Fluorescent method for monitoring cheese starter permeabilization and lysis

CAM/TMA-DPH as a promising alternative to SYTO9/PI for cell viability assessment in bacterial biofilms

Introduction

Accurately assessing biofilm viability is essential for evaluating both biofilm formation and the efficacy of antibacterial treatments. Traditional SYTO9 and propidium iodide (PI) live/dead staining in biofilm viability assays often ace challenges due to non-specific staining, limiting precise differentiation between live and dead cells. To address this limitation, we investigated an alternative staining method employing calcein acetoxymethyl (CAM) to detect viable cells based on esterase activity, and 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene p-toluenesulfonate (TMA-DPH) to assess the remaining biofilm population.

Methods

Biofilms of Pseudomonas aeruginosa, Klebsiella pneumoniae, Staphylococcus aureus, and Enterococcus faecium were matured and exposed to varying concentrations of antibiotics or sterile medium. Biofilm viability was assessed using CAM/TMA-DPH or SYTO9/PIstaining, followed by analysis with confocal laser scanning microscopy (CLSM) and ImageJ-based biofilm surface coverage quantification. Viability findings were compared with colony-forming units (CFU/mL), a standard microbial viability measure.

Results

CAM/TMA-DPH staining demonstrated strong positive correlations with CFU counts across all bacterial species (r = 0.59 - 0.91), accurately reflecting biofilm vitality. In contrast, SYTO9/PI staining consistently underestimated the viability of untreated biofilms, particularly in Klebsiella pneumoniae, where a negative correlation with CFU/mL was observed (r = -0.04). Positive correlations for SYTO9/PI staining were noted in other species (r = 0.65 - 0.79). These findings underscore the limitations of membrane integrity-based staining methods and highlight the advantages of metabolic-based probes like CAM/TMA-DPH.

Discussion

Our findings suggest that CAM/TMA-DPH staining provides a promising alternative to SYTO9/PI for cell viability assessment in bacterial biofilms, highlighting the advantages of metabolic-based probes over traditional membrane integrity assays. The consistency of CAM/TMA-DPH staining across different bacterial species underscores its potential to advance studies on biofilm and contribute to the development of more effective anti-biofilm treatments, which is essential for clinical management of biofilm-associated infections.

More than a contaminant: How zinc promotes carbonate-mineralizing bacteria metabolism and adaptation by reshaping precipitation conditions

Although microbial-induced carbonate precipitation (MICP) technology is both environmentally friendly and cost-effective, its efficiency is constrained by challenges such as low bacterial activity and heavy metal stress. This study explored the enhancement of mineralization efficiency by incorporating zinc (Zn) into the cultivation system of carbonate-mineralized bacteria. All Zn salts at a concentration of 30 μmol/L significantly enhanced the density and heavy metal resistance of bacterial cells, while also promoting CO2 hydration efficiency. The activities of urease and carbonic anhydrase (CA) were significantly elevated after treatment with 30 μmol/L ZnCl2 and Zn(C3H5O3)2 (ZnL) compared to the control. The results from qRT-PCR and ELISA confirmed that ZnL exhibited a stable biological effect on CA gene expression. Through the analysis of surface chemistry of cells and the subcellular distribution pattern of cadmium (Cd), it was observed that Zn supplementation maintained the cell surface stability and strengthened the cellular barrier against Cd uptake. SEM, FTIR and XRD results further confirmed that Zn supplementation significantly increased the complexity of the mineral morphology, resulting in a more stable crystal structure of CdCO3. This study offers additional theoretical and technical backing, opening a new avenue for the practical application of MICP technology in heavy metal remediation.

Facilitating effects of the synergy with zero-valent iron and peroxysulfate on the sludge anaerobic fermentation system: Combined biological enzyme, microbial community and fermentation mechanism assessment

This study evaluated a synergetic waste activated sludge treatment strategy with environmentally friendly zero-valent iron nanoparticles (Fe0) and peroxysulfate. To verify the feasibility of the synergistic treatment, Fe0, peroxysulfate, and the mixture of peroxysulfate and Fe0 (synergy treatment) were added to different sludge fermentation systems. The study demonstrated that the synergy treatment fermentation system displayed remarkable hydrolysis performance with 435.50 mg COD/L of protein and 197.67 mg COD/L of polysaccharide, which increased 1.13-2.85 times (protein) and 1.12-1.49 times (polysaccharide) for other three fermentation system. Additionally, the synergy treatment fermentation system (754.52 mg COD/L) exhibited a well acidification performance which was 1.35-41.73 times for other systems (18.08-557.27 mg COD/L). The synergy treatment fermentation system had a facilitating effect on the activity of protease, dehydrogenase, and alkaline phosphatase, which guaranteed the transformation of organic matter. Results also indicated that Comamonas, Soehngenia, Pseudomonas, and Fusibacter were enriched in synergy treatment, which was beneficial to produce SCFAs. The activation of Fe0 on peroxysulfate promoting electron transfer, improving the active groups, and increasing the enrichment of functional microorganisms showed the advanced nature of synergy treatment. These results proved the feasibility of synergy treatment with Fe0 and peroxysulfate to enhance waste activated sludge anaerobic fermentation.

Colonization potential of endophytes from halophytic plants growing in the "Runn of Kutch" salt marshes and their contribution to mitigating salt stress in tomato cultivation

Increasing soil salinity depreciates the quantity of the crop produce. Looking at the tremendous potential of plant-associated microorganisms in salinity stress mitigation, it would be very useful in exploring and deciphering salt-tolerant microorganisms from halophytic plants and their utilization in cultivated plants. With this aim, in the present study, four halophytic plants were taken from Rann of Kutch, and bacterial endophytes were isolated from different plant organs. These endophytes were characterized by plant growth and health promotion features. The molecular identification was done based on 16 s rRNA sequence similarity. It was found that the endophytic bacteria isolated from 4 different halophytes found sharing phylogenetic relatedness. Four potential endophytes Alkalihalobacillus gibsonii 2H2, Achromobacter insuavis 2H18, Terribacillus halophilus 2H20, and Bacillus siamensis 4H1 were tested in tomato for salinity stress alleviation. Changes in the levels of antioxidants were analyzed. Total chlorophyll, total phenolics, malondialdehyde, and proline content indicated reduced damage in the plant system due to salinity by the application of endophytes. All the treatments exhibited low levels of electrolyte leakage. The accumulation of enzymatic reactive oxygen species scavengers was assessed from the levels of peroxidase, catalase, superoxide dismutase, phenylalanine ammonia-lyase, ascorbate peroxidase, and guiacol peroxidase. The NBT and DAB staining confirmed the findings. The reduction in the accumulation of Na+ ions in tomato leaves was visualized using Sodium Green probes under CSLM and found to be lowest in Terribacillus halophilus 2H20 and Bacillus siamensis 4H1 inoculated plants. The endophyte Terribacillus halophilus 2H20 was the most promising isolate. The colonization in tomato roots was confirmed using a cell tracker system. Results showed that the endophytes were found to have salinity stress mitigation traits. The efficiency could be further improved with the combination of other endophytes tested earlier.

Study on the Collection Efficiency of Bioaerosol Nanoparticles by Andersen-Type Impactors

Airborne transmission is much more common than previously thought. Based on our knowledge about SARS-COV-2 (COVID-19) infection, the aerosol transmission routes for all respiratory infections must be reassessed. Thus far, the COVID-19 outbreak has caused catastrophic public health and economic crises, posing a serious threat to the lives and health of people around the world and directing public attention toward the airborne transmission of pathogens. The novel coronavirus transmission in the form of nanoaerosols in a wider range hinders prevention and early warning efforts. As a classical bioaerosol sampler, the Andersen six-stage sampler is widely used in the collection and research of aerosol particles. In this study, the physical and biological collection efficiency of the six-stage sampler was explored by qPCR and colony counting method. Results showed that the physical collection efficiency reached more than 50% when the particle size was larger than 0.75 μm. However, the overall biological collection efficiency was only 0.25%. In addition, fluorescence microscopy and flow cytometry were used to detect the microbial state after sampling, and the results showed that the proportion of the collected live bacteria was less than 15% of the total. This result is of great significance not only for the application of the Andersen six-stage sampler in collecting nanosized bioaerosols, but also provides reference for the selection of subsequent detection technologies for effective collection.

Purified lactases versus whole-cell lactases-the winner takes it all

Lactose-free dairy products are in great demand worldwide due to the high prevalence of lactose intolerance. To make lactose-free dairy products, commercially available β-galactosidase enzymes, also termed lactases, are used to break down lactose to its constituent monosaccharides, glucose and galactose. In this mini-review, the characteristics of lactase enzymes, their origin, and ways of use are discussed in light of their potential for hydrolyzing lactose. We also discuss whole-cell lactase catalysts, which appear to have great potential in terms of cost reduction and convenience, and which are more natural alternatives to purified enzymes. Lactic acid bacteria (LAB) already used in food fermentations seem to be optimal candidates for whole-cell lactases. However, they have not been industrially exploited yet due to technical hurdles. For whole-cell lactases to be efficient, the lactase enzymes inside the cells must be made available for lactose hydrolysis, and thus, cells need to be permeabilized or disrupted prior to use. Here we review state-of-the-art approaches for disrupting or permeabilizing microorganisms. Lastly, based on recent scientific achievements, we propose a novel, resource-efficient, and low-cost scenario for achieving lactose hydrolysis at a dairy plant using a LAB whole-cell lactase.Key points• Lactases (β-galactosidase) are essential for producing lactose-free dairy products• Novel permeabilization techniques facilitate the use of LAB lactases• Whole-cell lactase catalysts have great potential for reducing costs and resources Graphical abstract.

Effect of sodium dichloroisocyanurate treatment on enhancing the biodegradability of waste-activated sludge anaerobic fermentation

In the present study, a novel oxidant (sodium dichloroisocyanurate, NaCl₂(NCO)₃; SDIC) combined with microorganisms was employed to achieve a higher performance of waste-activated sludge (WAS) anaerobic fermentation. Four concentrations of SDIC (0, 0.3, 0.6, and 1.0 mg SDIC/mg SS) were studied in WAS fermentation systems. The results showed that the release of proteins and polysaccharides was enhanced by the addition of SDIC with values of 1002.25 mg COD/L and 680.25 mg COD/L, respectively, and these values increased 14.46-18.07 times (proteins) and 3.74-7.40 times (polysaccharides) compared with that of the blank test. Additionally, the short-chain fatty acids also increased 2.24 times. The rate of extraction of organic substances from the sludge increased from 3.03% to 33.33%. Furthermore, the fermented sludge with the SDIC treatment had higher hydrolytic acidification efficiencies for bovine serum albumin and glucose, increasing from 4.558% to 9.91% and 2.976%-6.764%, respectively. However, SDIC treatment of the conventional fermented sludge resulted in lower hydrolytic acidification efficiencies with values of 4.978%-1.781% and 3.334%-0.582%, respectively. Biological enzyme analysis also showed that SDIC enhanced α-glucosidase and protease activity but inhibited dehydrogenase, alkaline phosphatase, and acid phosphatase activity. Proteobacteria and Comamonas were the main microbial communities observed in the WAS anaerobic fermentation.

Antimicrobial Activity and Mode of Action of Celastrol, a Nortriterpen Quinone Isolated from Natural Sources

Species of the Celastraceae family are traditionally consumed in different world regions for their stimulating properties. Celastrol, a triterpene methylene quinone isolated from plants of celastraceas, specifically activates satiety centers in the brain that play an important role in controlling body weight. In this work, the antimicrobial activity and mechanism of action of celastrol and a natural derivative, pristimerin, were investigated in Bacillus subtilis. Celastrol showed a higher antimicrobial activity compared with pristimerin, being active against Gram-positive bacteria with minimum inhibitory concentrations (MICs) that ranged between 0.16 and 2.5 µg/mL. Killing curves displayed a bactericidal effect that was dependent on the inoculum size. Monitoring of macromolecular synthesis in bacterial populations treated with these compounds revealed inhibition in the incorporation of all radiolabeled precursors, but not simultaneously. Celastrol at 3 µg/mL and pristimerin at 10 µg/mL affected DNA and RNA synthesis first, followed by protein synthesis, although the inhibitory action on the uptake of radiolabeled precursors was more dramatic with celastrol. This compound also caused cytoplasmic membrane disruption observed by potassium leakage and formation of mesosome-like structures. The inhibition of oxygen consumption of whole and disrupted cells after treatments with both quinones indicates damage in the cellular structure, suggesting the cytoplasmic membrane as a potential target. These findings indicate that celastrol could be considered as an interesting alternative to control outbreaks caused by spore-forming bacteria.

Influence of the Spatial Distribution of Cationic Functional Groups at Nanoparticle Surfaces on Bacterial Viability and Membrane Interactions

While positively charged nanomaterials induce cytotoxicity in many organisms, much less is known about how the spatial distribution and presentation of molecular surface charge impact nanoparticle-biological interactions. We systematically functionalized diamond nanoparticle surfaces with five different cationic surface molecules having different molecular structures and conformations, including four small ligands and one polymer, and we then probed the molecular-level interaction between these nanoparticles and bacterial cells. Shewanella oneidensis MR-1 was used as a model bacterial cell system to investigate how the molecular length and conformation of cationic surface charges influence their interactions with the Gram-negative bacterial membranes. Nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS) demonstrate the covalent modification of the nanoparticle surface with the desired cationic organic monolayers. Surprisingly, bacterial growth-based viability (GBV) and membrane damage assays both show only minimal biological impact by the NPs functionalized with short cationic ligands within the concentration range tested, yet NPs covalently linked to a cationic polymer induce strong cytotoxicity, including reduced cellular viability and significant membrane damage at the same concentration of cationic groups. Transmission electron microscopy (TEM) images of these NP-exposed bacterial cells show that NPs functionalized with cationic polymers induce significant membrane distortion and the production of outer membrane vesicle-like features, while NPs bearing short cationic ligands only exhibit weak membrane association. Our results demonstrate that the spatial distribution of molecular charge plays a key role in controlling the interaction of cationic nanoparticles with bacterial cell membranes and the subsequent biological impact. Nanoparticles functionalized with ligands having different lengths and conformations can have large differences in interactions even while having nearly identical zeta potentials. While the zeta potential is a convenient and commonly used measure of nanoparticle charge, it does not capture essential differences in molecular-level nanoparticle properties that control their biological impact.

Effects of zinc oxide nanoparticles on sludge anaerobic fermentation: phenomenon and mechanism

Zinc oxide nanoparticles (ZnO NPs) production and usage might lead to a large discharge of ZnO NPs into the natural environment, raising concerns of pollution and ecological security. The effects of ZnO NPs on waste activated sludge hydrolytic acidification and microbial communities were studied in semi-continuous fermentation systems. The fermentation performance of eight ZnO NPs concentrations including ZnO NPs normal [0.01, 0.1, 1 and 10 mg/g mixed liquor suspended solids (MLSS)] and ZnO NPs shock (10, 1000, 1000 and 10,000 mg/g MLSS) were discussed, and their biodegradability was also analyzed. The experimental results showed that proteins, polysaccharides and short-chain fatty acids were enhanced by ZnO NPs, particularly by ZnO NPs shock. Low ZnO NPs concentrations inhibited coenzyme 420 (F420) and dehydrogenase activities but enhanced α-glucosidase and protease activities. Illumina MiSeq sequencing revealed that ZnO NPs addition enriched Azospira, Ottowia and Hyphomicrobium but not Anaerolineaceae.

Cultivable non-starter lactobacilli from ripened Parmigiano Reggiano cheeses with different salt content and their potential to release anti-hypertensive peptides

The impact of salt and fat intake on human health drives the consumer's attention towards dairy food with reduced salt and fat contents. How changes in salt and fat content modulate dairy LAB population and the associated proteolytic activities have been poorly studied. Here, non-starter LAB populations from 12 Parmigiano Reggiano (PR) cheeses (12-month ripened), clustered in low salt and fat content (LL-PR) and high salt and fat content (HH-PR) groups, were investigated and identified at specie-level with molecular assays. Lactobacillus rhamnosus was dominant in HH-PR samples, whereas Lactobacillus paracasei in LL-PR samples. (GTG)₅ rep-PCR analysis discriminated 11 and 12 biotypes for L. rhamnosus and L. paracasei isolates, respectively. Screening for proteolytic activity identified L. rhamnosus strains more proteolytic than L. paracasei, and, within L. rhamnosus species, HH-PR strains were generally more proteolytic than LL-PR strains. Two L. rhamnosus representatives, namely strain 0503 from LL-PR and strain 2006 from HH-PR, were functionally characterized in cow milk fermentation assay. HH-PR strain 2006 overcame LL-PR strain 0503 in acidification performance, leading to a fermented milk with higher angiotensin I-converting enzyme inhibitory and antioxidant activities. L. rhamnosus 2006 was more prone to release VPP, while L. rhamnosus 0503 released higher amount of IPP. This study provides evidences that salt/fat content affects NSLAB cultivable fraction and the associated proteolytic ability resulting in a complex occurrence of bioactive peptides featuring health-promoting properties.

Identification of cow, buffalo, goat and ewe milk species in fermented dairy products using synchronous fluorescence spectroscopy

In the dairy industry, substitution of high priced milk species with low priced ones is a common practice, and determination of milk species is critical. In this study, synchronous fluorescence spectroscopy (SFS) method was developed for identification of milk species in fermented dairy products (yoghurt and cheese). Three partial least square-discriminant analysis models were developed in order to identify pure-mixed samples, milk species and binary mixture type, and partial least square (PLS) model was utilized to quantify the mixing ratio in binary mixtures. PLS models used for yoghurt and cheese samples showed that detection limits of adulteration were below 3.3%. Apart from the buffalo-cow yoghurt and goat-cow cheese, precision of the measurements was found to be below 6.2. It can be said that SFS technique is applicable on yoghurt and cheese samples as it's a less destructive and a less costly method compared to DNA and protein based conventional methods.

Diminished inhibitory impact of ZnO nanoparticles on anaerobic fermentation by the presence of TiO2 nanoparticles: Phenomenon and mechanism

Engineered nanoparticle materials (ENMs) are widely and increasingly produced and employed in many sectors. The use of diverse ENMs potentially leads to the release of multiple ENMs into the environment. These ENMs after discharge will be end in wastewater treatment plant and present in sludge. This work investigated the effect of multi-ENMs systems of ZnO and TiO₂ on sludge anaerobic fermentation and the related toxicity mechanism. Results revealed that the toxicity of ZnO ENMs on anaerobic fermentation was reduced in the presence of TiO₂ ENMs. Investigation on the change of free Zn²⁺ and reactive oxygen species (OH and H₂O₂) suggested that both of free Zn²⁺ and ROS contributed to the toxicity mechanism. Zn²⁺ decrease was the main reason for the reduced toxicity in multi-ENMs systems. ROS mainly led to the reduction of cell viability in anaerobic fermentation systems. The presence of TiO₂ in the multi-ENMs systems promoted the recovery of enzyme activity, cell viability and bacteria abundance.

Rapid assessment of viable but non-culturable Bacillus coagulans MTCC 5856 in commercial formulations using Flow cytometry

Accurate enumeration of bacterial count in probiotic formulation is imperative to ensure that the product adheres to regulatory standards and citation in consumer product label. Standard methods like plate count, can enumerate only replicating bacterial population under selected culture conditions. Viable but non culturable bacteria (VBNC) retain characteristics of living cells and can regain cultivability by a process known as resuscitation. This is a protective mechanism adapted by bacteria to evade stressful environmental conditions. B. coagulans MTCC 5856(LactoSpore®) is a probiotic endospore which can survive for decades in hostile environments without dividing. In the present study, we explored the use of flow cytometry to enumerate the viable count of B. coagulans MTCC 5856 under acidic and alkaline conditions, high temperature and in commercial formulations like compressed tablets and capsules. Flow cytometry (FCM) was comparable to plate count method when the spores were counted at physiological conditions. We show that VBNC state is induced in B. coagulans MTCC 5856by high temperature and acidic pH. The cells get resuscitated under physiological conditions and FCM was sensitive to detect the VBNC spores. Flow cytometry showed excellent ability to assess the viable spore count in commercial probiotic formulations of B. coagulans MTCC 5856. The results establish Flow cytometry as a reliable method to count viable bacteria in commercial probiotic preparations. Sporulation as well as existence as VBNC could contribute to the extreme stability of B. coagulans MTCC 5856.

Influence of buttermilk powder or buttermilk addition on phospholipid content, chemical and bio-chemical composition and bacterial viability in Cheddar style-cheese

The effect of buttermilk powder addition post-curd formation or buttermilk addition to cheese milk on total and individual phospholipid content, chemical composition, enzyme activity, microbial populations and microstructure within Cheddar-style cheese was investigated. Buttermilk or buttermilk powder addition resulted in significant increases in total phospholipid content and their distribution throughout the cheese matrix. Addition of 10% buttermilk powder resulted in higher phospholipid content, moisture, pH and salt in moisture levels, and lower fat, fat in dry matter, L. helveticus and non-starter bacteria levels in cheeses. Buttermilk powder inclusion resulted in lower pH4.6/Soluble Nitrogen (SN) levels and significantly lower free amino acid levels in 10% buttermilk powder cheeses. Buttermilk addition provided a more porous cheese microstructure with greater fat globule coalescence and increased free fat pools, while also increasing moisture and decreasing protein, fat and pH levels. Addition of buttermilk in liquid or powdered form offers potential for new cheeses with associated health benefits.

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.

Colonial vs. planktonic type of growth: mathematical modeling of microbial dynamics on surfaces and in liquid, semi-liquid and solid foods

Predictive models are mathematical expressions that describe the growth, survival, inactivation, or biochemical processes of foodborne bacteria. During processing of contaminated raw materials and food preparation, bacteria are entrapped into the food residues, potentially transferred to the equipment surfaces (abiotic or inert surfaces) or cross-contaminate other foods (biotic surfaces). Growth of bacterial cells can either occur planktonically in liquid or immobilized as colonies. Colonies are on the surface or confined in the interior (submerged colonies) of structured foods. For low initial levels of bacterial population leading to large colonies, the immobilized growth differs from planktonic growth due to physical constrains and to diffusion limitations within the structured foods. Indeed, cells in colonies experience substrate starvation and/or stresses from the accumulation of toxic metabolites such as lactic acid. Furthermore, the micro-architecture of foods also influences the rate and extent of growth. The micro-architecture is determined by (i) the non-aqueous phase with the distribution and size of oil particles and the pore size of the network when proteins or gelling agent are solidified, and by (ii) the available aqueous phase within which bacteria may swarm or swim. As a consequence, the micro-environment of bacterial cells when they grow in colonies might greatly differs from that when they grow planktonically. The broth-based data used for modeling (lag time and generation time, the growth rate, and population level) are poorly transferable to solid foods. It may lead to an over-estimation or under-estimation of the predicted population compared to the observed population in food. If the growth prediction concerns pathogen bacteria, it is a major importance for the safety of foods to improve the knowledge on immobilized growth. In this review, the different types of models are presented taking into account the stochastic behavior of single cells in the growth of a bacterial population. Finally, the recent advances in the rules controlling different modes of growth, as well as the methodological approaches for monitoring and modeling such growth are detailed.

Spatial Distribution of Lactococcus lactis Colonies Modulates the Production of Major Metabolites during the Ripening of a Model Cheese

In cheese, lactic acid bacteria are immobilized at the coagulation step and grow as colonies. The spatial distribution of bacterial colonies is characterized by the size and number of colonies for a given bacterial population within cheese. Our objective was to demonstrate that different spatial distributions, which lead to differences in the exchange surface between the colonies and the cheese matrix, can influence the ripening process. The strategy was to generate cheeses with the same growth and acidification of a Lactococcus lactis strain with two different spatial distributions, big and small colonies, to monitor the production of the major ripening metabolites, including sugars, organic acids, peptides, free amino acids, and volatile metabolites, over 1 month of ripening. The monitored metabolites were qualitatively the same for both cheeses, but many of them were more abundant in the small-colony cheeses than in the big-colony cheeses over 1 month of ripening. Therefore, the results obtained showed that two different spatial distributions of L. lactis modulated the ripening time course by generating moderate but significant differences in the rates of production or consumption for many of the metabolites commonly monitored throughout ripening. The present work further explores the immobilization of bacteria as colonies within cheese and highlights the consequences of this immobilization on cheese ripening.

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

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

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.

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

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

In situ examination of cell growth and death of Lactococcus lactis

This study enables in situ studying of the growth and death of a large number of individual cells in a solid matrix. A wild type of Lactococcus lactis and several mutants with varying expression of GuaB was investigated. Large variability in the final size of individual microcolonies arising from clonal cells was observed. However, when growth was averaged over 16 locations in a specimen, the SEM was small and notable differences could be observed between the investigated strains, where mutants with lower expression of GuaB had a slower growth rate. The results show that the slow-growing mutants exhibited a lower fraction of dead cells, which indicate that slow-growing mutants are slightly more robust than the faster-growing strains. The large variability in the final size of individual microcolonies arising from clonal cells was quite surprising. We suggest that the control of the size of a microcolony is, at least partially, related to the actual microcolony depended on phenotypic heterogeneity. These findings are important to consider whenever a solid medium with discrete microcolonies is investigated.

The importance of rhamnolipid-biosurfactant-induced changes in bacterial membrane lipids of Bacillus subtilis for the antimicrobial activity of thiosulfonates

The antimicrobial properties of methyl (MTS) and ethyl (ETS) esters of thiosulfonic acid alone and in combination with rhamnolipid-biosurfactant (RL) have been characterized for their ability to disrupt the normal physiological functions of living pathogens. Bactericidal and fungicidal activities of MTS and ETS and their combination with rhamnolipid were demonstrated on strains of Pseudomonas aeruginosa, Bacillus subtilis, Alcaligenes faecalis, and Rhizopus ngtricans. It was found that the combination of rhamnolipid and thiosulfonic esters has a synergistic effect leading to decreasing of bactericidal and fungicidal concentrations of MTS and ETS. More extensively was studied the effect of rhamnolipid on the lipid composition of B. subtilis bacterial membrane. To our knowledge, in this article is reported for the first time a remarkable increase of negatively charged phospholipid cardiolipin in the presence of rhamnolipid. The capacity of RL as a surface-active substance was confirmed by scanning electron microscopy (SEM). The occurrence of surface infolds and blebs on B. subtilis shown by SEM, was not accompanied by changes in membrane permeability tested by a live/dead viability staining for fluorescence microscopy. When RL was applied in combination with MTS, a dramatic permeability shift for propidium iodide was observed in vegetative cells.

Influence of electric current on bacterial viability in wastewater treatment

Minimizing the influence of electric current on bacterial viability in the electro-technologies such as electrophoresis and electrocoagulation is crucial in designing and operating the electric hybrid wastewater treatment system. In this study the biomass from a membrane bioreactor (MBR) was subjected to constant direct current and the bacterial viability was monitored against electrical intensity, duration as well as the spatial vicinity related to the electrodes. It was found that the bacterial viability was not significantly affected (less than 10% of death percentage) when the applied electric current density (CD) was less than 6.2 A/m2 after 4 h. The percentage of live cell dropped by 15% and 29% at CD of 12.3 A/m2 and 24.7 A/m2, respectively. The pH of electrolytic biomass fluid has shifted to alkaline (from nearly neutral to around pH 10) at CD above 12.3 A/m2, which could have been the contributing factor for the bacterial inactivation. The temperature change in the electrolytic media at all current densities during 4 h of experiment was less than 2 °C, thus temperature effects were negligible. Bacteria experienced different micro-environments in the electrochemical reactor. Bacterial cells on the cathode surface exhibited highest death rate, whereas bacteria outside the space between electrodes were the least affected. It was concluded that in an electro-technology integrated wastewater treatment process, sufficient mixing should be used to avoid localized inactivation of bacterial cells.

Motility enhancement of bacteria actuated microstructures using selective bacteria adhesion

Microrobots developed by the technological advances are useful for application in various fields. Nevertheless, they have limitations with respect to their actuator and motility. Our experiments aim to determine whether a bioactuator using the flagellated bacteria Serratia marcescens would enhance the motility of microrobots. In this study, we investigate that the flagellated bacteria Serratia marcescens could be utilized as actuators for SU-8 microstructures by bovine serum albumin-selective patterning. Firstly, we analyze the adherence of the bacteria to the SU-8 micro cube by selective patterning using 5% BSA. The results show that number of attached-bacteria in the uncoated side of the selectively- coated micro cube with BSA increased by 200% compared with that in all sides of the non treated micro cube. Secondly, the selectively BSA coated micro cube had 210% higher motility than the uncoated micro cube. The results revealed that the bacteria patterned to a specific site using 5% BSA significantly increase the motility of the bacteria actuated microstructure.

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

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

The presence of pMRC01 promotes greater cell permeability and autolysis in lactococcal starter cultures

Conjugative transfer of plasmid-associated properties is routinely used to generate food-grade derivatives of lactococcal starter strains with improved technological traits. However, the introduction of one or more plasmids in a single strain is likely to impose a burden on regular cell metabolism and may affect the growth characteristics of the transconjugant culture. The aim of this study was to evaluate the impact of the 60.2-kb plasmid pMRC01 (encoding for an abortive infection bacteriophage resistance system and production of the anti-microbial, lacticin 3147) on starter performance. Five lactococcal strains (L. lactis HP, 255A, SK1, 712 and IL1403) and their pMRC01-containing derivatives were compared in terms of technological properties, including analysis of growth, acidification and autolysis rates. The transconjugants exhibited lower specific growth rates and higher generation times compared to the parental strains when grown at 30 degrees C in glucose-M17, but the presence of pMRC01 did not significantly affect the acidification capacity of strains in 11% reconstituted skimmed milk and synthetic media. Levels of lactate dehydrogenase were two-fold higher in supernatants of transconjugants than in those of parental strains, after 24 and 72 h of growth at 30 degrees C in glucose-M17, suggesting that the presence of pMRC01 somehow accelerates and promotes cellular autolysis. Analysis by flow cytometry following live/dead staining confirmed this result by showing larger populations of injured and dead cells in pMRC01-carrying cultures compared to the parental strains. The results of this study reveal that the plasmid pMRC01 places a burden on lactococcal host metabolism, which is associated with an increased cell permeability and autolysis, without significantly affecting the acidification capacity of the starter. While the magnitude of these effects appears to be strain dependent, the production of the bacteriocin lacticin 3147 may not be involved.