LIVE/DEAD BacLight : application of a new rapid staining method for direct enumeration of viable and total bacteria in drinking water

Antibacterial hydrogels of amino acid-based cationic amphiphiles

Development of biomaterials, which are inherently antibacterial having broad-spectrum activity against both Gram-positive and Gram-negative bacteria with considerable biocompatibility, is of tremendous importance in biomedicinal chemistry. Microbial infections are still of great concern, often originated from indwelling medical devices typically in hospitalized patients. To this end, hydrogelating soft materials particularly from low-molecular-weight (LMW) gelators have generated significant interest in preparing and modifying biomedicinal implants. Herein, we have developed L-tryptophan based cationic amphiphilic hydrogelators with varying degree of hydrophobicity that exhibited remarkable bactericidal activity against wide range of Gram-positive (MIC = 0.1-75 microg/mL) and Gram-negative bacteria (MIC = 0.5-5 microg/mL). Antimicrobial efficacy of the amphiphiles was greatly influenced by their alkyl chain length. This bactericidal effect of cationic hydrogelators is quite comparable or in some cases markedly better than that of clinically available antibiotics. Most excitingly, they selectively attack the bacterial pathogens while remain biocompatible to the mammalian cells. Thus, we have developed LMW biocompatible, inherently antibacterial hydrogels having potential applications in biomedicines.

Adhesion to and viability of Listeria monocytogenes on food contact surfaces

Listeria monocytogenes is an important pathogen responsible for major outbreaks associated with food products. Adhesion to surfaces leads to significant modifications in cell physiology. The aim of this work was to determine the adhesion ability of 10 isolates of L. monocytogenes to eight materials commonly used in kitchens and to evaluate the viability of the adhered cells. The materials assayed were stainless steel 304, marble, granite, glass, polypropylene from a bowl and from a cutting board, and two kinds of silestone. All L. monocytogenes strains attached to all surfaces, although to different extents. L. monocytogenes adhered most tightly to granite and marble, followed by stainless steel 304, glass, silestones, and finally polypropylene surfaces. Surfaces at the threshold between hydrophobicity and hydrophilicity, with high electron acceptor capability and a regular pattern of roughness, were more prone to attachment. Polypropylene surfaces displayed the highest percentage of viable bacteria (nearly 100%), whereas marble and granite had a lower percentage of cultivable cells, 69.5 and 78.7%, respectively. The lowest percentage of culturable bacteria was found on white silestone (18.5%). These results indicate that there are differences in adhered cell viability on different materials. Cell viability assays are important to better understand the cross-contamination process because only adhered bacteria that remain viable are responsible for postprocess contamination.

Comparison of uranium(VI) removal by Shewanella oneidensis MR-1 in flow and batch reactors

To better understand the interactions among metal contaminants, nutrients, and microorganisms in subsurface fracture-flow systems, biofilms of pure culture of Shewanella oneidensis MR-1 were grown in six fracture-flow reactors (FFRs) of different geometries. The spatial and temporal distribution of uranium and bacteria were examined using a tracer dye (brilliant blue FCF) and microscopy. The results showed that plugging by bacterial cells was dependent on the geometry of the reactor and that biofilms grown in FFRs had a limited U(VI)-reduction capacity. To quantify the U(VI)-reduction capacity of biofilms, batch experiments for U(VI) reduction were performed with repetitive U(VI) additions. U(VI)-reduction rates of stationary phase cultures decreased after each U(VI) addition. After the fourth U(VI) addition, stationary phase cultures treated with U(VI) with and without spent medium yielded gray and black precipitates, respectively. These gray and black U precipitates were analyzed using high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. Data for randomly selected areas of black precipitates showed that reduced U particles (3-6 nm) were crystalline, whereas gray precipitates were a mixture of crystalline and amorphous solids. Results obtained in this study, including a dramatic limitation of S. oneidensis MR-1 and its biofilms to reduce U(VI) and plugging of FFRs, suggest that alternative organisms should be targeted for stimulation for metal immobilization in subsurface fracture-flow systems.

Viability, diversity and composition of the bacterial community in a high Arctic permafrost soil from Spitsbergen, Northern Norway

The viable and non-viable fractions of the bacterial community in a 2347-year-old permafrost soil from Spitsbergen were subjected to a comprehensive investigation using culture-independent and culture-dependent methods. LIVE/DEAD BacLight staining revealed that 26% of the total number of bacterial cells were viable. Quantitatively, aerobic microcolonies, aerobic colony-forming units and culturable anaerobic bacteria comprised a minor fraction of the total number of viable bacteria, which underlines the necessity for alternative cultivation approaches in bacterial cryobiology. Sulfate reduction was detected at temperatures between -2 degrees C and 29 degrees C while methanogenesis was not detected. Bacterial diversity was high with 162 operational taxonomic units observed from 800 16S rDNA clone sequences. The 158 pure cultures isolated from the permafrost soil affiliated with 29 different bacterial genera, the majority of which have not previously been isolated from permafrost habitats. Most of the strains isolated were affiliated to the genera Cellulomonas and Arthrobacter and several of the pure cultures were closely related to bacteria reported from other cryohabitats. Characterization of viable bacterial communities in permafrost soils is important as it will enable identification of functionally important groups together with the as yet undescribed adaptations that bacteria have evolved for surviving subzero temperatures for millennia.

The evolution of contact-dependent inhibition in non-growing populations of Escherichia coli

In the course of liquid culture, serial passage experiments with Escherichia coli K-12 bearing a mutator gene deletion (DeltamutS) we observed the evolution of strains that appeared to kill or inhibit the growth of the bacteria from where they were derived, their ancestors. We demonstrate that this inhibition occurs after the cells stop growing and requires physical contact between the evolved and ancestral bacteria. Thereby, it is referred to as stationary phase contact-dependent inhibition (SCDI). The evolution of this antagonistic relationship is not anticipated from existing theory and experiments of competition in mass (liquid) culture. Nevertheless, it occurred in the same way (parallel evolution) in the eight independent serial transfer cultures, through different single base substitutions in a gene in the glycogen synthesis pathway, glgC. We demonstrate that the observed mutations in glgC, which codes for ADP-glucose pyrophosphorylase, are responsible for both the ability of the evolved bacteria to inhibit or kill their ancestors and their immunity to that inhibition or killing. We present evidence that without additional evolution, mutator genes, or known mutations in glgC, other strains of E. coli K-12 are also capable of SCDI or sensitive to this inhibition. We interpret this, in part, as support for the generality of SCDI and also as suggesting that the glgC mutations responsible for the SCDI, which evolved in our experiments, may suppress the action of one or more genes responsible for the sensitivity of E. coli to SCDI. Using numerical solutions to a mathematical model and in vitro experiments, we explore the population dynamics of SCDI and postulate the conditions responsible for its evolution in mass culture. We conclude with a brief discussion of the potential ecological significance of SCDI and its possible utility for the development of antimicrobial agents, which unlike existing antibiotics, can kill or inhibit the growth of bacteria that are not growing.

Numeration methods for targeting photoactive materials in the UV-A photocatalytic removal of microorganisms

This tutorial review reports on the different numeration methods for evaluating the efficiency of the photocatalytic action on microorganisms. Here we put forward the advantages and drawbacks of the standard methods such as the plate count, the fluorescence techniques and the Most Probable Number method for determining the biocidal photocatalytic activity and thus selecting efficient photocatalytic materials among complex systems. We highlight that bacterial spores are a representative and suitable tool for meeting the restrictive requirements resulting from the complex use of living matter instead of chemical targets.

Survival of gastric and enterohepatic Helicobacter spp. in water: implications for transmission

Part of the reason for rejecting aquatic environments as possible vectors for the transmission of Helicobacter pylori has been the preference of this microorganism to inhabit the human stomach and hence use a direct oral-oral route for transmission. On the other hand, most enteric bacterial pathogens are well known for being able to use water as an environmental reservoir. In this work, we have exposed 13 strains of seven different Helicobacter spp. (both gastric and enterohepatic) to water and tracked their survival by standard plating methods and membrane integrity assessment. The influence of different plating media and temperatures and the presence of light on recovery was also assessed. There was good correlation between cultivability and membrane integrity results (Pearson's correlation coefficient = 0.916), confirming that the culture method could reliably estimate differences in survival among different Helicobacter spp. The species that survived the longest in water was H. pylori (>96 h in the dark at 25 degrees C), whereas H. felis appeared to be the most sensitive to water (<6 h). A hierarchical cluster analysis demonstrated that there was no relationship between the enterohepatic nature of Helicobacter spp. and an increased time of survival in water. This work assesses for the first time the survival of multiple Helicobacter spp., such has H. mustelae, H. muridarum, H. felis, H. canadensis, H. pullorum, and H. canis, in water under several conditions and concludes that the roles of water in transmission between hosts are likely to be similar for all these species, whether enterohepatic or not.

Evaluating the flow-cytometric nucleic acid double-staining protocol in realistic situations of planktonic bacterial death

Since heterotrophic prokaryotes play an important biogeochemical role in aquatic ecosystems and have a high capacity to survive in extreme environments, easy-to-perform protocols that probe their physiological states and the effects of environmental variables on those states are highly desired. Some methodologies combine a general nucleic acid stain with a membrane integrity probe. We calibrated one of these, the nucleic acid double-staining (NADS) protocol (G. Grégori, S. Citterio, A. Ghiani, M. Labra, S. Sgorbati, S. Brown, and M. Denis, Appl. Environ. Microbiol. 67:4662-4670, 2001), determining the optimal stain concentrations in seawater and the response to conditions that generate prokaryote death (such as heat) and to conditions that are known to produce death in plankton, such as nutrient limitation or flagellate grazing. The protocol was validated by comparison to two methods used to detect viability: active respiration by 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) and incorporation of tritiated leucine. We show that concentrations in the range of 5 to 20 microg ml(-1) of propidium iodide, simultaneous to a 10x concentration of Sybr green I, are best for detecting two separated populations of "live" (green cells) and "dead" (red cells) organisms. During exposure to heat and UVC, we observed that the number of live cells declined concurrently with that of actively respiring cells (CTC positive) and with total leucine incorporation. In seawater mesocosms, the NADS protocol allowed detection of bacterioplankton starvation-related death and flagellate predation. The protocol was also tested in deep profiles in the northwest Atlantic, demonstrating its potential for routine characterization of this fraction of the physiological diversity of marine heterotrophic prokaryotic plankton.

Detection of viable but nonculturable Escherichia coli O157:H7 bacteria in drinking water and river water

A sensitive method for specific detection of viable Escherichia coli O157:H7 cells, including viable but nonculturable (VBNC) cells, in water samples was developed. This method involved capture of the bacterial cells on a low-protein-binding membrane and direct extraction and purification of RNA followed by reverse transcription-PCR and electronic microarray detection of the rfbE and fliC genes of E. coli O157:H7. It detected as few as 1 CFU of E. coli O157:H7 in diluted cultures, 3 to 4 CFU/liter in tap water, 7 CFU/liter in river water, and 50 VBNC cells in 1 liter of river water, demonstrating the best limit of detection reported to date for VBNC cells in environmental water samples.

Respiration and Carbon Balance of the Bacterium Pseudomonas sp., a Protozoan Tetrahymena thermophila, and a Fungus Trichoderma viride in a Food Chain System with Glass Beads

The respiration rates per unit of microbial biomass (qCO(2)) of live cells were evaluated in inorganic medium using glass beads to which glucose was added at a rate of 50 μg C vial(-1) day(-1). The qCO(2) of live cells (g CO(2)-C (g biomass-C)(-1) day(-1)) was 0.23 in a pure culture of bacteria, 0.67 in a mixed culture of bacteria and protozoa, 0.16 in a mixed culture of bacteria and fungi, and 0.20 in a mixed culture of bacteria, protozoa and fungi. A culture system containing glass beads is a useful tool to estimate qCO(2) of live cells.

Flow-cytometric total bacterial cell counts as a descriptive microbiological parameter for drinking water treatment processes

There are significantly more microbial cells in drinking water than what can be cultured on synthetic growth media. Nonetheless, cultivation-based heterotrophic plate counts (HPCs) are used worldwide as a general microbial quality parameter in drinking water treatment and distribution. Total bacterial cell concentrations are normally not considered during drinking water treatment as a design, operative or legislative parameters. This is mainly because easy and rapid methods for quantification of total bacterial cell concentrations have, up to now, not been available. As a consequence, the existing lack of data does not allow demonstrating the practical value of this parameter. In this study, we have used fluorescence staining of microbial cells with the nucleic acid stain SYBR((R)) Green I together with quantitative flow cytometry (FCM) to analyse total cell concentrations in water samples from a drinking water pilot plant. The plant treats surface water (Lake Zürich) through sequential ozonation, granular active carbon (GAC) filtration and membrane ultrafiltration (UF). The data were compared with adenosine tri-phosphate (ATP) measurements and conventional HPCs performed on the same water samples. We demonstrated that the impact of all three major treatment steps on the microbiology in the system could accurately be described with total cell counting: (1) ozonation caused chemical destruction of the bacterial cells; (2) GAC filtration facilitated significant regrowth of the microbial community; and (3) membrane UF physically removed the bacterial cells from the water. FCM typically detected 1-2 log units more than HPC, while ATP measurements were prone to interference from extracellular ATP released during the ozonation step in the treatment train. We have shown that total cell concentration measured with FCM is a rapid, easy, sensitive and importantly, a descriptive parameter of several widely applied drinking water treatment processes.

Bactericidal effects of bioactive glasses on clinically important aerobic bacteria

Bioactive glasses (BAGs) have been studied for decades for clinical use, and they have found many dental and orthopedic applications. BAGs have also been shown to have an antibacterial effect e.g., on some oral microorganisms. In this extensive work we show that six powdered BAGs and two sol-gel derived materials have a clear antibacterial effect on 29 clinically important bacterial species. We also incorporated a rapid and accurate flow cytometric (FCM) method to calculate and standardize the numbers of viable bacteria inoculated in the suspensions used in the tests for antibacterial activity. In all materials tested growth inhibition could be demonstrated, although the concentration and time needed for the effect varied depending on the BAG. The most effective glass was S53P4, which had a clear growth-inhibitory effect on all pathogens tested. The sol-gel derived materials CaPSiO and CaPSiO II also showed a strong antibacterial effect. In summary, BAGs were found to clearly inhibit the growth of a wide selection of bacterial species causing e.g., infections on the surfaces of prostheses in the body after implantation.

Effects of chlorination and heat disinfection on long-term starved Legionella pneumophila in warm water

AIMS

To characterize the efficacy of widely accepted heat and chlorination on culturable and non-culturable Legionella pneumophila in starved and warm water.

METHODS AND RESULTS

For L. pneumophila starved for 1 day (S1), heating at 60 degrees C or more for 30 min or chlorination at 0.5-20 mg l(-1) for 60 min, a loss of 6-8 log culturability was observed, whereas only 17-47% of cells had membrane damage. Non-culturability was also observed after heating or chlorinating the cells starved for 14 days (S14). The effect of heating on membrane deterioration was reduced for S14 cells while the chlorination effect remained. Legionella pneumophila entered a non-culturable phase after being starved for 33-40 days. The disinfection effects of both heating and chlorination on non-culturable N4 and N35 cells (which were collected on the fourth and the 35th days of the non-culturability phase respectively) decreased, indicating the development of disinfection resistance among non-culturable cells that had been subjected to starvation for 1-2 months.

CONCLUSIONS

Heating and chlorination significantly reduce the culturability of starved L. pneumophila, and damage cell membrane to a much less extent.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows the ability of long-term starved L. pneumophila to resist against disinfection treatments, which has implications in terms of public health.

Activity and mechanism of the action of zeylasterone against Bacillus subtilis

AIMS

To investigate the antimicrobial properties of 6-oxophenolic triterpenoids isolated from Maytenus blepharodes against different micro-organisms and the mode of action on Bacillus subtilis.

METHODS AND RESULTS

The activity of zeylasterone and demethylzeylasterone was evaluated by microdilution method. Zeylasterone showed a higher activity, being active against Gram-positive bacteria (minimum inhibitory concentration 3-20 microg ml(-1)) and Candida albicans (10 microg ml(-1)). Killing curves revealed a bacteriostatic effect on B. subtilis that was dependent on the growth phase and inoculum size. Zeylasterone caused cell membrane alterations in B. subtilis, as shown by potassium leakage and formation of mesosome-like structures. However, membrane disruption was not revealed by either LIVE/DEAD Baclight assay or measurement of intracellular constituent efflux. Zeylasterone showed an early effect on N-acetyl-glucosamine and uridine incorporation and later on that of thymidine and leucine.

CONCLUSIONS

Diverse micro-organisms exhibit sensitivities towards compounds studied. The permeability changes in the cytoplasmic membrane and nonsimultaneous ceasing of macromolecular synthesis suggest that zeylasterone could act on multiple targets on B. subtilis.

SIGNIFICANCE AND IMPACT OF THE STUDY

The activity showed against B. subtilis as a model of spore-forming bacteria would provide valuable information for further studies in the development of 6-oxophenolic triterpenoids as antiseptic and disinfectant properties.

Inherent antibacterial activity of a peptide-based beta-hairpin hydrogel

Among several important considerations for implantation of a biomaterial, a main concern is the introduction of infection. We have designed a hydrogel scaffold from the self-assembling peptide, MAX1, for tissue regeneration applications whose surface exhibits inherent antibacterial activity. In experiments where MAX1 gels are challenged with bacterial solutions ranging in concentrations from 2 x 10(3) colony forming units (CFUs)/dm2 to 2 x 10(9) CFUs/dm2, gel surfaces exhibit broad-spectrum antibacterial activity. Results show that the hydrogel surface is active against Gram-positive (Staphylococcus epidermidis, Staphylococcus aureus, and Streptococcus pyogenes) and Gram-negative (Klebsiella pneumoniae and Escherichia coli) bacteria, all prevalent in hospital settings. Live-dead assays employing laser scanning confocal microscopy show that bacteria are killed when they engage the surface. In addition, the surface of MAX1 hydrogels was shown to cause inner and outer membrane disruption in experiments that monitor the release of beta-galactosidase from the cytoplasm of lactose permease-deficient E. coli ML-35. These data suggest a mechanism of antibacterial action that involves membrane disruption that leads to cell death upon cellular contact with the gel surface. Although the hydrogel surface exhibits bactericidal activity, co-culture experiments indicate hydrogel surfaces show selective toxicity to bacterial versus mammalian cells. Additionally, gel surfaces are nonhemolytic toward human erythrocytes, which maintain healthy morphologies when in contact with the surface. These material attributes make MAX1 gels attractive candidates for use in tissue regeneration, even in nonsterile environments.

Comparison of multiple methods for quantification of microbial biofilms grown in microtiter plates

In the present study six assays for the quantification of biofilms formed in 96-well microtiter plates were optimised and evaluated: the crystal violet (CV) assay, the Syto9 assay, the fluorescein diacetate (FDA) assay, the resazurin assay, the XTT assay and the dimethyl methylene blue (DMMB) assay. Pseudomonas aeruginosa, Burkholderia cenocepacia, Staphylococcus aureus, Propionibacterium acnes and Candida albicans were used as test organisms. In general, these assays showed a broad applicability and a high repeatability for most isolates. In addition, the estimated numbers of CFUs present in the biofilms show limited variations between the different assays. Nevertheless, our data show that some assays are less suitable for the quantification of biofilms of particular isolates (e.g. the CV assay for P. aeruginosa).

Analysis of the survival of H. pylori within a laboratory-based aquatic model system using molecular and classical techniques

Despite the significance of Helicobacter pylori infection for man, its transmission is not clearly known. The human stomach is considered the reservoir of this pathogen, and one of the accepted routes is fecal-oral, in which water acts as a vector. However, although H. pylori epidemiology associates its transmission with water, only molecular and not cultural analysis detects the bacteria in water. This study was carried out to understand these data through studying the survival of H. pylori in a laboratory water model using cultural, morphological, and molecular methods. A mineral water system spiked with H. pylori and stored at 7 +/- 1 degrees C in the dark was analyzed by different methods over a period of 3 weeks. The total number of cells observed by DAPI staining and their DNA content remained constant over this study period. In contrast, cells could no longer be cultured after 5 days. Cell viability, which was determined via the LIVE/DEAD BacLight kit, decreased up to day 14, and at day 21 all cell membranes were damaged. In addition, a gradual conversion from spiral to coccal morphology occurred from day 3 onward. However, polymerase chain reaction (PCR) technique detected H. pylori DNA at day 21 and 3 months later. A study of the cell morphology of a young colony demonstrated the coexistence of bacilli and cocci. The results of this study show that H. pylori survives in water but loses its culturability and bacillar morphology rapidly, although it remains viable for longer periods and its DNA is still detectable much later. Thus, interpreting H. pylori's behavior in water differs according to the type of analysis. Consequently, we suggest that the presence of H. pylori infective cells is overestimated by PCR, whereas, in contrast, culture techniques underestimate it. Nevertheless, H. pylori should be considered a waterborne pathogen during its viable period, independently of its shape and culturability, as its presence in water may be risky for human health.

Conjugal transfer of plasmid R6K gamma ori minireplicon derivatives from Escherichia coli to various genera of pathogenic bacteria

Three R6K-derived gamma ori minireplicons were successfully transferred by conjugation from Escherichia coli to several species of pathogenic bacteria. The pFL129 replicon encodes the wild-type initiation replication protein pi, while plasmids pFL130 and pAG101 encode mutant forms of the pi protein conferring the plasmid copy-up phenotype. Plasmids could be transferred to all recipient species tested, although high efficiency conjugal transfer was only obtained with genera of the Enterobacteriaceae. The efficiency of plasmid transfer to all recipients was lower for the copy-up derivatives, pFL130 and pAG101, than for pFL129. The three gamma ori replicons were stably maintained in all transconjugants except pFL129 in Listeria monocytogenes. The two mutant plasmids retained their copy-up phenotype in the new bacterial hosts.

Viable but non-culturableListeria monocytogeneson parsley leaves and absence of recovery to a culturable state

AIMS

To investigate the presence of viable but non-culturable Listeria monocytogenes during survival on parsley leaves under low relative humidity (RH) and to evaluate the ability of L. monocytogenes to recover from VBNC to culturable state under satured humidity.

METHODS AND RESULTS

Under low RH (47-69%) on parsley leaves, the initial number of L. monocytogenes populations counted on non selective media (10(9) L. monocytogenes per leaf on TSA) was reduced by 6 log10 scales in 15 days, whereas number of viable L. monocytogenes counted under the microscope was reduced by 3-4 log10 scales, indicating the presence of VBNC cells. This was demonstrated on three L. monocytogenes strains (EGDe, Bug 1995 and LmP60). Changing from low to 100% RH permitted an increase of the culturable counts of L. monocytogenes and this growth was observed only when residual culturable cells were present. Moreover, VBNC L. monocytogenes inoculated on parsley leaves did not become culturable after incubation under 100% RH.

CONCLUSIONS

Dry conditions induced VBNC L. monocytogenes on parsley leaves but these VBNC were likely unable to recover culturability after transfer to satured humidity.

SIGNIFICANCE AND IMPACT OF STUDY

Enumeration on culture media presumably under-estimates the number of viable L. monocytogenes on fresh produce after exposure to low RH.

Laser-induced fluorescence-capillary electrophoresis and fluorescence microplate reader measurement: two methods to quantify the effect of antibiotics

Putative antibiotic drugs have to be classified according to their bactericidal potential. Two new methods by means of fluorescence spectroscopy (FS) using a fluorescence microplate reader (FMR) and laser-induced fluorescence capillary electrophoresis (LIF-CE), respectively, were developed for the assessment of the bactericidal efficiency using the LIVE/DEAD BacLight viability kit composed of the two fluorescent nucleic acid stains, SYTO9 (stains all cells green) and propidium iodide (stains cells with damaged membrane red). By correlation of the ratio of green and red fluorescence with the percentage of live cells by employing FS and LIF-CE, respectively, dose response curves of drug-treated Pseudomonas spp. and Streptococcus spp. samples were calculated, which allowed for the assessment of ED50 values. Both methods led to similar results which were in agreement with the minimum inhibitory concentrations (MICs) obtained by conventional broth microdilution. The application of the BacLight viability kit on drug-treated bacteria cultures presents a rapid method of assessing the antibiotic potency which is of great importance for high throughput screening in the development of new antibiotics. Additionally, the new LIF-CE method, which based on the use of a second unlabeled bacteria injection as a stacking front, allowed drawing conclusions from the electrophoretic profile about the constitution of the bacterial population. Thus, the tendency of bacterial chain formation and alterations in the live/dead ratio of the bacterial composition can be directly observed in the presence of different antibiotics.

Mechanism of cell surface expression of the Streptococcus mitis platelet binding proteins PblA and PblB

PblA and PblB are prophage-encoded proteins of Streptococcus mitis strain SF100 that mediate binding to human platelets. The mechanism for surface expression of these proteins has been unknown, as they do not contain signal sequences or cell wall sorting motifs. We therefore assessed whether expression of these proteins was linked the lytic cycle of the prophage. Deletion of either the holin or lysin gene resulted in retention of PblA and PblB in the cytoplasm, and loss of these proteins from the cell wall. Flow cytometric analysis revealed that induction of phage replication in SF100 produced a subpopulation of cells with increased permeability. This effect was abrogated by disruption of the holin and lysin genes. Treatment of these mutants with exogenous PblA and PblB restored surface expression, apparently via binding of the proteins to cell wall choline. Loss of PblA and PblB expression was associated with decreased platelet binding in vitro, and reduced virulence in an animal model of endocarditis. Thus, expression of PblA and PblB occurs via a novel mechanism, whereby phage induction increases bacterial permeability and release of the proteins, followed by their binding to surface of viable cells. This mechanism may be important for endovascular infection.

Effects of surfactant protein D on growth, adhesion and epithelial invasion of intestinal Gram-negative bacteria

Surfactant protein D (SP-D) interacts with various different microorganisms and plays an important role in pulmonary innate immunity. SP-D expression has also been detected in extrapulmonary tissues, including the gastro-intestinal tract. However, its function in the intestine is unknown and may differ considerably from SP-D functions in the lung. Therefore, the effects of porcine SP-D (pSP-D) on several strains of intestinal bacteria were studied by means of bacterial growth assays, colony-count assays, radial diffusion assays and differential fluorescent staining. Furthermore, the effect of pSP-D on the adhesion- and invasion-characteristics was investigated. All bacterial strains tested in this study were aggregated by pSP-D, but only Escherichia coli K12 was susceptible to pSP-D-mediated growth inhibition. Bacterial membrane integrity of E. coli K12 was affected by pSP-D, but this did not lead to a reduced bacterial viability. Therefore, it is unlikely that pSP-D has a direct antimicrobial effect, and the observed effects are most likely due to pSP-D-mediated bacterial aggregation. The effects of pSP-D on bacterial adhesion and invasion were studied with the porcine intestinal epithelial cell line IPI-2I. Preincubation with pSP-D results in a several-fold increase in adhesion (E. coli and Salmonella) and invasion (Salmonella), but did not affect the IL-8 production induced by the bacteria. Results obtained in this study suggest that pSP-D promotes uptake of pathogenic bacteria by epithelial cells. This may reflect a scavenger function for pSP-D in the intestine, which enables the host to generate a more rapid response to infectious bacteria.

Highly chlorinatedEscherichia colicannot be stained by propidium iodide

Several studies have shown that the staining by fluorochromes (DAPI, SYBR Green II, and TOTO-1) of bacteria is altered by chlorination. To evaluate the effect of chlorine (bleach solution) on propidium iodide (PI) staining, we studied Escherichia coli in suspension and biomolecules in solution (DNA, RNA, BSA, palmitic acid, and dextran) first subjected to chlorine and then neutralized by sodium thiosulphate. The suspensions and solutions were subsequently stained with PI. The fluorescence intensity of the PI-stained DNA and RNA in solution dramatically decreased with an increase in the chlorine concentration applied. These results explain the fact that for chlorine concentrations higher than 3 μmol/L Cl2, the E. coli cells were too damaged to be properly stained by PI. In the case of highly chlorinated bacteria, it was impossible to distinguish healthy cells (with a PI-impermeable membrane and undamaged nucleic acids), which were nonfluorescent after PI staining, from cells severely injured by chlorine (with a PI-permeable membrane and damaged nucleic acids) that were also nonfluorescent, as PI penetrated but did not stain chlorinated nucleic acids. Our results suggest that it would be prudent to be cautious in interpreting the results of PI staining, as PI false-negative cells (cells with compromised membranes but not stained by PI because of nucleic acid damage caused by chlorine) are obtained as a result of nucleic acid damage, leading to an underestimation of truly dead bacteria.

Assessment of toxicity of a glyphosate-based formulation using bacterial systems in lake water

A new Aeromonas bioassay is described to assess the potential harmful effects of the glyphosate-based herbicide, Roundup, in the Albufera lake, a protected area near Valencia. Viability markers as membrane integrity, culturability and beta-galactosidase production of Aeromonas caviae were studied to determine the influence of the herbicide in the bacterial cells. Data from the multifactor analysis of variance test showed no significant differences (P>0.05) between A. caviae counts of viability markers at the studied concentrations (0, 50 and 100 mg l-1 of glyphosate). The effects of Roundup on microbial biota present in the lake were assessed by measuring the number of indigenous mesophilic Aeromonas in presence of different amounts of the herbicide at 0, 50 and 100 mg l-1 of glyphosate. In samples containing 50 and 100 mg l-1 of glyphosate a significant (P<0.05) increase in Aeromonas spp. counts and accompanying flora was observed. The acute toxicity of Roundup and of Roundup diluted with Albufera lake water to Microtox luminescent bacterium (Vibrio fischeri) also was determined. The EC50 values obtained were 36.4 mg l-1 and 64.0 mgl-1 of glyphosate respectively. The acidity (pH 4.5) of the herbicide formulation was the responsible of the observed toxicity.

Adhesion of water stressed Helicobacter pylori to abiotic surfaces

AIM

The main aim of this work was to study and compare the adhesion of water exposed Helicobacter pylori to six different substrata and correlate any changes in morphology, physiology, ability to form aggregates and cultivability when in the planktonic or in the sessile phase.

METHODS AND RESULTS

The number of total cells adhered for different water exposure times and modifications in the cell shape were evaluated using epifluorescence and scanning electron microscopy, and physiology assessed using Syto9 and propidium iodide (PI) cellular uptake. All abiotic surfaces were rapidly colonized by H. pylori, and colonization appeared to reach a steady state after 96 h with levels ranging from 2.3 x 10(6) to 3.6 x 10(6) total cells cm(-2). Cell morphology was largely dependent on the support material, with spiral bacteria, associated with the infectious form of H. pylori, subsisting in a higher percentage on nonpolymeric substrata. Also, sessile bacteria were generally able to retain the spiral shape for longer when compared with planktonic bacteria, which became coccoid more quickly. The formation of large aggregates, which may act as a protection mechanism against the negative impact of the stressful external environmental conditions, was mostly observed on the surface of copper coupons. However, Syto9 and PI staining indicates that most of H. pylori attached to copper or SS304 have a compromised cell membrane after only 48 h. Cultivability methods were only able to detect the bacteria up to the 2 h exposure-time and at very low levels (up to 500 CFU cm(-2)).

CONCLUSIONS

The fact that the pathogen is able to adhere, retain the spiral morphology for longer and form large aggregates when attached to different plumbing materials appeared to point to pipe materials in general, and copper plumbing in particular, as a possible reservoir of virulent H. pylori in water distribution systems. However, the Syto9/PI staining results and cultivability methods indicate that the attached H. pylori cells quickly enter in a nonviable physiological state.

SIGNIFICANCE AND IMPACT OF THE STUDY

This represents the first study of H. pylori behaviour in water-exposed abiotic surfaces. It suggests that co-aggregation with the autochthonous heterotrophic consortia present in water is necessary for a longer survival of the pathogen in biofilms associated to drinking water systems.

Assessment and interpretation of bacterial viability by using the LIVE/DEAD BacLight Kit in combination with flow cytometry

The commercially available LIVE/DEAD BacLight kit is enjoying increased popularity among researchers in various fields of microbiology. Its use in combination with flow cytometry brought up new questions about how to interpret LIVE/DEAD staining results. Intermediate states, normally difficult to detect with epifluorescence microscopy, are a common phenomenon when the assay is used in flow cytometry and still lack rationale. It is shown here that the application of propidium iodide in combination with a green fluorescent total nucleic acid stain on UVA-irradiated cells of Escherichia coli, Salmonella enterica serovar Typhimurium, Shigella flexneri, and a community of freshwater bacteria resulted in a clear and distinctive flow cytometric staining pattern. In the gram-negative bacterium E. coli as well as in the two enteric pathogens, the pattern can be related to the presence of intermediate cellular states characterized by the degree of damage afflicted specifically on the bacterial outer membrane. This hypothesis is supported by the fact that EDTA-treated nonirradiated cells exhibit the same staining properties. On the contrary, this pattern was not observed in gram-positive Enterococcus faecalis, which lacks an outer membrane. Our observations add a new aspect to the LIVE/DEAD stain, which so far was believed to be dependent only on cytoplasmic membrane permeability.

Surfaces of fluorinated pyridinium block copolymers with enhanced antibacterial activity

Polystyrene-b-poly(4-vinylpyridine) copolymers were quaternized with 1-bromohexane and 6-perfluorooctyl-1-bromohexane. Surfaces prepared from these polymers were characterized by contact angle measurements, near-edge X-ray absorption fine structure spectroscopy and X-ray photoelectron spectroscopy. The fluorinated pyridinium surfaces showed enhanced antibacterial activity compared to their nonfluorinated counterparts. Even a polymer with a relatively low molecular weight pyridinium block showed high antimicrobial activity. The bactericidal effect was found to be related to the molecular composition and organization in the top 2-3 nm of the surface and increased with increasing hydrophilicity and pyridinium concentration of the surface.

Bacterial adhesion to and viability on positively charged polymer surfaces

Secondary and tertiary amino groups were introduced into polymer chains grafted onto a polyethylene flat-sheet membrane to evaluate the effects of surface properties on the adhesion and viability of a strain of the Gram-negative bacterium Escherichia coli and a strain of the Gram-positive bacterium Bacillus subtilis. The characterization of the surfaces containing amino groups, i.e. ethylamino (EA) and diethylamino (DEA) groups, revealed that the membrane potentials are proportional to amino-group densities and contact angle hysteresis. A high bacterial adhesion rate constant k was observed at high membrane potential, which indicates that membrane potential could be used as an indicator for estimating bacterial adhesion to the EA and DEA sheets, especially in B. subtilis. The bacterial adhesion rate constant of E. coli markedly increased at a membrane potential higher than −7.8 mV, whereas that of B. subtilis increased at a membrane potential higher than −8.3 mV, at which the dominant effect on bacterial adhesion is expected to change. The viability experiments revealed that approximately 80 % of E. coli cells adhering to the sheets with high membrane potential were inactivated after a contact time of 8 h, whereas 60 % of B. subtilis cells were inactivated. Furthermore, E. coli viability significantly decreased at a membrane potential higher than −8 mV, whereas B. subtilis viability decreased as membrane potential increased, which reflects differences in cell wall structure between E. coli and B. subtilis.

Effect of 2,4-D contamination on soil functional stability evaluated using the relative soil stability index (RSSI)

Soil functional stability is the capacity of soil functions to resist and recover from an environmental perturbation and can be used to evaluate soil health. It can be influenced by the presence of xenobiotics such as herbicides. The impact of a fresh 2,4-D contamination (36 mg kg(-1) dry soil) on soil functional stability was evaluated by comparing the capacity of soil enzyme activities to resist and recover from a heat perturbation for both a clean and 2,4-D-contaminated soil. The functional stabilities of the soils (uniform sands, pH 6.9, 7% (w/w) organic matter) were calculated using the relative soil stability index (RSSI). The RSSI scores indicate the proportion of potential enzyme activity the soil retains after a perturbation compared to the potential activity of an unperturbed soil. Six extra-cellular enzyme activities (acid and alkaline phosphatases, arylsulfatase, urease, protease and beta-glucosidase) were monitored in soil microcosms during a 15-day period. During this period, a 60 degrees C heat perturbation was applied to the soil for 24 h. The activities of arylsulfatase and protease were found to be the most stable following heat perturbation obtaining the highest RSSI scores (87% and 77%, respectively). Urease activity showed the lowest RSSI score (38%). Although all enzyme activities were inhibited by the presence of 2,4-D, the RSSI results indicated that contamination lowered the stability of only three enzyme activities (arylsulfatase, beta-glucosidase and urease). The RSSI adequately described resistance, recovery and recovery rate parameters and enabled differentiation between functional stabilities of clean and contaminated soil and between different soil types.

Measurement of the Growth of a Floc Forming Bacterium Pseudomonas putida CP1

Pseudomonas putida CP1 formed clumps of cells when grown on mono-chlorophenols but not on phenol or glucose. An increase in cell numbers for the organism grown on mono-chlorophenols was accompanied by a decrease in the dry weight. The change in shape of the bacterium from rod shape to coccus shape coupled with a reduction in cell size when the organism was grown under nutritional stress was found. This result together with cell aggregation affected the measurement of growth parameters in the system by conventional methods (optical density measurements, dry weight measurements and the plate count technique). Monitoring growth of Pseudomonas putida CP1 by a direct microscopic count technique was found to be more representative than conventional methods including optical density measurements, dry weight measurements and the plate count technique when grown on phenolics.

The effect of a biofilm on solute diffusion in fractured porous media

At sites in fractured rock where contamination has been exposed to the rock matrix for extended periods of time, the amount of contaminant mass residing in the matrix can be considerable. Even though it may be possible to diminish concentrations by the advection of clean water through the fracture features, back diffusion from mass held in the matrix will lead to a continuing source of contamination. In such an event, the development of a biofilm (a thin film of microbial mass) on the wall of the fractures may act to limit or prevent the back diffusion process. The objective of this preliminary study is to explore the influence imparted by the presence of a biofilm on the process of matrix diffusion. The investigation was conducted using radial diffusion cells constructed from rock core in which biofilm growth was stimulated in a central reservoir. Once biofilms were developed, forward diffusion experiments were conducted in which a conservative solute migrated from the central reservoir into the intact rock sample. Diffusion experiments were performed in a total of 11 diffusion cell pairs where biofilm growth was stimulated in one member of the pair and inhibited in the other. The effect of the presence of a biofilm on tracer diffusion was determined by comparison of the diffusion curves produced by each cell pair. A semi-analytical model that accounts for the presence of a biofilm was used to investigate the effect of the biofilm on mass transfer due to changes in the effective porosity, effective diffusion coefficient, and the depth of penetration of the biofilm into the intact rock. The results show that the biofilm acted to plug the rock matrix, rather than forming a discrete layer on the reservoir surface. The reduction in effective porosity due to the biofilm ranged from 6% to 52% with the majority of the samples in the 30% to 50% range. Based on the present results, with more efficient biofilm stimulation, it is reasonable to assume that a more complete plugging of the microcrack porosity might be possible, leaving a much thicker and efficient barrier than could be achieved via a surface biofilm.

Fluorescence microscopy for studying the viability of micro-organisms in natural whey starters

AIMS

The aim of this work was to study the viability and cultivability of microbial populations of different natural whey starters and to evaluate their resistance to thermal treatments (such as exposure to high or low temperatures).

METHODS AND RESULTS

Twenty-three natural whey starters for Grana Padano cheese were investigated and subsequently pH measurement, plate count agar using Man-Rogasa-Sharpe (MRS) pH 5.4 agar and whey agar medium (WAM) were performed using these samples. LIVE/DEAD BacLight bacterial viability kit was used. Total count and viability of all the 23 samples were high and similar to each other (CV 20%). However, the cultivable population was lower in terms of cfu ml(-1) and number of cells per millilitre than the viable fraction and highly variable, although its count value was higher in WAM than in MRS pH 5.4. The heating (60 degrees C for 5 min and 54 degrees C for 1 h) and freezing (-20 and -80 degrees C) treatments affected the cultivability and viability of the microbial population.

CONCLUSIONS

This study demonstrated the effectiveness of LIVE/DEAD BacLight bacterial viability kit, which has already been used to evaluate bacterial populations, in investigating microbial viability in a complex ecosystem such as a natural whey starter.

SIGNIFICANCE AND IMPACT OF THE STUDY

The aim of this study was to quantify the presence of damaged nonviable bacterial cells in natural whey starters. The Thoma Glass is a useful method to obtain fluorescence microscopy counts to evaluate the technological performance of natural whey starters.

Rapid test for distinguishing membrane-active antibacterial agents

In the search for antibacterial agents with a novel mode-of-action (MOA) many targeted cellular and cell-free assays are developed and used to screen chemical and natural product libraries. Frequently, hits identified by the primary screens include compounds with nonspecific activities that can affect the integrity and function of bacterial membrane. For a rapid dereplication of membrane-active compounds, a simple method was established using a commercially available Live/Dead(R) Bacterial Viability Kit. This method utilized two fluorescent nucleic acid stains, SYTO9 (stains all cells green) and propidium iodide (stains cells with damaged membrane red) for the drug-treated bacterial cells. The cells were then either examined visually by fluorescence microscopy or their fluorescence emissions were recorded using a multi-label plate reader set to measure emissions at two different wavelengths. The ratio of green versus red was compared to a standard curve indicating the percentage of live versus dead bacteria. Nine known antibiotics and 14 lead compounds from various antibacterial screens were tested with results consistent with their MOA.

Survival strategy of Erwinia amylovora against copper: induction of the viable-but-nonculturable state

Copper compounds, widely used to control plant-pathogenic bacteria, have traditionally been employed against fire blight, caused by Erwinia amylovora. However, recent studies have shown that some phytopathogenic bacteria enter into the viable-but-nonculturable (VBNC) state in the presence of copper. To determine whether copper kills E. amylovora or induces the VBNC state, a mineral medium without copper or supplemented with 0.005, 0.01, or 0.05 mM Cu(2+) was inoculated with 10(7) CFU/ml of this bacterium and monitored over 9 months. Total and viable cell counts were determined by epifluorescence microscopy using the LIVE/DEAD kit and by flow cytometry with 5-cyano-2,3-ditolyl tetrazolium chloride and SYTO 13. Culturable cells were counted on King's B nonselective solid medium. Changes in the bacterial morphology in the presence of copper were observed by scanning electron microscopy. E. amylovora entered into the VBNC state at all three copper concentrations assayed, much faster when the copper concentration increased. The addition of different agents which complex copper allowed the resuscitation (restoration of culturability) of copper-induced VBNC cells. Finally, copper-induced VBNC cells were virulent only for the first 5 days, while resuscitated cells always regained their pathogenicity on immature fruits over 9 months. These results have shown, for the first time, the induction of the VBNC state in E. amylovora as a survival strategy against copper.

New methodology for viability testing in environmental samples

Environmental samples can be complex and are comprised of microorganisms and a matrix of decaying organic matter as well as an inorganic phase such as sand or precipitated material (waste water, sludge, soils, etc.). Nucleic acid dyes have recently been developed to address the growing need for environmental analyses (cell staining, counting, viability testing and specific organism identification). However, certain dyes may not be ideally suited for testing of environmental samples, because they readily adhere to the substrate material as well as their target molecule, resulting in increased non-specific binding and background fluorescence. The aim of this study was to address the limitations of the widely used and commercially available Live/Dead BacLight Bacterial Viability kit (Molecular Probes, Eugene, OR). A new combination of nucleic acid dyes, i.e. SYTO13 and SYTOX Orange (Molecular Probes, Eugene, OR), was proposed as an alternative. The dyes were carefully chosen for their spectral separation and increase of fluorescence quantum yield. A protocol for this combination was first designed and optimized and the two staining assays were compared against suspensions of live and dead E. coli, mixed in different proportions and it was shown that both protocols performed equally on pure cultures. However, when testing activated sludge samples, the commercial kit showed greater background fluorescence and non-specific binding than the alternate combination. Therefore, the proposed dye combination and its corresponding protocol are deemed more suitable for use on complex environmental samples than the Live/Dead BacLight Bacterial Viability kit.

Effect of a bacteriocin-activated polythene film on Listeria monocytogenes as evaluated by viable staining and epifluorescence microscopy

AIMS

To evaluate the effect of a bacteriocin-activated polythene film on resting and growing populations of Listeria monocytogenes.

METHODS AND RESULTS

The active polythene films were industrially obtained by coating a solution of bacteriocin 32Y from Lactobacillus curvatus upon the surface of the film to be in contact with the packaged material. The behaviour of live Listeria populations was examined in liquid suspensions directly in contact with the bacteriocin-activated film, packed in antimicrobial film, and in a challenge test of storage of frankfurters superficially contaminated by L. monocytogenes and packed in antimicrobial film. In all the experiments, live and dead cells of L. monocytogenes were counted in epifluorescence microscopy after viable staining, which proved to be a suitable method to evaluate the action of bacteriocins on populations of L. monocytogenes. The results showed that the direct contact between active film surface and L. monocytogenes cells is effective for a fast and irreversible inactivation of the population by determining a direct cell disruption. This was confirmed by the results of the challenge test indicating that the antimicrobial package was effective in inhibiting the growth and survival of the pathogen on the surface of frankfurters during storage.

CONCLUSIONS

The use of the antimicrobial film is encouraged especially for solid food products where the superficial contaminants come immediately in contact with the antimicrobial film.

SIGNIFICANCE AND IMPACT OF THE STUDY

A fast inactivation of the bacterial population, coupled with appropriate conditions of storage, can improve the quality and safety and prolong the shelf-life of the food products packed in antimicrobial films.

A conceptual model of water's role as a reservoir in Helicobacter pylori transmission: a review of the evidence

Helicobacter pylori infection plays a role in the development of chronic gastritis, peptic ulcer and gastric cancer, yet the route of transmission into susceptible hosts remains unknown. Studies employing microbiological techniques have demonstrated that H. pylori has the ability to survive when introduced into water and that H. pylori is present in water and other environmental samples all over the world. Epidemiological studies have shown that water source and exposures related to water supply, including factors related to sewage disposal and exposure to animals, are risk factors for infection. This review describes the microbiological and epidemiological evidence for, and proposes a model of, waterborne H. pylori transmission outlining important features in the transmission cycle. In the model, humans and animals shed the bacteria in their faeces and the mechanisms for entry into water, and for survival, ingestion and infection are dependent upon a range of environmental influences. Verification of the proposed model pathways has important implications for public-health prevention strategies.

Use of the modified robbins device and fluorescent staining to screen plant extracts for the inhibition of S. mutans biofilm formation

Streptococcus mutans plays an important role in the formation of dental plaque. To study biofilm growth on hydroxyapatite (HA) in vitro, a flow system based on a Modified Robbins Device (MRD) and a method for the quantification of the biomass using fluorescent staining with SYTO(R) 9 were developed. The combined approach was used to assess the inhibitory effect of plant extracts on biofilm formation in concentrations below their minimal inhibitory concentrations.