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

Alternative food-preservation technologies: efficacy and mechanisms

High-pressure processing, ionizing radiation, pulsed electric field and ultraviolet radiation are emerging preservation technologies designed to produce safe food, while maintaining its nutritional and sensory qualities. A sigmoid inactivation pattern is observed in most kinetic studies. Damage to cell membranes, enzymes or DNA is the most commonly cited cause of death of microorganisms by alternative preservation technologies.

Confocal microscopy and microbial viability detection for food research

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

Copper-induced inhibition of growth of Desulfovibrio desulfuricans G20: assessment of its toxicity and correlation with those of zinc and lead

The toxicity of copper [Cu(II)] to sulfate-reducing bacteria (SRB) was studied by using Desulfovibrio desulfuricans G20 in a medium (MTM) developed specifically to test metal toxicity to SRB (R. K. Sani, G. Geesey, and B. M. Peyton, Adv. Environ. Res. 5:269-276, 2001). The effects of Cu(II) toxicity were observed in terms of inhibition in total cell protein, longer lag times, lower specific growth rates, and in some cases no measurable growth. At only 6 microM, Cu(II) reduced the maximum specific growth rate by 25% and the final cell protein concentration by 18% compared to the copper-free control. Inhibition by Cu(II) of cell yield and maximum specific growth rate increased with increasing concentrations. The Cu(II) concentration causing 50% inhibition in final cell protein was evaluated to be 16 microM. A Cu(II) concentration of 13.3 microM showed 50% inhibition in maximum specific growth rate. These results clearly show significant Cu(II) toxicity to SRB at concentrations that are 100 times lower than previously reported. No measurable growth was observed at 30 microM Cu(II) even after a prolonged incubation of 384 h. In contrast, Zn(II) and Pb(II), at 16 and 5 microM, increased lag times by 48 and 72 h, respectively, but yielded final cell protein concentrations equivalent to those of the zinc- and lead-free controls. Live/dead staining, based on membrane integrity, indicated that while Cu(II), Zn(II), and Pb(II) inhibited growth, these metals did not cause a loss of D. desulfuricans membrane integrity. The results show that D. desulfuricans in the presence of Cu(II) follows a growth pattern clearly different from the pattern followed in the presence of Zn(II) or Pb(II). It is therefore likely that Cu(II) toxicity proceeds by a mechanism different from that of Zn(II) or Pb(II) toxicity.

Fluorescent method for monitoring cheese starter permeabilization and lysis

A fluorescence method to monitor lysis of cheese starter bacteria using dual staining with the LIVE/DEAD BacLight bacterial viability kit is described. This kit combines membrane-permeant green fluorescent nucleic acid dye SYTO 9 and membrane-impermeant red fluorescent nucleic acid dye propidium iodide (PI), staining damaged membrane cells fluorescent red and intact cells fluorescent green. For evaluation of the fluorescence method, cells of Lactococcus lactis MG1363 were incubated under different conditions and subsequently labeled with SYTO 9 and PI and analyzed by flow cytometry and epifluorescence microscopy. Lysis was induced by treatment with cell wall-hydrolyzing enzyme mutanolysin. Cheese conditions were mimicked by incubating cells in a buffer with high protein, potassium, and magnesium, which stabilizes the cells. Under nonstabilizing conditions a high concentration of mutanolysin caused complete disruption of the cells. This resulted in a decrease in the total number of cells and release of cytoplasmic enzyme lactate dehydrogenase. In the stabilizing buffer, mutanolysin caused membrane damage as well but the cells disintegrated at a much lower rate. Stabilizing buffer supported permeabilized cells, as indicated by a high number of PI-labeled cells. In addition, permeable cells did not release intracellular aminopeptidase N, but increased enzyme activity was observed with the externally added and nonpermeable peptide substrate lysyl-p-nitroanilide. Finally, with these stains and confocal scanning laser microscopy the permeabilization of starter cells in cheese could be analyzed.

Assessment of Changes in Biodiversity when a Community of Ultramicrobacteria Isolated from Groundwater Is Stimulated to Form a Biofilm

The stimulation of groundwater bacteria to form biofilms, for the remediation of polluted aquifers, is subjected to environmental regulations that include measurement of effects on microbial biodiversity. Groundwater microorganisms contain a proportion of unidentified and uncharacterized ultramicrobacteria (UMB) that might play a major role in the bioclogging of geological materials. This study aimed to assess the changes in genetic and metabolic biodiversity when a community of UMB, isolated from groundwater, is stimulated to form biofilms on a ceramic surface. UMB were stimulated with aerobic conditions and injection of molasses, in reactors reproducing groundwater composition and temperature. Concentration of planktonic viable UMB, secretion of extracellular polymeric substances (EPS), and biofilm thickness were monitored. The assessment of changes in biodiversity was achieved by comparing the initial UMB community to the biofilm community, using the single strand conformational polymorphism (SSCP) method, the cloning and sequencing of 16S rRNA gene (16S rDNA) sequences, and the Biolog microplate system. The hypothesis stating that indigenous UMB would play a significant role of in the biofilm development was corroborated. Within 13 days of stimulation, the UMB produced 700 mg L?1 of planktonic EPS and formed a biofilm up to a thickness of 1100 mm. This stimulation led to a decrease in genetic diversity and an increase in metabolic diversity. The decrease in genetic diversity was shown by a reduced number of single strand DNA fragments in the SSCP profiles. As such, 16S rDNA sequences from the biofilm revealed the predominance of four bacterial groups: Zoogloea, Bacillus/Paenibacillus, Enterobacteriaceae, and Pseudomonads. A significant increase in metabolic diversity was shown by a highest substrate richness profile and a lower substrate evenness profile of the biofilm bacterial population (p = 0.0 and p = 0.09, respectively). This higher metabolic diversity might be a consequence of the stimulation that seemed to favor the growth of bacteria having a high nutritional versatility. Stimulation of UMB, isolated from groundwater, was effective to form a biofilm having a high metabolic biodiversity. This combination of molecular-based and metabolic-based methods expanded the insight into monitoring the changes in bacterial biodiversity.

Adequacy of in situ glass slides and direct sand extractions to assess the microbiota within sand columns used for drinking water treatment

Historically, Cholodny-Rossi buried glass slide techniques have been used to study the microbiota of subsurface environments, yet the bias of such a technique has not been compared against direct sand extraction using modern in situ probing. Over a period of 34 wk, four separate 4-m-deep sand columns receiving raw lake water were examined to compare direct extraction of sand filter biofilm material against in situ glass slide biofilms. Significantly different DAPI direct counts and fluorescent in situ hybridization signals for major phylogenetic groups were observed. Not only were lower proportions (P < 0.001) of EUB338-probed DAPI cells observed on in situ glass slides, but also fewer γ-Proteobacteria (12%–21%) and more α-Proteobacteria (16%–33%) when compared to direct sand extracts. Hence, investigators of the microbial ecology of even simple sand biofilms must consider the inherent biases from "accepted" methods and seek further independent methods to identify those which may be most accurate.Key words: sand filter, biofilms, in situ hybridization, groundwater recharge.

Clogging of a limestone fracture by stimulating groundwater microbes

Biological clogging is promoted in aquifers either to contain or to remediate groundwater. In this study, an apparatus able to detect small changes in hydraulic conductivity (K) was developed to measure the clogging of a single fracture in limestone, following microbial stimulation. The fracture had a 2.5 mm2 section and was 50 cm long. Prior to the inoculation of the limestone, the sequencing of representative clones from 16S rRNA genes isolated from groundwater, showed significant affiliation with Cytophaga spp., Arcobacter spp. and Rhizobium spp. These bacteria are known to secrete extracellular polymeric substances and form biofilms. When nutrients were added to the inoculated limestone, a decrease in K occurred after 8 days, reaching 0.8% of its initial value after 22 days (Kfi = 340 cm min-1). This study showed that a stimulation of indigenous microbes from groundwater effectively clogged a macrofracture in limestone, suggesting the potential application of biobarriers in fractured rock aquifers.

Improved methods for the enumeration of heterotrophic bacteria in bottled mineral waters

At this time the European Union regulations require that the heterotrophic plate counts (HPC) of mineral waters be assessed at two recovery temperatures: 22 degrees C for 72 h and 37 degrees C for 24 h. This procedure is time consuming and expensive. Development of new rapid methods for microbiological assessment of the microbial flora in the bottled water is an industry-driven need. The objectives of this work were to develop a method for the HPC that utilises only one recovery temperature and one incubation period and evaluate the use of, the LIVE/DEAD(R) BacLight Bacterial Viability Kit, 5-cyano-2,3-ditotyl tetrazolium chloride (CTC) and impedance methods to enumerate viable bacteria in bottled mineral water. Results showed that incubation at 30 degrees C could be used instead of incubation at 22 degrees C and 37 degrees C. Good correlation exists between counts at 30 degrees C and counts at 22 degrees C (r>0.90) and all the pathogens important in mineral water analyses grow similarly at 30 degrees C and 37 degrees C during 24 h. It was demonstrated that impedance methods might be useful to the mineral water industry as a rapid indicator of microbiological quality of the water. Results obtained with BacLight and CTC were similar to those obtained with plate counts.

Use of fluorescent probes to assess physiological functions of bacteria at single-cell level

A wide diversity of fluorescent probes is currently available to assess the physiological state of microorganisms. The recent development of techniques such as solid-phase cytometry, the increasing sensitivity of fluorescence tools and multiparametric approaches combining taxonomic and physiological probes have improved the effectiveness of direct methods in environmental and industrial microbiology.

Survival of Escherichia coli in freshwater: β-D-Glucuronidase activity measurements and characterization of cellular states

A rapid enzyme assay measuring β-D-glucuronidase activity of Escherichia coli was tested in survival experiments after discharge of E. coli in river water. Enzyme activity was compared with several analyses performed to characterize cellular states under stressful conditions. Enzyme activity remained stable under starvation and light stress conditions despite losses of culturability, respiratory activity, and cytoplasmic membrane integrity. β-D-Glucuronidase activity followed the pattern of genetic and morphologic cell integrity. The tested enzyme assay seems well adapted to study the fate of fecal coliforms in survival experiments, and appears to be a rapid and efficient way to estimate the microbiological quality of surface waters.Key words: β-D-glucuronidase activity, Escherichia coli, survival, freshwater, cellular states.

Bioaugmentation of activated sludge by an indigenous 3-chloroaniline-degrading Comamonas testosteroni strain, I2gfp

A strain identified as Comamonas testosteroni I2 was isolated from activated sludge and found to be able to mineralize 3-chloroaniline (3-CA). During the mineralization, a yellow intermediate accumulated temporarily, due to the distal meta-cleavage of chlorocatechol. This strain was tested for its ability to clean wastewater containing 3-CA upon inoculation into activated sludge. To monitor its survival, the strain was chromosomally marked with the gfp gene and designated I2gfp. After inoculation into a lab-scale semicontinuous activated-sludge (SCAS) system, the inoculated strain maintained itself in the sludge for at least 45 days and was present in the sludge flocs. After an initial adaptation period of 6 days, complete degradation of 3-CA was obtained during 2 weeks, while no degradation at all occurred in the noninoculated control reactor. Upon further operation of the SCAS system, only 50% 3-CA removal was observed. Denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes revealed a dynamic change in the microbial community structure of the activated sludge. The DGGE patterns of the noninoculated and the inoculated reactors evolved after 7 days to different clusters, which suggests an effect of strain inoculation on the microbial community structure. The results indicate that bioaugmentation, even with a strain originating from that ecosystem and able to effectively grow on a selective substrate, is not permanent and will probably require regular resupplementation.