects; however, the antimicrobial and antibiofilm activities of CoQ₀ against Salmonella enterica serovar Typhimurium are unknown. Thus, we investigated the bacteriostatic and antibiofilm activities, along with the underlying mechanism, of CoQ₀ against S. Typhimurium. The minimum inhibitory concentration (MIC) of CoQ₀ against S. enterica serovars Typhimurium was 0.1–0.2 mg/mL (549–1098 µM), and CoQ₀ at MIC and 2MIC decreased viable S. Typhimurium counts below detectable limits within 6 and 4 h, respectively. CoQ₀ at 20MIC (4 mg/mL) reduced S. Typhimurium on raw chicken by 1.5 log CFU/cm³ within 6 h. CoQ₀ effectively disrupted cell membrane integrity and induced morphological changes in the cell, resulting in hyperpolarization, decreased intracellular ATP concentrations, and cellular constituents leakage. Biofilm-associated S. Typhimurium cells were killed by CoQ₀ treatment. These findings suggest that CoQ₀ could be applied as a natural antibacterial substance for use against S. Typhimurium by the food industry.

A pattern-triggered immunity-related phenolic, acetosyringone, boosts rapid inhibition of a diverse set of plant pathogenic bacteria

BACKGROUND

Acetosyringone (3,5-dimethoxy-4-hydroxyacetophenone, AS) is a syringyl-type phenolic compound rarely found in plants in free form. It has been shown earlier to inhibit the growth of Pseudomonas bacteria in the presence of hydrogen peroxide and peroxidase (AS mix).

RESULTS

We detected elevated levels of free AS in Nicotiana tabacum and N. benthamiana plants after inducing pattern-triggered immunity (PTI) by injecting bacterial elicitor flg22, or pathogenicity-mutant Pseudomonas syringae pv. syringae 61 hrcC- bacteria; but not after inoculations with compatible or incompatible pathogens at the time of PTI onset. In this study, we demonstrate that the antibacterial effect of the AS mix is general, as growth of several Gram-negative and -positive phytopathogenic bacteria was characteristically inhibited. The inhibition of bacterial metabolism by the AS mix was rapid, shown by the immediate drop of luminescence intensity of P. syringae pv. tomato DC3000 lx strain after addition of AS mix. The mechanism of the bacteriostatic effect was investigated using fluorescent reporter dye assays. SYTOX Green experiments supported others' previous findings that the AS mix does not result in membrane permeabilization. Moreover, we observed that the mode of action could be depolarization of the bacterial cell membrane, as shown by assays carried out with the voltage sensitive dye DIBAC4(3).

CONCLUSIONS

Level of free acetosyringone is elevated during plant PTI responses in tobacco leaves (N. tabacum and N. benthamiana). When combined with hydrogen peroxide and peroxidase (AS mix), components of the mix act synergistically to inhibit bacterial metabolism and proliferation rapidly in a wide range of plant pathogens. This effect is related to depolarization rather than to permeabilization of the bacterial cell membrane. Similar AS mixture to the in vivo model might form locally at sites of invading bacterial attachment to the plant cells and the presence of acetosyringone might have an important role in the inhibition of bacterial proliferation during PTI.

Polymeric Materials with Antibacterial Activity: A Review

Infections caused by bacteria are one of the main causes of mortality in hospitals all over the world. Bacteria can grow on many different surfaces and when this occurs, and bacteria colonize a surface, biofilms are formed. In this context, one of the main concerns is biofilm formation on medical devices such as urinary catheters, cardiac valves, pacemakers or prothesis. The development of bacteria also occurs on materials used for food packaging, wearable electronics or the textile industry. In all these applications polymeric materials are usually present. Research and development of polymer-based antibacterial materials is crucial to avoid the proliferation of bacteria. In this paper, we present a review about polymeric materials with antibacterial materials. The main strategies to produce materials with antibacterial properties are presented, for instance, the incorporation of inorganic particles, micro or nanostructuration of the surfaces and antifouling strategies are considered. The antibacterial mechanism exerted in each case is discussed. Methods of materials preparation are examined, presenting the main advantages or disadvantages of each one based on their potential uses. Finally, a review of the main characterization techniques and methods used to study polymer based antibacterial materials is carried out, including the use of single force cell spectroscopy, contact angle measurements and surface roughness to evaluate the role of the physicochemical properties and the micro or nanostructure in antibacterial behavior of the materials.

Species Dependence of SYTO 9 Staining of Bacteria

SYTO 9 is a fluorescent nucleic acid stain that is widely used in microbiology, particularly for fluorescence microscopy and flow cytometry analyzes. Fluorimetry-based analysis, i.e., analysis of fluorescence intensity from a bulk sample measurement, is more cost effective, rapid and accessible than microscopy or flow cytometry but requires application-specific calibration. Here we show the relevance of SYTO 9 for food safety analysis. We stained four bacterial species of relevance to food safety (Bacillus cereus, Escherichia coli, Salmonella enterica subspecies enterica ser. Typhimurium, Staphylococcus aureus) with different concentrations of SYTO 9, with and without the presence of ethylenediaminetetraacetic acid (EDTA), for varying amounts of time, to investigate the effect of these treatment parameters on fluorescence intensity. The addition of EDTA and an increased staining duration did not significantly affect fluorescence intensity, and over the bacterial cell concentration range investigated (∼10⁵–10⁸ CFU/ml) there was no significant difference in using 0.5 or 1 μM SYTO 9. The effect of bacterial cell concentration on fluorescence intensity was species specific. At different bacterial cell concentrations, the effect of species on fluorescence intensity is different. This interaction complicates the development of a general fluorimetry-based protocol for the determination of bacterial cell concentration in a mixed bacterial suspension, as would be expected from samples taken from food safety settings.

The Immunomodulatory Properties of Extracellular Vesicles Derived from Probiotics: A Novel Approach for the Management of Gastrointestinal Diseases

Probiotics, included in functional foods, nutritional supplements, or nutraceuticals, exhibit different beneficial effects on gut function. They are extensively used to improve the digestive processes as well as reduce the symptoms and progression of different diseases. Probiotics have shown to improve dysbiosis and modulate the immune response of the host by interacting with different cell types. Probiotics and the host can interact in a direct way, but it is becoming apparent that communication occurs also through extracellular vesicles (EVs) derived from probiotics. EVs are key for bacteria-bacteria and bacteria-host interactions, since they carry a wide variety of components that can modulate different signaling pathways, including those involved in the immune response. Interestingly, EVs are recently starting to be considered as an alternative to probiotics in those cases for which the use of live bacteria could be dangerous, such as immunocompromised individuals or situations where the intestinal barrier is impaired. EVs can spread through the mucus layer and interact with the host, avoiding the risk of sepsis. This review summarizes the existing knowledge about EVs from different probiotic strains, their properties, and their potential use for the prevention or treatment of different gastrointestinal diseases.

Rapid and cost-effective evaluation of bacterial viability using fluorescence spectroscopy

A rapid and easy method that takes advantage of an inexpensive and portable fibre-based spectroscopic system (optrode) to determine the ratio of live to dead bacteria is proposed. Mixtures of live and dead Escherichia coli with proportions of live:dead cells varying from 0 to 100% were stained using SYTO 9 and propidium iodide (PI) and measured using the optrode. We demonstrated several approaches to obtaining the proportions of live:dead E. coli in a mixture of both live and dead, from analyses of the fluorescence spectra collected by the optrode. To find a suitable technique for predicting the percentage of live bacteria in a sample, four analysis methods were assessed and compared: SYTO 9:PI fluorescence intensity ratio, an adjusted fluorescence intensity ratio, single-spectrum support vector regression (SVR) and multi-spectra SVR. Of the four analysis methods, multi-spectra SVR obtained the most reliable results and was able to predict the percentage of live bacteria in 10⁸ bacteria/mL samples between c. 7 and 100% live, and in 10⁷ bacteria/mL samples between c. 7 and 73% live. By demonstrating the use of multi-spectra SVR and the optrode to monitor E. coli viability, we raise points of consideration for spectroscopic analysis of SYTO 9 and PI and aim to lay the foundation for future work that uses similar methods for different bacterial species.

Near real-time enumeration of live and dead bacteria using a fibre-based spectroscopic device

A rapid, cost-effective and easy method that allows on-site determination of the concentration of live and dead bacterial cells using a fibre-based spectroscopic device (the optrode system) is proposed and demonstrated. Identification of live and dead bacteria was achieved by using the commercially available dyes SYTO 9 and propidium iodide, and fluorescence spectra were measured by the optrode. Three spectral processing methods were evaluated for their effectiveness in predicting the original bacterial concentration in the samples: principal components regression (PCR), partial least squares regression (PLSR) and support vector regression (SVR). Without any sample pre-concentration, PCR achieved the most reliable results. It was able to quantify live bacteria from 108 down to 106.2 bacteria/mL and showed the potential to detect as low as 105.7 bacteria/mL. Meanwhile, enumeration of dead bacteria using PCR was achieved between 108 and 107 bacteria/mL. The general procedures described in this article can be applied or modified for the enumeration of bacteria within populations stained with fluorescent dyes. The optrode is a promising device for the enumeration of live and dead bacterial populations particularly where rapid, on-site measurement and analysis is required.

Droplet Microarray Based on Superhydrophobic-Superhydrophilic Patterns for Single Cell Analysis

Single-cell analysis provides fundamental information on individual cell response to different environmental cues and is a growing interest in cancer and stem cell research. However, current existing methods are still facing challenges in performing such analysis in a high-throughput manner whilst being cost-effective. Here we established the Droplet Microarray (DMA) as a miniaturized screening platform for high-throughput single-cell analysis. Using the method of limited dilution and varying cell density and seeding time, we optimized the distribution of single cells on the DMA. We established culturing conditions for single cells in individual droplets on DMA obtaining the survival of nearly 100% of single cells and doubling time of single cells comparable with that of cells cultured in bulk cell population using conventional methods. Our results demonstrate that the DMA is a suitable platform for single-cell analysis, which carries a number of advantages compared with existing technologies allowing for treatment, staining and spot-to-spot analysis of single cells over time using conventional analysis methods such as microscopy.

Inactivation of Cronobacter sakazakii in reconstituted infant formula by combination of thymoquinone and mild heat

AIMS

The objective of this study was to determine the combined effect of thymoquinone (TQ) and mild heat on Cronobacter sakazakii in reconstituted infant formula.

METHODS AND RESULTS

Reconstituted infant formula samples inoculated with a mixture of four C. sakazakii strains (approx. 6·5 log CFU ml(-1) ) were prepared with various concentrations of TQ (0, 5, 10, 20 and 30 mmol l(-1) ) and were heated to 45, 50 and 55°C for 0, 10, 20, 30, 60 and 120 min, and the surviving populations of C. sakazakii at each sampling time were enumerated. To elucidate the mode of action of TQ, membrane integrity and changes in cell morphology were examined by LIVE/DEAD(®) BacLight(™) bacterial viability kit and field emission scanning electron microscope respectively. TQ at 30 mmol l(-1) reduced the pathogen to undetectable level in between 60 and 120 min at 45°C, 60 min at 50°C and 10 min at 55°C respectively.

CONCLUSIONS

Our results demonstrated that the combined treatments significantly reduced (P < 0·05) the population of C. sakazakii, compared to the control. Cronobacter sakazakii numbers were reduced much more rapidly with higher temperatures and increased concentrations of TQ. And combined treatment inactivated pathogen partly by causing cell membrane disruption.

SIGNIFICANCE AND IMPACT OF THE STUDY

These findings suggested that TQ, together with mild heat, may have potential application in infant formula to control C. sakazakii before consumption and therefore is a possible way to prevent infections associated with C. sakazakii in infant formula.

A review of practical tools for rapid monitoring of membrane bioreactors

The production of high quality effluent from membrane bioreactors (MBRs) arguably requires less supervision than conventional activated sludge (CAS) processes. Nevertheless, the use of membranes brings additional issues of activated sludge filterability, cake layer formation and membrane fouling. From a practical standpoint, process engineers and operators require simple tools which offer timely information about the biological health and filterability of the mixed liquor as well as risks of membrane fouling. To this end, a range of analytical tools and biological assays are critically reviewed from this perspective. This review recommends that Capillary Suction Time (CST) analysis along with Total Suspended and Volatile Solids (TSS/VSS) analysis is used daily. For broad characterisation, total carbon and nitrogen analysis offer significant advantages over the commonly used chemical and biological oxygen demand (COD/BOD) analyses. Of the technologies for determining the vitality of the microbial biomass the most robust and reproducible, are the second generation adenosine-5'-triphosphate (ATP) test kits. Extracellular polymer concentrations are best monitored by measurement of turbidity after centrifugation. Taken collectively these tools can be used routinely to ensure timely intervention and smoother operation of MBR systems.

Antimicrobial Activity of Ferulic Acid Against Cronobacter sakazakii and Possible Mechanism of Action

Cronobacter sakazakii is an opportunistic pathogen transmitted by food that affects mainly newborns, infants, and immune-compromised adults. In this study, the antibacterial activity of ferulic acid was tested against C. sakazakii strains. Minimum inhibitory concentration of ferulic acid against C. sakazakii strains was determined using the agar dilution method. Changes in intracellular pH, membrane potential and intracellular ATP concentration were measured to elucidate the possible antibacterial mechanism. Moreover, SYTO 9 nucleic acid staining was used to assess the effect of ferulic acid on bacterial membrane integrity. Cell morphology changes were observed under a field emission scanning electron microscope. The minimum inhibitory concentrations of ferulic acid against C. sakazakii strains ranged from 2.5 to 5.0 mg/mL. Addition of ferulic acid exerted an immediate and sustained inhibition of C. sakazakii proliferation. Ferulic acid affected the membrane integrity of C. sakazakii, as evidenced by intracellular ATP concentration decrease. Moreover, reduction of intracellular pH and cell membrane hyperpolarization were detected in C. sakazakii after exposure to ferulic acid. Reduction of green fluorescence indicated the injury of cell membrane. Electronic microscopy confirmed that cell membrane of C. sakazakii was damaged by ferulic acid. Our results demonstrate that ferulic acid has moderate antimicrobial activity against C. sakazakii. It exerts its antimicrobial action partly through causing cell membrane dysfunction and changes in cellular morphology. Considering its antimicrobial properties, together with its well-known nutritional functions, ferulic acid has potential to be developed as a supplement in infant formula or other foods to control C. sakazakii.

Characterisation of the Poly-(Vinylpyrrolidone)-Poly-(Vinylacetate-Co-Crotonic Acid) (PVP:PVAc-CA) Interpolymer Complex Matrix Microparticles Encapsulating a Bifidobacterium lactis Bb12 Probiotic Strain

The method of producing poly-(vinylpyrrolidone)-poly-(vinylacetate-co-crotonic acid) (PVP:PVAc-CA) interpolymer complex matrix microparticles in supercritical carbon dioxide (scCO2), encapsulating bacteria, has recently been developed. This study was aimed at probing the external and internal structure of these microparticles, which can be used in food. The encapsulation efficiency and distribution of encapsulated Bifidobacterium lactis Bb12 within these microparticles were also investigated. Scanning electron microscopy (SEM) revealed irregular, mostly small, smooth microparticles with no visible bacterial cells on the surface. However, some of the microparticles appeared to have porous surfaces. The results of a Microtrac S3500 particle size analyzer showed that the PVP:PVAc-CA interpolymer complex matrix microparticles encapsulating B. lactis Bb12 had an average particle size of 166.1 μm (<350 μm designated standard size for microparticles). The D 10, D 50 and D 90 values for these microparticles were 48.16, 166.06 and 382.55 μm, respectively. Both SEM and confocal laser scanning microscopy showed a high density of bacterial cells within the microparticles. An average encapsulation efficiency of 96% was achieved. Consequently, the microparticles have the potential to be evenly distributed in foods, deliver adequate amounts of probiotics and produce minimal adverse effects on the texture and mouth feel of the foods into which they are incorporated.

Antimicrobial activity of syringic acid against Cronobacter sakazakii and its effect on cell membrane

Syringic acid (SA) has been reported to exhibit antibacterial ability against various microorganisms, but little work has been done on its effect on Cronobacter sakazakii. In this study, minimum inhibitory concentrations (MICs) of SA against various C. sakazakii strains were determined. Moreover, changes in intracellular ATP concentration, intracellular pH (pHin), membrane potential and membrane integrity were measured to evaluate the influence of SA on cell membrane. Finally, field emission scanning electron microscope (FESEM) was used to assess the morphological changes of bacterial cells caused by SA. It was shown that the MICs of SA against all tested C. sakazakii strains were 5mg/mL. SA retarded bacterial growth, and caused cell membrane dysfunction, which was evidenced by intracellular ATP concentration decrease, pHin reduction, cell membrane hyperpolarization and changes in cellular morphology. These findings indicated that SA has potential to be developed as a natural preservative to control C. sakazakii in foods associated with this pathogen and prevent related infections.

Lyophilised Vegetal BM 297 ATO-Inulin lipid-based synbiotic microparticles containing Bifidobacterium longum LMG 13197: design and characterisation

This study aimed at the manufacturing and characterisation of Vegetal BM 297 ATO-inulin-Bifidobacterium longum LMG 13197 microparticles prepared by freeze drying. Emulsions containing 1%, 1.5%, 2%, 3.5% or 5% w/v inulin were prepared, with or without centrifugation before freeze drying. Morphological properties, particle size distribution, encapsulation efficiency of the microparticles and their ability to preserve viability of the enclosed B. longum LMG 13197 cells were evaluated. The microparticles produced from both formulations without a centrifugation step were irregular, porous with concavities and contained high number of bacterial cells. Formulations with or without inulin had average particle sizes of 33.4-81.0 μm with encapsulation efficiencies of 82% and 88%, respectively. Vegetal-inulin microparticles have the morphology and size that will enable their even distribution in final food products, and hence, they have the potential for use as a functional food additive because they are likely to deliver sufficient numbers of viable bacteria.

Dual Roles of Capsular Extracellular Polymeric Substances in Photocatalytic Inactivation of Escherichia coli: Comparison of E. coli BW25113 and Isogenic Mutants

The dual roles of capsular extracellular polymeric substances (EPS) in the photocatalytic inactivation of bacteria were demonstrated in a TiO2-UVA system, by comparing wild-type Escherichia coli strain BW25113 and isogenic mutants with upregulated and downregulated production of capsular EPS. In a partition system in which direct contact between bacterial cells and TiO2 particles was inhibited, an increase in the amount of EPS was associated with increased bacterial resistance to photocatalytic inactivation. In contrast, when bacterial cells were in direct contact with TiO2 particles, an increase in the amount of capsular EPS decreased cell viability during photocatalytic treatment. Taken together, these results suggest that although capsular EPS can protect bacterial cells by consuming photogenerated reactive species, it also facilitates photocatalytic inactivation of bacteria by promoting the adhesion of TiO2 particles to the cell surface. Fluorescence microscopy and scanning electron microscopy analyses further confirmed that high capsular EPS density led to more TiO2 particles attaching to cells and forming bacterium-TiO2 aggregates. Calculations of interaction energy, represented by extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) potential, suggested that the presence of capsular EPS enhances the attachment of TiO2 particles to bacterial cells via acid-base interactions. Consideration of these mechanisms is critical for understanding bacterium-nanoparticle interactions and the photocatalytic inactivation of bacteria.

Confusion over live/dead stainings for the detection of vital microorganisms in oral biofilms--which stain is suitable?

BACKGROUND

There is confusion over the definition of the term "viability state(s)" of microorganisms. "Viability staining" or "vital staining techniques" are used to distinguish live from dead bacteria. These stainings, first established on planctonic bacteria, may have serious shortcomings when applied to multispecies biofilms. Results of staining techniques should be compared with appropriate microbiological data.

DISCUSSION

Many terms describe "vitality states" of microorganisms, however, several of them are misleading. Authors define "viable" as "capable to grow". Accordingly, staining methods are substitutes, since no staining can prove viability.The reliability of a commercial "viability" staining assay (Molecular Probes) is discussed based on the corresponding product information sheet: (I) Staining principle; (II) Concentrations of bacteria; (III) Calculation of live/dead proportions in vitro. Results of the "viability" kit are dependent on the stains' concentration and on their relation to the number of bacteria in the test. Generally this staining system is not suitable for multispecies biofilms, thus incorrect statements have been published by users of this technique.To compare the results of the staining with bacterial parameters appropriate techniques should be selected. The assessment of Colony Forming Units is insufficient, rather the calculation of Plating Efficiency is necessary. Vital fluorescence staining with Fluorescein Diacetate and Ethidium Bromide seems to be the best proven and suitable method in biofilm research.Regarding the mutagenicity of staining components users should be aware that not only Ethidium Bromide might be harmful, but also a variety of other substances of which the toxicity and mutagenicity is not reported.

SUMMARY

- The nomenclature regarding "viability" and "vitality" should be used carefully.- The manual of the commercial "viability" kit itself points out that the kit is not suitable for natural multispecies biofilm research, as supported by an array of literature.- Results obtained with various stains are influenced by the relationship between bacterial counts and the amount of stain used in the test. Corresponding vitality data are prone to artificial shifting.- As microbiological parameter the Plating Efficiency should be used for comparison.- Ethidium Bromide is mutagenic. Researchers should be aware that alternative staining compounds may also be or even are mutagenic.

Recommendations for the viability assessment of probiotics as concentrated cultures and in food matrices

Due to the fact that probiotic cells need to be alive when they are consumed, culture-based analysis (plate count) is critical in ascertaining the quality (numbers of viable cells) of probiotic products. Since probiotic cells are typically stressed, due to various factors related to their production, processing and formulation, the standard methodology for total plate counts tends to underestimate the cell numbers of these products. Furthermore, products such as microencapsulated cultures require modifications in the release and sampling procedure in order to correctly estimate viable counts. This review examines the enumeration of probiotic bacteria in the following commercial products: powders, microencapsulated cultures, frozen concentrates, capsules, foods and beverages. The parameters which are specifically examined include: sample preparation (rehydration, thawing), dilutions (homogenization, media) and plating (media, incubation) procedures. Recommendations are provided for each of these analytical steps to improve the accuracy of the analysis. Although the recommendations specifically target the analysis of probiotics, many will apply to the analysis of commercial lactic starter cultures used in food fermentations as well.

Development of a real-time RT-PCR method for enumeration of viable Bifidobacterium longum cells in different morphologies

Viability of probiotic bacteria is traditionally assessed by plate counting which has several limitations, including underestimation of cells in aggregates or chains morphology. We describe a quantitative PCR (qPCR)-based method for an accurate enumeration of viable cells of Bifidobacterium longum NCC2705 exhibiting different morphologies by measuring the mRNA levels of cysB and purB, two constitutively expressed housekeeping genes. Three primer-sets targeting short fragments of 57-bp of cysS and purB and one 400-bp fragment of purB were used. Cell quantification of serially diluted samples showed a good correlation coefficient of R(2) 0.984 +/- 0.003 between plate counts and qRT-PCR for all tested primer sets. Loss of viable cells exposed to a lethal heat stress (56 degrees C, 10, 20 and 30 min) was estimated by qRT-PCR and plate counts. No significant difference was observed using qRT-PCR targeting the 400-bp fragment of purB compared to plate counts indicating that this fragment is a suitable marker of cell viability. In contrast, the use of the 57-bp fragments led to a significant overestimation of viable cell counts (18 +/- 3 and 7 +/- 2 fold for cysB and purB, respectively). Decay of the mRNA fragments was studied by treatment of growing cells with rifampicin prior qRT-PCR. The 400-bp fragment of purB was faster degraded than the 57-bp fragments of cysB and purB. The 400-bp fragment of purB was further used to enumerate viable cells in aggregate state. Cell counts were more than 2 log(10) higher using the qRT-PCR method compared to plate counts. Growing interest in probiotic characteristics of aggregating bacteria cells make this technique a valuable tool to accurately quantify viable probiotic bacteria exhibiting heterogeneous morphology.

Effect of the fermentation pH on the storage stability of Lactobacillus rhamnosus preparations and suitability of in vitro analyses of cell physiological functions to predict it

AIMS

To investigate how cell physiological functions can predict the stability of freeze-dried probiotics. In addition, the effect of the fermentation pH on the stability of probiotics was investigated.

METHODS AND RESULTS

Fermenter-grown (pH 5.8 or 5.0) Lactobacillus rhamnosus cells were freeze-dried and their survival was evaluated during storage at 37 degrees C, in apple juice and during acid [hydrochloric acid (HCl) and malic acid] and bile exposure. Cells grown at pH 5.0 were generally coping better with acid-stress than cells grown at pH 5.8. Cells were more sensitive to malic acid compared with HCl. Short-term stability results of Lact. rhamnosus cells in malic acid correlated well with the long-term stability results in apple juice, whereas the results of cell membrane integrity studies were in accordance with bile exposure results.

CONCLUSIONS

Malic acid exposure can prove useful in evaluating the long-term stability of probiotic preparations in apple juice. Fermentation at reduced pH may ensure a better performance of Lact. rhamnosus cells during the subsequent acid-stress.

SIGNIFICANCE AND IMPACT OF THE STUDY

The beneficial effect of lowered fermentation pH to Lact. rhamnosus stability during storage in apple juice and the usefulness of malic acid test in predicting the stability were shown.

In Vitro Assessment of the Gastrointestinal Transit Tolerance of Taxonomic Reference Strains from Human Origin and Probiotic Product Isolates of Bifidobacterium

Next to health promoting effects, the functional aspect of probiotic strains also involves their capacity to reach the colon as viable metabolically active cells. The present study aimed to assess the potential of 24 probiotic product isolates and 42 human reference strains of Bifidobacterium to survive gastrointestinal transit under in vitro conditions. The survival capacity of exponential and stationary phase cultures upon exposure to gastric and small intestinal juices was determined using a recently developed microplate-based assay in combination with the LIVE/DEAD BacLight Bacterial Viability kit. All 66 strains tested displayed a considerable loss in viability during exposure to an acidic pepsin containing solution (pH 2.0). Among the 10 taxa tested, cultures of B. animalis ssp. lactis appeared to be most capable to survive gastric transit. Although to a lesser extent, the presence of bile salts also affected the viability of most of the strains tested. Except for 3 strains, all 66 strains showed bile salt hydrolase activity using an agar-based assay. In contrast, the bifidobacterial strains used in this study appeared to possess a natural ability to survive the presence of pancreatin (pH 8.0). Although the effect was not significant, a slightly enhanced tolerance to gastrointestinal transit was observed when cells were in the stationary phase, especially when exposed to acid, compared with cells being in the exponential phase. Survival in the gastrointestinal tract appeared to be largely strain-dependent and hence implies that different strains will likely display a different behavior in functionality. The assay used in this study allows an initial assessment of strains for use as probiotic cultures prior to selecting potential candidate strains for further investigation in vivo.

Evaluation of the response of Lactobacillus rhamnosus VTT E-97800 to sucrose-induced osmotic stress

Environmental osmotic changes are one of the stresses live probiotics may encounter either in their natural habitats or as a result of usage in food formulations and processing. Response to osmotic stress, induced by sucrose, of the probiotic strain Lactobacillus rhamnosus VTT E-97800 (E800) was investigated. The fluorescence-based approach used, by combined staining with caboxyfluorescein (cFDA) and propidium iodide (PI) could give insights on the osmotic-induced changes of microbial esterase activity and membrane integrity; also the extrusion of intracellular accumulated carboxyfluorescein (cF) upon energizing with glucose. Comparison of the flowcytometric viability assessment with the conventional culture techniques revealed that sucrose-stressed cells had a slight loss of culturability (logN/N(0) approximately -0.3) at 1.2 and 1.5M sucrose concentration though they could perform an enzymatic conversion of cFDA into cF. The presence of such metabolically active bacteria in food might be critical as they may excrete toxic or food spoilage metabolites. Moreover, the perturbation of cF extrusion activities became a limiting factor for reproductive capacities. There was no change in the cell morphology. These results proved the ability of the strain of study to tolerate sucrose, even at extreme concentrations and these must be taken into consideration for its usage in the formulation/processing of sugar-based foods, e.g. jams, candies, etc.

A fluorescence assay to determine the viable biomass of microcosm dental plaque biofilms

Dental plaque bacteria form complex and robust cell aggregates which cannot be counted accurately using epifluorescence microscopy. This causes a significant problem for quantifying their viability. The aim of the investigation was to develop a fluorescence assay to quantify the viable biomass of dental plaque biofilms. Using an artificial mouth system, microcosm plaques were grown under a range of fluoride and mineralizing conditions, and were treated with the oral antiseptics chlorhexidine (CHX) and Listerine. Plaques were harvested, made into suspension and stained in microtitre plates with a di-chromatic fluorescent stain (Live/Dead BacLight). The percentage of viable biomass was calculated from the regression data generated from a viability standard. The standard was constructed using different proportions of viable (green fluorescence) and non-viable (red fluorescence) plaque bacteria, and growth conditions for optimizing green fluorescence were investigated. The results from the assay showed that fluoride at 1000 and 3000 ppm promoted plaque viability by at least 15%, from approximately 45 to 60%, and at 5000 ppm to approximately 87% (P<0.05). Plaques treated with Listerine and CHX from d 0 yielded insufficient biomass to be tested for viability, however 14 d post-treatment, viability was comparable to untreated plaques (approximately 55%, P>0.05). Treatment with Listerine and CHX from d 3 reduced biomass but not viability. Development of this assay enabled viability of plaque bacteria which cannot be resolved with epifluorescence microscopy to be evaluated. It offers a rapid alternative to epifluorescence microscopy and could be applied to nonoral bacteria.