Bacterial characterization by flow cytometry

Characterization of bacteria by multiparameter flow cytometry

An arc-lamp based flow cytometer was used to obtain high resolution measurements of the light scattering characteristics and DNA contents of eight different bacteria. Light scatter profiles of bacteria are a useful first step when flow cytometry is used to characterize organisms. Scanning and transmission electron microscopy of the bacterial samples demonstrate that the structural basis of the light scattering profiles is not always clear, i.e. some organisms appear to have anomalous light scattering characteristics. The use of a third measurement parameter, DNA content, allowed much better discrimination of the organisms. Flow cytometry shows great promise as a method for the rapid discrimination and identification of bacterial populations.

Quantifying heterogeneity: flow cytometry of bacterial cultures

Flow cytometry is a technique which permits the characterisation of individual cells in populations, in terms of distributions in their properties such as DNA content, protein content, viability, enzyme activities and so on. We review the technique, and some of its recent applications to microbiological problems. It is concluded that cellular heterogeneity, in both batch and continuous axenic cultures, is far greater than is normally assumed. This has important implications for the quantitative analysis of microbial processes.

Fluorogenic and chromogenic substrates used in bacterial diagnostics

Methods based on the application of chromogenic and fluorogenic substrates enable specific and rapid detection of a variety of bacterial enzymatic activities. By using these techniques, enzymatic reactions can be examined simultaneously or individually, either directly on the isolation plate or in cell suspensions. For this purpose, various testing principles and test kits for clinical and food microbiology have been introduced successfully during the last few years. In this paper we present a survey of different enzymes of microbial origin that are utilized for microbiological identification and differentiation and the corresponding methods. Particular emphasis is given to the examination of Escherichia coli and the description of the different techniques as used in routine analysis.

Detection and analysis by dual-laser flow cytometry of bacteriophage T4 DNA inside Escherichia coli

Bacteriophage T4 DNA was detected and analyzed inside E. coli by dual-laser flow cytometry using a dye combination of Hoechst 33258 (H33258) and chromomycin A3 (CA3) which bind to A-T- and G-C-rich regions of DNA, respectively. An exponentially-growing culture of E. coli ATCC 11303 was infected with T4 bacteriophage at a 1:1 multiplicity of infection. Samples were taken immediately and at 5 min intervals and placed on ice to interrupt viral replication. The samples were then centrifuged, ethanol-fixed, stained with H33258 and CA3, and analyzed by flow cytometry. Twenty-five minutes post-infection, a population of cells which contained T4 DNA began to appear on both a bivariate contour plot and a frequency histogram plot of the data. By 35 min, T4 DNA-containing cells could be distinguished from E. coli cells containing little or no T4 DNA. The ratio of CA3:H33258 fluorescence was then used to calculate the % G + C value for T4 DNA inside E. coli. A value of 35.6 +/- 0.2% was obtained, which agrees with % G + C values determined by traditional methods. These results demonstrate that dual-laser flow cytometry can be used to study viral DNA inside the bacterial host.

Binding of viridans group streptococci to human platelets: a quantitative analysis

The binding of viridans group streptococci with human platelets was analyzed by two-color flow cytometry. Binding was detected within 15 s of mixing bacteria and platelets. At ratios of bacteria to platelets of 1:1, 10:1, 100:1, and 1,000:1, the percentages of bound streptococci (mean +/- standard deviation) were 93.2% +/- 5.4%, 80.0% +/- 8.6%, 39.8% +/- 11.1%, and 12.5% +/- 2.0%, respectively. Binding of labeled bacteria was reversed by adding a 500-fold excess of unlabeled streptococci. These results demonstrate that streptococcus-platelet binding is rapid, reversible, and saturable, which suggests a specific receptor-ligand interaction.

Validation of flow cytometry for rapid enumeration of bacterial concentrations in pure cultures

Flow cytometry was investigated as a rapid detection and counting method for bacteria in pure cultures. A simple two-parameter detection scheme was employed: particle size was measured by forward angle light scatter and nucleic acid content by fluorescence of the DNA/RNA-binding dye ethidium bromide. The technique gave results that correlated exceptionally well with conventional plate counting for four species of bacteria, and concentrations in the range 10(2) to 10(7) cfu/ml. Cytometric counts were obtained in a few minutes, as compared with 2 d required for the plate counts. Under ideal conditions, each bacterial species examined exhibited a characteristic 'signature' on the cytometer, which could be explained by its known properties and morphology.

Flow cytometric identification of microorganisms by dual staining with FITC and PI

The identification of microorganisms by flow cytometry was evaluated by using a double staining technique with propidium iodide and fluorescein isothiocyanate and a two dimensional analysis. A diverse group of 19 different species and strains of microorganisms was tested to determine if they could be differentiated by flow cytometry. The organisms tested displayed characteristic and distinct two dimensional fluorescent patterns which allowed ready grouping and differentiation into subsets of organisms. The slopes and correlation coefficients of the histograms and the ratio of red to green signals expressed these differences quantitatively and allowed organisms to be placed into one of three groups based on these values. In some instances, as with Streptococcus pneumoniae and pyogenes and Staphylococcus aureus and epidermidis, it was possible to distinguish between species of bacteria from the same genus. The use of dual dye labeling and flow cytometry provided a rapid method of identifying selected microorganisms and may be broadly applicable for the detection and identification of many bacteria and fungi.

Rapid flow cytometric bacterial detection and determination of susceptibility to amikacin in body fluids and exudates

A flow cytometry-based method for rapid and quantitative detection of bacteria in various clinical specimens and for rapid determination of antibiotic effect is described. Achieving such a measurement with high sensitivity required discrimination between bacteria and other particles which were often present in clinical samples in high concentrations. This discrimination was facilitated by detecting the bacterial characteristic light scatter and fluorescence signals following staining, e.g., with the fluorescent nucleic acid-binding dye ethidium bromide, as well as by measuring bacterial proliferation during short time intervals. Antibiotic susceptibility was measured by observing the inhibition of such proliferation. The method was applied to 43 clinical specimens from various sources, such as wound exudates, bile, serous cavity fluids, and bronchial lavage. Bacterial detection, achieved in less than 2 h, agreed with results of conventional methods with a sensitivity of 74% and a specificity of 88%. Susceptibility to amikacin was detected in 1 h in 92% of 13 positive specimens.

Characterizing aquatic bacteria according to population, cell size, and apparent DNA content by flow cytometry

Flow cytometry offers a rapid method for characterizing aquatic populations according to the properties of individual cells. This technology has been extended to aquatic bacteria by using high-intensity UV excitation, condensing the laser beam onto a small area, using blemish-free flow cells, optimizing organism staining protocol, segregating the optical signal produced with high-transmittance optical filters, collecting the signal with sensitive photomultipliers, and expanding the range of data displayed from individual samples with calibrated circuitry. Bacteria could be counted according to event frequency, and populations agreed with direct counts by epifluorescence microscopy. Forward scatter intensity was a linear function of volume for bacterial cells between 1.3 and 0.25 micron 3 as calibrated by Coulter impedance. Plastic spheres down to 0.014 micron 3, 0.3 micron in diameter, were resolved. Aquatic bacteria 0.05 micron 3 in volume were clearly resolved according to DNA content by staining with DAPI. The observed signal was DNA-dependent because DNase treatment eliminated most fluorescence. These procedures are suitable for direct analysis of the bacteria in marine and freshwater samples without interference from algae, sediment, or most DNA-free organic particles. Cytograms indicated one or more clearly resolved subpopulations of bacteria of substantially smaller size and DNA content than the laboratory organisms typically classified.

Flow cytometric screening and isolation of Escherichia coli clones which express surface antigens of the oil-degrading microorganism Acinetobacter calcoaceticus RAG-1

Flow cytometry (FCM) in conjunction with immunocytochemical-labeling was used to analyze and screen a population of Escherichia coli clones containing a genomic library from the oil-degrading microorganism Acinetobacter calcoaceticus RAG-1 surface antigens. Reconstruction experiments using mixed populations indicated that RAG-1 cells could be clearly distinguished at a ratio of one RAG-1 cell to 500 Escherichia coli cells. Using this technique two clones, WM143 and WM191, were isolated and shown by restriction endonuclease cleavage and Southern hybridization to contain plasmids carrying inserts of RAG-1 DNA of 9.4 and 9.8 kb respectively.

Pyrolysis mass spectrometry of bacteria from infected human urine. I. Influence of culturing and antibiotics

Curie-point pyrolysis mass spectrometry was performed on three bacterial isolates obtained from infected human urine using a novel, direct isolation method. Bacterial samples were analysed directly after isolation, as well as after free growth in broth and after exposure to solutions containing penicillin and gentamicin. The spectra of bacteria directly from urine showed no detectable contamination from urinary constituents. Discriminant analysis indicated genetic strain differences to be greater than the combined variances due to sample preparation or the growth phase. Characteristic biochemical changes related to growth or non-growth were detectable after only 2 h of incubation. The potential usefulness of pyrolysis mass spectrometry techniques for rapid susceptibility testing is discussed.

Flow cytometry analysis of recombinant Saccharomyces cerevisiae populations

A new fluorescent stain has been developed for detecting cloned beta-galactosidase activity in individual cells of Saccharomyces cerevisiae by flow cytometry. The staining reaction is based on enzymatic cleavage of alpha-naphthol-beta-D-galactopyranoside by intracellular beta-galactosidase and trapping of the liberated naphthol by hexazoniumpararosaniline yielding a fluorescent, insoluble end product. This stain, in connection with an appropriate host strain, has been applied for detecting plasmids encoding inducible beta-galactosidase in unstable recombinant cell populations carrying plasmids with different origins of replication. The method enables rapid determination of the fraction of plasmid-containing cells as well as quantitation of intracellular beta-galactosidase content by kinetic enzyme assay. Inducibility of the marker enzyme is important for maintaining correlation between enzyme and gene content.

Detection of Escherichia coli in blood using flow cytometry

A rapid method for the detection of Escherichia coli in blood has been developed. The method employs blood cell lysis, staining of bacteria with ethidium bromide, and detection of stained bacteria using flow cytometry. The detection protocol requires less than 2 h sample handling time and is not dependent on bacterial growth. This method has been applied to human donor blood specimens seeded with various E. coli concentrations and to two rabbit model systems. Bacterial detection is evident from the in vitro human blood studies at levels of 10 E. coli/ml and from in vivo rabbit model studies at less than 100 E. coli/ml.

Spectrofluorometric determination of bacterial DNA base composition

A spectrofluorometric technique for bacterial DNA base composition has been developed. This fast and simple technique requires two fluorescent dyes and a few inexpensive reagents. The data from this assay indicate that the guanine-cytosine content obtained was within acceptable statistical limits in comparison to commonly cited literature values. The spectrofluorometric technique is reliable and reproducible.

Automated immunofluorescent speciation of oral bacteria using flow cytometry

Mixtures containing two bacterial species were analyzed using flow cytometric techniques. Light scattering characteristics of Streptococcus mutans and Actinomyces viscosus show unique profiles for pure cultures. However, the light scatter analysis of a mixture containing these two species demonstrates overlapping near the origin. Thus, light scatter analysis was not sufficient to speciate bacteria with different morphologies. Labeling of samples with species-specific immunofluorescent antibodies permitted speciation of mixtures. As the percentage of the bacterium to which the antibody is directed increased in a two-component mixture, fluorescent flow cytometric analysis showed a corresponding increase in the percentage of cells displaying fluorescent labeling. These methods could permit the rapid identification of bacteria from oral sites without culturing.