Individual Escherichia coli cells studied from light scattering with the scanning flow cytometer

Analysis of very low bacterial counts in small sample volumes using angle-resolved light scattering

Because of its high sensitivity to even small objects and the quick measurement principle, angle-resolved scattering (ARS) measurements exhibit a promising potential as a rapid analysis tool for bacterial cells at small sample sizes and very low numbers of cells. In this study, investigations on scattered light from various bacterial cell samples revealed applicability down to single cell levels, which is a huge benefit compared to conventional methods that depend on time-consuming cellular growth over several hours or even days. With the proposed setup and data analysis method, it is possible to detect scatter differences among cell types, together with the cell concentration.

Scattering angle resolved optical coherence tomography measures morphological changes in Bacillus subtilis colonies

Significance

An unmet need is recognized for early detection and diagnosis of neurological diseases. Many psychological markers emerge years after disease onset. Mitochondrial dysfunction and corresponding neurodegeneration occur before onset of large-scale cell and tissue pathology. Early detection of subcellular morphology changes could serve as a beacon for early detection of neurological diseases. This study is on bacterial colonies, Bacillus subtilis, which are similar in size to mitochondria.

Aim

This study investigates whether morphological changes can be detected in Bacillus subtilis using scattering angle resolved optical coherence tomography (SAR-OCT).

Approach

The SAR-OCT was applied to detect scattering angle distribution changes in Bacillus subtilis. The rod-to-coccus shape transition of the bacteria was imaged, and the backscattering angle was analyzed by recording the distribution of the ratio of low- to medium angle scattering (L/M ratio). Bacillus orientation at different locations in colonies was analytically modeled and compared with SAR-OCT results.

Results

Significant differences in the distribution of backscattering angle were observed in Bacillus subtilis transitioning from rod-to-coccus shapes. In Bacillus subtilis, the C -parameter of the Burr distribution of the SAR-OCT-derived L/M ratio was significantly smaller in coccus compared with rod-shaped bacteria. SAR-OCT-derived L/M ratio varied with bacterial position in the colony and is consistent with predicted orientations from previous studies.

Conclusions

Study results support the potential of utilizing SAR-OCT to detect bacterial morphological changes.

In vitro optoacoustic flow cytometry with light scattering referencing

Morphological and functional optoacoustic imaging is enhanced by dedicated transgene reporters, in analogy to fluorescence methods. The development of optoacoustic reporters using protein engineering and directed evolution would be accelerated by high-throughput in-flow screening for intracellular, genetically encoded, optoacoustic contrast. However, accurate characterization of such contrast is impeded because the optoacoustic signals depend on the cell's size and position in the flow chamber. We report herein an optoacoustic flow cytometer (OA-FCM) capable of precise measurement of intracellular optoacoustic signals of genetically-encoded chromoproteins in flow. The novel system records light-scattering as a reference for the detected optoacoustic signals in order to account for cell size and position, as well as excitation light flux in the focal volume, which we use to reference the detected optoacoustic signals to enhance the system's precision. The OA-FCM was calibrated using micrometer-sized particles to showcase the ability to assess in-flow objects in the size range of single-cells. We demonstrate the capabilities of our OA-FCM to identify sub-populations in a mixture of two E. coli stocks expressing different reporter-proteins with a precision of over 90%. High-throughput screening of optoacoustic labels could pave the way for identifying genetically encoded optoacoustic reporters by transferring working concepts of the fluorescence field such as directed evolution and activated cell sorting.

Improved synchronous light scattering method for measuring baker's yeast biomass using thickened suspensions

Measuring yeast biomass is important in the processes of microbial fermentations. It has been demonstrated that synchronous light scattering (SLS) signals could be applied for the quantification of model bioparticles such as Saccharomyces cerevisiae. In this study, an improved synchronous light scattering method was developed for yeast biomass estimation. The settlement of yeast cells during SLS signals measuring process was studied, and hydrolysis anionic polyacrylamide was added into yeast suspensions to increase the stability of the cells in liquid environment. By simultaneously scanning both the excitation and emission monochromators of a common spectrofluorometer with same starting excitation and emission wavelength (namely, ∆λ = 0), the SLS intensity was found to be proportional to the yeast concentration in the range from 0 to 4.9 × 10(6) cell/mL (R (2) = 0.9907), the detection limit is 8.1 × 10(3) cell/mL. The developed method exhibited good stability and sensitivity in the recovery test and growth curve drawing process, demonstrating the potential of the method in practical application of biomass estimation.

High speed classification of individual bacterial cells using a model-based light scatter system and multivariate statistics

We describe a model-based instrument design combined with a statistical classification approach for the development and realization of high speed cell classification systems based on light scatter. In our work, angular light scatter from cells of four bacterial species of interest, Bacillus subtilis, Escherichia coli, Listeria innocua, and Enterococcus faecalis, was modeled using the discrete dipole approximation. We then optimized a scattering detector array design subject to some hardware constraints, configured the instrument, and gathered experimental data from the relevant bacterial cells. Using these models and experiments, it is shown that optimization using a nominal bacteria model (i.e., using a representative size and refractive index) is insufficient for classification of most bacteria in realistic applications. Hence the computational predictions were constituted in the form of scattering-data-vector distributions that accounted for expected variability in the physical properties between individual bacteria within the four species. After the detectors were optimized using the numerical results, they were used to measure scatter from both the known control samples and unknown bacterial cells. A multivariate statistical method based on a support vector machine (SVM) was used to classify the bacteria species based on light scatter signatures. In our final instrument, we realized correct classification of B. subtilis in the presence of E. coli,L. innocua, and E. faecalis using SVM at 99.1%, 99.6%, and 98.5%, respectively, in the optimal detector array configuration. For comparison, the corresponding values for another set of angles were only 69.9%, 71.7%, and 70.2% using SVM, and more importantly, this improved performance is consistent with classification predictions.

Erythrocyte lysis in isotonic solution of ammonium chloride: theoretical modeling and experimental verification

A mathematical model of erythrocyte lysis in isotonic solution of ammonium chloride is presented in frames of a statistical approach. The model is used to evaluate several parameters of mature erythrocytes (volume, surface area, hemoglobin concentration, number of anionic exchangers on membrane, elasticity and critical tension of membrane) through their sphering and lysis measured by a scanning flow cytometer (SFC). SFC allows measuring the light-scattering pattern (indicatrix) of an individual cell over the angular range from 10 degrees to 60 degrees . Comparison of the experimentally measured and theoretically calculated light scattering patterns allows discrimination of spherical from non-spherical erythrocytes and evaluation of volume and hemoglobin concentration for individual spherical cells. Three different processes were applied for erythrocytes sphering: (1) colloid osmotic lysis in isotonic solution of ammonium chloride, (2) isovolumetric sphering in the presence of sodium dodecyl sulphate and albumin in neutrally buffered isotonic saline, and (3) osmotic fragility test in hypotonic media. For the hemolysis in ammonium chloride, the evolution of distributions of sphered erythrocytes on volume and hemoglobin content was monitored in real-time experiments. The analysis of experimental data was performed in the context of a statistical approach, taking into account that parameters of erythrocytes vary from cell to cell.

Investigation of morphometric parameters for granulocytes and lymphocytes as applied to a solution of direct and inverse light-scattering problems

Quantitative data on cell structure, shape, and size distribution are obtained by optical measurement of normal peripheral blood granulocytes and lymphocytes in a cell suspension. The cell nuclei are measured in situ. The distribution laws of the cell and nuclei sizes are estimated. The data gained are synthesized to construct morphometric models of a segmented neutrophilic granulocyte and a lymphocyte. Models of interrelation between the cell and nucleus metric characteristics for granulocyte and lymphocyte are obtained. The discovered interrelation decreases the amount of cell-nucleus size combinations that have to be considered under simulation of cell scattering patterns. It allows faster analysis of light scattering to discriminate cells in a real-time scale. Our morphometric data meet the requirements of scanning flow cytometry dealing with the high rate analysis of cells in suspension. Our findings can be used as input parameters for the solution of the direct and inverse light-scattering problems in scanning flow cytometry, dispensing with a costly and time-consuming immunophenotyping of the cells, as well as in turbidimetry and nephelometry. The cell models developed can ensure better interpretations of scattering patterns for an improvement of discriminating capabilities of immunophenotyping-free scanning flow cytometry.

Biophysical modeling of forward scattering from bacterial colonies using scalar diffraction theory

A model for forward scattering from bacterial colonies is presented. The colonies of interest consist of approximately 10(12) - 10(13) individual bacteria densely packed in a configuration several millimeters in diameter and approximately 0.1-0.2 mm in thickness. The model is based on scalar diffraction theory and accounts for amplitude and phase modulation created by three macroscopic properties of the colonies: phase modulation due to the surface topography, phase modulation due to the radial structure observed from some strains and species, and diffraction from the outline of the colony. Phase contrast and confocal microscopy were performed to provide quantitative information on the shape and internal structure of the colonies. The computed results showed excellent agreement with the experimental scattering data for three different Listeria species: Listeria innocua, Listeria ivanovii, and Listeria monocytogenes. The results provide a physical explanation for the unique and distinctive scattering signatures produced by colonies of closely related Listeria species and support the efficacy of forward scattering for rapid detection and classification of pathogens without tagging.

Bacteriological examination of ballast water in Singapore Harbour by flow cytometry with FISH

In this study the concentrations of total bacteria, enterobacteria, Vibrio spp., and E. coli have been compared for ballast water samples taken from ships in Singapore Harbour. The cell concentrations were enumerated using FISH and flow cytometry. The data were highly variable, reflecting the many influences upon ballast water as it is utilized in the shipping industry. The concentration of bacterial species was determined as a proportion of the total concentration of cells for the ballast water sampled. For the ballast water sampled these concentrations were 0.67-39.55% for eubacteria, 0-2.46% for enterobacteria, 0.18-35.82% for Vibrio spp., and 0-2.46% for E. coli. Using FISH and flow cytometry, an informative determination of the bacterial hazards of ship ballast water can be made.

Angular distribution of light scattered by single biological cells and oriented particle agglomerates

We used a flow cytometer together with an intensified CCD camera to record spatially resolved light scattering from micrometer-sized single particles and single oriented particle agglomerates. Experimental differential cross sections of an oriented dumbbell made from two identical polystyrene spheres were compared with theoretical values calculated within the discrete dipole approximation, and good agreement was achieved. Furthermore, characteristic two-dimensional patterns of the scattered-light intensity were recorded for single blood cells, yielding information on the cells' shape and volume. Besides flow cytometry, we observed and analyzed differential light scatter of particle clusters of known size, shape, and orientation located within an optical trap.

Flow cytometry and conventional enumeration of microorganisms in ships' ballast water and marine samples

Conventional methods for bacteriological testing of water quality take long periods of time to complete. This makes them inappropriate for a shipping industry that is attempting to comply with the International Maritime Organization's anticipated regulations for ballast water discharge. Flow cytometry for the analysis of marine and ship's ballast water is a comparatively fast and accurate method. Compared to a 5% standard error for flow cytometry analysis the standard methods of culturing and epifluorescence analysis have errors of 2-58% and 10-30%, respectively. Also, unlike culturing methods, flow cytometry is capable of detecting both non-viable and viable but non-culturable microorganisms which can still pose health risks. The great variability in both cell concentrations and microbial content for the samples tested is an indication of the difficulties facing microbial monitoring programmes. The concentration of microorganisms in the ballast tank was generally lower than in local seawater. The proportion of aerobic, microaerophilic, and facultative anaerobic microorganisms present appeared to be influenced by conditions in the ballast tank. The gradual creation of anaerobic conditions in a ballast tank could lead to the accumulation of facultative anaerobic microorganisms, which might represent a potential source of pathogenic species.

Separation, identification, and characterization of microorganisms by capillary electrophoresis

The use of capillary electrophoresis (CE) for the analysis, identification, and characterization of microorganisms has been gaining in popularity. The advantages of CE, such as small sample requirements, minimal sample preparation, rapid and simultaneous analysis, ease of quantitation and identification, and viability assessment, make it an attractive technique for the analysis of microbial analytes. As this instrumental method has evolved, higher peak efficiencies have been achieved by optimizing CE conditions, such as pH, ionic strength, and polymer additive concentration. Experimental improvements have allowed better quantitation and more accurate results. Many practical applications of this technique have been investigated. Viability and identification of microbes can be accomplished in a single analysis. This is useful for evaluation of microbial analytes in consumer products. Diagnosis of microbe-based diseases is now possible, in some cases, without the need for culture methods. Microbe-molecule, virus-antibody, or bacteria-antibiotic interactions can be monitored using CE, allowing for the screening of possible drug candidates. Fermentation can be monitored using this system. This instrumental approach can be adapted to many different applications, including assessing the viability of sperm cells. Progress has been made in the development of microelectrophoresis instrumentation. These advances will eventually allow the development of small, dedicated devices for the rapid, repetitive analyses of specific microbial samples. Although these methods may never fully replace traditional approaches, they are proving to be a valuable addition to the collection of techniques used to analyze, quantitate, and characterize microbes. This review outlines the recent developments in this rapidly growing field.