Changes in viability and macromolecular content of long-term batch cultures of Salmonella typhimurium measured by flow cytometry

Removal of Microcystis aeruginosa by ultrasound: Inactivation mechanism and release of algal organic matter

The efficacy of ultrasonic irradiation for removal of Microcystis aeruginosa and release of algal organic matter (AOM) was investigated under different ultrasound conditions, including ultrasonic frequency, power density, and time. Laboratory results suggested that the ultrasonic efficiency and the release of AOM were influenced by frequency, power density, and time. The mechanism of AOM algae removal by ultrasound was systematically explored. The inactivation of algae resulted from mechanical and chemical effects caused by ultrasound. Mechanical destruction and free-radical oxidation considerably affected the structure and physiological function of algal cells. The SEM and TEM images indicated that ultrasound could damage the cell membrane, wall, and organelle. Flow cell cytometry results showed decreases in the size, internal granularity, integrity, and activity of algal cells, revealing that ultrasound exerted severe damage to the structure and function of algal cells. The activity of the antioxidant system of algal cells was then studied by investigating changes in MDA, SOD, and CAT concentration after ultrasound to confirm the inactivation of the cells. The release of AOM was explored by determining changes in water quality indices (UV254, DOC, and SUVA) at 10 min and 48 h after ultrasound. This study provides information about the safety of ultrasound usage on algae removal and references for ultrasonic parameters to be selected to ensure effective and safe algae removal.

Extreme slow growth as alternative strategy to survive deep starvation in bacteria

Bacteria can become dormant or form spores when they are starved for nutrients. Here, we find that non-sporulating Bacillus subtilis cells can survive deep starvation conditions for many months. During this period, cells adopt an almost coccoid shape and become tolerant to antibiotics. Unexpectedly, these cells appear to be metabolically active and show a transcriptome profile very different from that of stationary phase cells. We show that these starved cells are not dormant but are growing and dividing, albeit with a doubling time close to 4 days. Very low nutrient levels, comparable to 10,000-fold diluted lysogeny broth (LB), are sufficient to sustain this growth. This extreme slow growth, which we propose to call ‘oligotrophic growth state’, provides an alternative strategy for B. subtilis to endure nutrient depletion and environmental stresses. Further work is warranted to test whether this state can be found in other bacterial species to survive deep starvation conditions.

Bacteria can become dormant or form spores when starved for nutrients. Here, Gray et al. describe an alternative strategy, or ‘oligotrophic growth state’, showing that non-sporulating Bacillus subtilis cells can survive deep starvation conditions by adopting an almost coccoid shape and extremely low growth rates.

Bio-organic materials in the atmosphere and snow: measurement and characterization

Bio-organic chemicals are ubiquitous in the Earth's atmosphere and at air-snow interfaces, as well as in aerosols and in clouds. It has been known for centuries that airborne biological matter plays various roles in the transmission of disease in humans and in ecosystems. The implication of chemical compounds of biological origins in cloud condensation and in ice nucleation processes has also been studied during the last few decades, and implications have been suggested in the reduction of visibility, in the influence on oxidative potential of the atmosphere and transformation of compounds in the atmosphere, in the formation of haze, change of snow-ice albedo, in agricultural processes, and bio-hazards and bio-terrorism. In this review we critically examine existing observation data on bio-organic compounds in the atmosphere and in snow. We also review both conventional and cutting-edge analytical techniques and methods for measurement and characterisation of bio-organic compounds and specifically for microbial communities, in the atmosphere and snow. We also explore the link between biological compounds and nucleation processes. Due to increased interest in decreasing emissions of carbon-containing compounds, we also briefly review (in an Appendix) methods and techniques that are currently deployed for bio-organic remediation.

Nonmultiplying bacteria are profoundly tolerant to antibiotics

Bacteria survive treatments with antimicrobial agents; they achieve this in two ways. Firstly, bacteria quickly become tolerant to these agents. This tolerance is temporary, reversible, and associated with slowing of the multiplication rate. Secondly, bacteria can undergo genetic mutations leading to permanent clonal resistance to antimicrobial agents. In patients with infections, nonmultiplying bacteria, some of which may be viable but nonculturable, exist side by side with multiplying bacteria. Current antibiotics capable of killing actively multiplying bacteria have very limited or no effect against nonmultiplying bacteria. Treatment of such infections requires a regimen of multiple antimicrobial agents in order to control nonmultiplying persistent bacteria. This is especially important in tuberculosis where there is co-existence of slowly multiplying tolerant bacteria with fast growing sensitive organisms. For this reason, a prolonged length of chemotherapy, lasting 6 months, is necessary to achieve cure. This long duration of treatment is due to the slow, inadequate effect of antibiotics on nonmultiplying persistent bacteria. Similar problems with eradication of persistent bacteria are evident in the treatment of biofilms. These bacteria serve as a pool for recurrent infections. Extended courses of antibiotics increase the likelihood of genetic resistance, raise the cost of treatments, and lead to more side effects.

Discriminating multi-species populations in biofilms with peptide nucleic acid fluorescence in situ hybridization (PNA FISH)

BACKGROUND

Our current understanding of biofilms indicates that these structures are typically composed of many different microbial species. However, the lack of reliable techniques for the discrimination of each population has meant that studies focusing on multi-species biofilms are scarce and typically generate qualitative rather than quantitative data.

METHODOLOGY/PRINCIPAL FINDINGS

We employ peptide nucleic acid fluorescence in situ hybridization (PNA FISH) methods to quantify and visualize mixed biofilm populations. As a case study, we present the characterization of Salmonella enterica/Listeria monocytogenes/Escherichia coli single, dual and tri-species biofilms in seven different support materials. Ex-situ, we were able to monitor quantitatively the populations of ∼56 mixed species biofilms up to 48 h, regardless of the support material. In situ, a correct quantification remained more elusive, but a qualitative understanding of biofilm structure and composition is clearly possible by confocal laser scanning microscopy (CLSM) at least up to 192 h. Combining the data obtained from PNA FISH/CLSM with data from other established techniques and from calculated microbial parameters, we were able to develop a model for this tri-species biofilm. The higher growth rate and exopolymer production ability of E. coli probably led this microorganism to outcompete the other two [average cell numbers (cells/cm(2)) for 48 h biofilm: E. coli 2,1 × 10(8) (± 2,4 × 10(7)); L. monocytogenes 6,8 × 10(7) (± 9,4 × 10(6)); and S. enterica 1,4 × 10(6) (± 4,1 × 10(5))]. This overgrowth was confirmed by CSLM, with two well-defined layers being easily identified: the top one with E. coli, and the bottom one with mixed regions of L. monocytogenes and S. enterica.

SIGNIFICANCE

While PNA FISH has been described previously for the qualitative study of biofilm populations, the present investigation demonstrates that it can also be used for the accurate quantification and spatial distribution of species in polymicrobial communities. Thus, it facilitates the understanding of interspecies interactions and how these are affected by changes in the surrounding environment.

Gamma radiation used as hygienization technique for foods does not induce viable but non-culturable state (VBNC) in Salmonella enterica subsp. enterica serovar Typhimurium

Gamma radiation has been widely used for hygienization of food products. Whether gamma radiation stress induces VBNC state in Salmonella is of great concern. Therefore, the study was carried out to determine whether gamma radiation exposure induces VBNC state in Salmonella enterica subsp. enterica serovar Typhimurium (S. typhimurium). The parameters tested were culturability on agar medium, transcriptional activity by RT-PCR, cytoplasmic membrane integrity, and direct viable count using LIVE/DEAD BacLight bacterial viability kit. The LIVE/DEAD BacLight counts for S. typhimurium cells treated with 0.5 and 1.0 kGy radiation dose were 0.8 and 0.1% of the control, respectively. Plate counts for S. typhimurium cells treated with 0.5 and 1.0 kGy radiation dose were 0.7 and 0.05% of the control, respectively. No viable cells of S. typhimurium were detected by both plate count and LIVE/DEAD BacLight after radiation treatment with 2 kGy. No transcriptional activity was detected in cells treated with 2 kGy radiation dose. If there were VBNC cells present, then significant differences in the counts between the LIVE/DEAD BacLight microscopic counts and plate agar counts must be observed. No significant difference (P > 0.05) in the counts were observed. Thus, it can be concluded that treatment with 2 kGy results in complete killing and does not induce VBNC state in S. typhimurium.

Nucleic acids as viability markers for bacteria detection using molecular tools

A large set of nucleic acid detection methods with good sensitivity and specificity are now available for the detection of pathogens in clinical, food and environmental samples. Given increasing demand, many efforts have been made to combine these methods to assess viability. Genomic DNA PCR amplification has been shown to be inappropriate for distinguishing viable from dead bacteria owing to DNA stability. Many authors have tried to bypass this difficulty by switching to RNA amplification methods such as reverse transcription-PCR and nucleic acid sequence-based amplification. More recently, researchers have developed methods combining specific sample pretreatment with nucleic acid detection methods, notably ethidium or propidium monoazide pretreatment coupled with PCR DNA detection or direct viable count methods and subsequent fluorescent in situ hybridization of 16S rRNA. This review evaluates the performance of these different methods for viability assessment.

Flow cytometry and environmental microbiology

This survey unit discusses many of the issues involved for flow cytometry in the field of microbiology, particularly the preparative procedures, which are far more stringent than many other procedures using larger cells. For instance, it is often necessary to filter laboratory agents multiple times to obtain the true particle-free solutions needed for flow cytometry of microbes. It is difficult enough to recognize bacteria in cell extracts from soil, sediment, or sludge given the background of same-size particles. This unit provides an excellent overview of a potentially large application area in flow cytometry and is written by one of the most respected scientists in the field.

Antibacterial properties of dermaseptin S4 derivatives under extreme incubation conditions

Antibacterial properties of the frog-derived peptide dermaseptin S4 and a series of synthetic derivatives against the food pathogen Escherichia coli O157:H7 were investigated under extreme incubation conditions. The 28-mer analog K4K20S4 (P28) displayed an MIC of 8 microM and rapid bactericidal kinetics under standard culture conditions. Potent bactericidal properties were maintained at high salt concentrations, under acidic or basic conditions, and at extreme temperatures. The N-terminal 14-mer sequence (P14) displayed higher potency (MIC, 4 microM) but only within a narrow range of incubation conditions, pointing to the importance of the C-terminal domain of P28. The potency range was reextended upon conjugation of aminododecanoic acid to P14. The resulting lipopeptide was even more potent (MIC, 2 microM) and affected bacterial viability under most of the conditions tested, including in commercial apple juice. The mechanistic implications of peptides' hydrophobicity, charge, structure, and binding to an idealized membrane were probed and are discussed here. Collectively, the data indicate interest in simple peptide-based compounds for design of antimicrobials that affect pathogens under a variable range of incubation conditions.

Effect of trophic status on the culturability and activity of bacteria from a range of lakes in the English Lake District

The bacterioplankton from a number of lakes that differed in nutrient status in the English Lake District was examined with a number of techniques for enumeration and activity assessment. Natural water samples showed a clear correlation between total counts and trophic status. Esterase activity measurements with Chemchrome B were able to distinguish high- and low-nutrient-status lakes, whereas tetrazolium salt (5-cyano-2,3-ditoyltetrazolium chloride) reduction, the direct viable count-cell elongation assay, and culturability measurements could not. Tetrazolium salt reduction and esterase activity measurements labeled a significant number of cells from water of all nutrient levels, whereas the direct viable count-cell elongation method was of use only in oligotrophic waters. Size fractionation of samples showed that the culturable cells were retained by the larger filters, especially in nutrient-rich waters. Esterase activity measurements also favored the larger cells. The differences observed between assays using water that differed in trophic status raise questions about the use of these tests as a definitive measure of viability.

Stationary phase mutagenesis: mechanisms that accelerate adaptation of microbial populations under environmental stress

Microorganisms are exposed to constantly changing environmental conditions. In a growth-restricting environment (e.g. during starvation), mutants arise that are able to take over the population by a process known as stationary phase mutation. Genetic adaptation of a microbial population under environmental stress involves mechanisms that lead to an elevated mutation rate. Under stressful conditions, DNA synthesis may become more erroneous because of the induction of error-prone DNA polymerases, resulting in a situation in which DNA repair systems are unable to cope with increasing amounts of DNA lesions. Transposition may also increase genetic variation. One may ask whether the rate of mutation under stressful conditions is elevated as a result of malfunctioning of systems responsible for accuracy or are there specific mechanisms that regulate the rate of mutations under stress. Evidence for the presence of mutagenic pathways that have probably been evolved to control the mutation rate in a cell will be discussed.

Molecular methods for the assessment of bacterial viability

A significant number of pathogenic microorganisms can be found in environmental reservoirs (air, water, soil). It is important to assess the viability status of these organisms to determine whether they pose a threat to public health. Classical methods for determining viability are time consuming. Hence, molecular methods have been developed to address this problem. Molecular methods offer speed, sensitivity and specificity. Both DNA and RNA have been analysed using molecular amplification methods such as polymerase chain reaction (PCR), reverse transcriptase PCR (RT-PCR) and nucleic acid sequence-based amplification (NASBA). However, due to the variable persistence of nucleic acids in cells post-death, the correlation between presence of DNA and RNA and viability is not clear-cut. Similarly, the choice of target and sensitivity of the method can significantly affect the validity of the viability assay. This review assesses the molecular methods currently available and evaluates their ability to assess cell viability with emphasis on environmental pathogens.

Correlation of the rate of protein synthesis and the third power of the RNA : protein ratio in Escherichia coli and Mycobacterium tuberculosis

In order to further understand the different physiological states of the tubercle bacillus, a frame of reference was sought by first correlating the macromolecular compositions of Escherichia coli with specific growth rates and also with the rates of protein synthesis. Data for DNA : protein : RNA were converted to the average amounts of DNA [m(DNA(av))], protein [m(p(av))] and RNA [m(RNA(av))] per cell. The specific growth rate micro was found to be directly proportional to m(RNA(av))/m(p(av)). The specific protein synthesis rate per average cell [omega(p(av))] was shown to be directly proportional to the third power of the ratio m(RNA(av))/m(p(av)) which reflects the ribosome concentration. The equations derived were shown apply to both E. coli ( micro =1.73 h(-1)) and Mycobacterium bovis BCG ( micro =0.029 h(-1)).

Cold and carbon dioxide used as multi-hurdle preservation do not induce appearance of viable but non-culturable Listeria monocytogenes

AIMS

To study whether the exposure to cold (4 degrees C) and carbon dioxide which results in the elongation of Listeria cells, induces a viable but nonculturable (VBNC) state.

METHODS AND RESULTS

When cold and CO2 stressed L. monocytogenes were observed under a fluorescence microscope, using the LIVE/DEAD BacLight bacteria viability kit (Molecular Probes, Eugene, OR, USA), the healthy, mildly injured, and the putative VBNC cells accounted for 31.0% of the stressed cell population. By using the selective plate count, 31.4% of the same stressed cell population was found to be healthy and mildly injured (putative VBNC cells not included). If there were VBNC state cells present, we should have observed a significant difference between the above two numbers. In fact, there was no significant difference between the results obtained from those two methods.

CONCLUSIONS

There were no VBNC state cells observed in the stressed cell population. We conclude that cold and CO2 do not induce L. monocytogenes to enter a VBNC state.

SIGNIFICANCE AND IMPACT OF THE STUDY

Cold and modified atmospheres are widely used in fresh muscle food and fruit preservation. Whether they would induce L. monocytogenes into a VBNC state is of a great concern for microbial food safety.

Cell-cell interactions influence resistance and survival of Salmonella serotype Typhimurium to environmental stress

AIMS

The aim of this work was to study the effects of prolonged nutrient stress on survival, cell interactions and resistance to inimical processes in Salmonella serotype Typhimurium.

METHODS AND RESULTS

Salmonella Typhimurium cells were subjected to prolonged incubation in the stationary phase of growth and the properties of starved cells (old) were investigated with reference to those of exponentially-growing cells (young). Competition experiments between old and young cells revealed cell-cell interactions that influenced stationary phase survival and response of the bacterium to heat stress. During prolonged incubation of cells, cycles of resistance and sensitivity to heat stress were identified. Competition experiments between old and young cells revealed that the resistance of young cells to heat increased to levels more like those of stationary phase cells than growing cells. The presence of old cells influenced the phenotype of young cells, possibly by means of cell-cell interactions. There was no evidence for the involvement of any extracellularly-produced factors in this phenomenon, but a requirement that the old competitor cells be viable could be demonstrated.

CONCLUSIONS

It is proposed that the complex interactions within stationary phase cultures of Salm. Typhimurium may be due to cycles of mutation in concert with an as yet undefined interaction between old cells and growing ones.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides evidence for active and diverse responses to nutrient stress within populations of Salm. Typhimurium that promote survival and that may be important for the success of this bacterium as a pathogen.

Adoption of the transiently non-culturable state — a bacterial survival strategy?

Microbial culturability can be ephemeral. Cells are not merely either dead or alive but can adopt physiological states in which they appear to be (transiently) non-culturable under conditions in which they are known normally to be able to grow and divide. The reacquisition of culturability from such states is referred to as resuscitation. We here develop the idea that this "transient non-culturability" is a consequence of a special survival strategy, and summarise the morphological, physiological and genetic evidence underpinning such behaviour and its adaptive significance.

Simultaneous fluorescent gram staining and activity assessment of activated sludge bacteria

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

Salmonella typhimurium displays cyclical patterns of sensitivity to UV-C killing during prolonged incubation in the stationary phase of growth

Stationary phase cells of Salmonella typhimurium were more resistant to killing by UV-C irradiation than those from the exponential phase. Analysis of the tolerance of cells taken at different stages of prolonged incubation as batch cultures to 60 or 100 J m(2) doses of UV-C revealed cycles of resistance and tolerance. The possible involvement of rpoS-controlled functions in mediating these cycles could be discounted because they were also detected in an rpoS minus mutant of S. typhimurium. The results are discussed in the context of heterogeneity in cells of stationary phase cultures of S. typhimurium.