Dual-parameter scatter-flow immunofluorescence analysis of Bacillus spores

Using a commercial flow cytometer (Cyto-fluorograf), narrow-forward-angle (NFA) light-scatter signals were detected for spore preparations of Bacillus anthracis Vollum, B. anthracis Sterne, B. cereus NCTC 8035, and B. subtilis var niger. In the flow immunofluorescence (FIF) analysis of spores stained with fluorescein-conjugated hyperimmune antibody to B. anthracis Vollum spores, fluorescence histograms could be acquired by selecting on NFA scatter. Fluorescence data selected on ninety degree scatter were rather noisier. Fluorescence analysis by dual parameter NFA scatter-FIF techniques was shown to have several advantages over the subtraction FIF method reported earlier. The implication from FIF analysis of spore suspensions and corresponding cell-free supernatants that the peak in the fluorescence histogram was caused by signals from fluorescing spores, was confirmed by use of the cell sorter and subsequent microscopy of the sorted samples. Although a proportion of spore aggregates was present in samples sorted from the right-hand tail of the fluorescence histogram, it was demonstrated that the majority of the observed distribution of fluorescence was not due to the formation of aggregates but was rather an expression of variation in the degree of staining of individual spores.     

Radioactive labels for Protein A: evaluation in the indirect immunoradiometric assay (IRMA) for Bacillus anthracis spores

P hillips , A.P. & M artin , K.L. 1984. Radioactive labels for Protein A: evaluation in the indirect immunoradiometric assay (IRMA) for Bacillus anthracis spores. Journal of Applied Bacteriology 56 , 449–456.
Staphylococcus aureus Protein A (SpA) labelled with [¹²⁵I] by the Bolton & Hunter (1973) method performed about as well as labelled sheep anti-rabbit globulin (SAR) in an indirect immunoradiometric assay (IRMA) for Bacillus anthracis spores immobilized on multispot microscope slides. SpA labelled with [³H] by propi-onylation also performed well but would be expensive to use. SpA labelled with [³H] fluorodinitrobenzene, or labelled with [¹²⁵I] by the chloramine T reaction gave erratic assay results, high noise values and low signal-to-noise ratios, indicating substantial direct binding of labelled SpA to the slide surface and to the bacterial preparation. The uptake of radioactively labelled SpA in the IRMA was compared with the fluorescence intensity of individual spores in a micro-fluorometric immunofluorescence (IF) test involving dual labelled fluorescein-[¹²⁵I]-SpA. The maximum number of SAR molecules bound to the mixture of spores and cell-free antigens in the B. anthracis IRMA was about twice the maximum number of radioactively labelled SpA molecules bound. The SAR : SpA saturation binding ratio on the surface of the spores, however, was approximately the inverse of this. It is concluded that radioactively-labelled SpA is not recommended in preference to anti-species antibody reagents in bacterial IRMA tests but fiuorescein-conjugated SpA deserves further consideration for use in microscope-based IF tests for bacterial antigens.     

Radioactive labels for Protein A: evaluation in the indirect immunoradiometric assay (IRMA) for Bacillus anthracis spores

Staphylococcus aureus Protein A (SpA) labelled with [125I] by the Bolton & Hunter (1973) method performed about as well as labelled sheep anti-rabbit globulin (SAR) in an indirect immunoradiometric assay (IRMA) for Bacillus anthracis spores immobilized on multispot microscope slides. SpA labelled with [3H] by propionylation also performed well but would be expensive to use. SpA labelled with [3H] fluorodinitrobenzene, or labelled with [125I] by the chloramine T reaction gave erratic assay results, high noise values and low signal-to-noise ratios, indicating substantial direct binding of labelled SpA to the slide surface and to the bacterial preparation. The uptake of radioactively labelled SpA in the IRMA was compared with the fluorescence intensity of individual spores in a microfluorometric immunofluorescence (IF) test involving dual labelled fluorescein-[125I]-SpA. The maximum number of SAR molecules bound to the mixture of spores and cell-free antigens in the B. anthracis IRMA was about twice the maximum number of radioactively labelled SpA molecules bound. The SAR:SpA saturation binding ratio on the surface of the spores, however, was approximately the inverse of this. It is concluded that radioactively-labelled SpA is not recommended in preference to anti-species antibody reagents in bacterial IRMA tests but fluorescein-conjugated SpA deserves further consideration for use in microscope-based IF tests for bacterial antigens.     

The flow cytometry of Bacillus anthracis spores revisited

BACKGROUND: The potential use of Bacillus anthracis spores as a weapon of terror has rekindled interest in the rapid detection and identification of the spores of these bacteria. Prior efforts to utilize flow cytometry (FCM) for this purpose resulted in tedious and time-consuming protocols. Advances in rapid immunoassays suggest a reinvestigation of the use of FCM because this may allow for the development of a rapid and sensitive system for detection and/or identification of spores in suspect samples. METHODS: In this study, antiserum was raised in goats using three different strains of B. anthracis spores as the immunogen. The resultant antibodies were purified, labeled with fluorescein, and evaluated for use in an immunoassay on a Coulter Epics XL flow cytometer. In the protocol that was developed, fluorescein-labeled antibodies are simply mixed with the sample, allowed to incubate, and then analyzed on the flow cytometer. Washes and centrifugation were eliminated. RESULTS: The results showed that a rapid (5 min) and sensitive immunological analysis was feasible. The detection limit (approximately 10(3) colony-forming units [CFU]/ ml) varied with strain, but there was no difference in the detection limit between live and irradiated spores. In addition, the power of FCM was utilized to minimize false-positive reactions among similar species of Bacillus by placing constraints on scatter and fluorescence intensity. The data also suggest that scatter might be useful to determine spore viability. CONCLUSION: This study shows that FCM may be an effective platform on which to perform immunological analysis for the detection and/or presumptive identification of B. anthracis spores. Published 2000 Wiley-Liss, Inc.     

Bacterial fingerprinting by flow cytometry: bacterial species discrimination.

BACKGROUND: A flow cytometric measurement (FCM) technique has been developed to size DNA fragments. Individual fragments of a restriction digest of genomic DNA, stained with an intercalating dye, are passed through an ultrasensitive cytometer. The measured fluorescence intensity from each fragment is proportional to the fragment length. METHODS: The isolation of bacterial genomic DNA and digestion by restriction enzymes were performed inside an agarose plug. Rare cutting enzymes were employed to produce a manageable number of DNA fragments. Electroelution was used to move the DNA fragments from the agarose plug into a solution containing polyamines to protect the DNA from shear-induced breakage. The DNA was stained with the bisintercalating dye thiazole orange homodimer and introduced into our ultrasensitive flow cytometer. A histogram of the fluorescence intensities (fingerprint) was constructed. RESULTS: Gram-positive Bacillus globigii and gram-negative bacteria Escherichia coli and Erwinia herbicola were distinguished by the fingerprint pattern of restriction fragments of their genomic DNA. DNA sizes determined by FCM are in good agreement with pulsed-field gel electrophoresis (PFGE) analysis. Flow cytometry requires only picogram quantities of purified DNA and takes less than 10 min for data collection and analysis. When the total sample preparation time is included, the analysis times for PFGE and FCM are similar ( approximately 3 days). CONCLUSIONS: FCM is an attractive technique for the identification of bacterial species. It is more sensitive and potentially much faster than PFGE.     

Identification of Bacillus anthracis by polyclonal antibodies against extracted vegetative cell antigens

The extractable protein antigens EA1 and EA2 of Bacillus anthracis were prepared from electrophoresis transblots of SDS extracts of vegetative bacteria of the Sterne strain. Hyperimmune guinea-pig antiserum against EA2 failed to react with B. anthracis cells in immunofluorescence (IF) tests. Guinea-pig antiserum against EA1 (anti-EA1) reacted strongly in IF tests with non-encapsulated vegetative cell of 10 of 12 strains of B. anthracis and with cells of strains of B. cereus and B. thuringiensis. The unreactive B. anthracis strains were delta-Vollum-1B-1 and Texas. Encapsulated cells of B. anthracis stained poorly except for small bright regions. Absorption of anti-EA1 with cells of B. cereus NCTC 8035 and NCTC 9946 removed activity towards all B. cereus strains tested, but only partly reduced cross-reaction with B. thuringiensis strains. Absorption of anti-EA1 with B. thuringiensis 4041 removed activity towards this strain and B. cereus strains. Evidence is produced that B. thuringiensis cells grown on nutrient agar possess more cross-reacting antigens than cells grown in nutrient broth. The reaction of anti-EA1 with Bacillus spores immobilized in clumps on microscope slides was attributed to contaminating vegetative debris because well-separated individual spores failed to react. A rapid IF test was developed allowing identification of B. anthracis sampled from overnight cultures on blood plates. When sodium dodecyl sulphate extracts of B. anthracis vegetative cells were analysed on immunoblots (Western blots) by reaction with anti-EA1, a number of bands were visualized in addition to the expected 91 kiloDalton EA1 band. Prior absorption of anti-EA1 with B. cereus or B. thuringiensis cells resulted in the disappearance of most or all of the brands in blots of these species, but had less effect on blots of the B. anthracis strains. All six B. anthracis strains that were blotted including delta-Vollum-1B-1 and Texas, could thus be distinguished from B. cereus and B. thuringiensis by their differential reaction with unabsorbed and absorbed anti-EA1.     

A microtiter fluorometric assay to detect the germination of Bacillus anthracis spores and the germination inhibitory effects of antibodies

Bacillus anthracis spore germination is usually detected in vitro by alterations in spore refractility, heat resistance, and stainability. We developed a more quantitative, sensitive, and semi-automated procedure for detecting germination by using a microtiter kinetic reader for fluorescence spectrophotometry. The procedure was based on the increase in fluorescence of spores with time during their incubation in germination medium containing a fluorescent nucleic acid-binding dye which stained germinated B. anthracis but not ungerminated (UG) spores. Spore germination in the presence of several germinants was characterized. Although L-alanine and inosine alone stimulated rapid germination in this assay, a medium containing optimal concentrations of L-alanine, adenosine, and casamino acids gave low background fluorescence, stimulated germination completely, and at a reasonable rate. Suspensions of heat-activated, UG spores of B. anthracis strain Ames were preincubated with antibodies (Abs) against whole spores to assess their effect on germination. Analyses of the germination data obtained revealed significant differences between spores pretreated with these Abs and those treated with non-immune sera or IgG. Germination inhibitory activity (GIA) was detected for several polyclonal rabbit anti-spore Ab preparations. These included anti-Ames strain spore antisera, IgG purified from the latter, and spore affinity-purified Abs from antisera elicited against four strains of B. anthracis. Abs elicited against UG as well as completely germinated Ames spores inhibited germination. Abs were ranked according to their GIA, and those specific for UG spores usually exhibited greater GIA. Direct binding to spores of these Abs was detected by an ELISA with whole un-germinated Ames spores. Although specific binding to spores by the anti-spore Abs was shown, their titers did not correlate with their GIA levels. Current efforts are focused on identifying the spore antigens recognized by the anti-spore Abs, characterizing the role of these targeted antigens in disease pathogenesis, and evaluating the ability of specific anti-spore Abs to protect against infection with B. anthracis.     

Monoclonal antibodies against spore antigens of Bacillus anthracis

Abstract A murine monoclonal antibody produced against heat inactivated spores of Bacillus anthracis Ames, reacted with live or inactivated spores of several anthrax strains in indirect immunofluorescence (IF) tests. The reactive anthrax strain gave only a moderate degree of reaction. No staining of anthrax vegetative cells was observed. The monoclonal did not react with spores of non-anthrax Bacillus strains that gave cross reactions with mouse hyperimmune antiserum raised against Ames spores. The staining of individual spores in B. anthracis preparations was more heterogeneous with the monoclonal antibody than with the hyperimmune serum. Evidence is produced that the epitope for this monoclonal is not stable during long-term storage of inactivated spore preparations, and is not fully available for reaction with antibody until late in spore maturation. The monoclonal did not react by immunoblotting (Western blotting) of spore extracts.     

Monoclonal antibodies against spore antigens of Bacillus anthracis

A murine monoclonal antibody produced against heat inactivated spores of Bacillus anthracis Ames, reacted with live or inactivated spores of several anthrax strains in indirect immunofluorescence (IF) tests. The reactive anthrax strain gave only a moderate degree of reaction. No staining of anthrax vegetative cells was observed. The monoclonal did not react with spores of non-anthrax Bacillus strains that gave cross reactions with mouse hyperimmune antiserum raised against Ames spores. The staining of individual spores in B. anthracis preparations was more heterogeneous with the monoclonal antibody than with the hyperimmune serum. Evidence is produced that the epitope for this monoclonal is not stable during long-term storage of inactivated spore preparations, and is not fully available for reaction with antibody until late in spore maturation. The monoclonal did not react by immunoblotting (Western blotting) of spore extracts. A monoclonal antibody produced against Ames spore extracts reacted with about 1% of Ames spores in IF tests, but not reproducible reactions with other anthrax strains were recorded. This monoclonal interacted with three bands in Western blots of anthrax spore extracts.     

Membrane-based on-line optical analysis system for rapid detection of bacteria and spores

We report here the adaptation of our electronic microchip technology towards the development of a new method for detecting and enumerating bacterial cells and spores. This new approach is based on the immuno-localization of bacterial spores captured on a membrane filter microchip placed within a flow cell. A combination of microfluidic, optical, and software components enables the integration of staining of the bacterial species with fully automated assays. The quantitation of the analyte signal is achieved through the measurement of a collective response or alternatively through the identification and counting of individual spores and particles. This new instrument displays outstanding analytical characteristics, and presents a limit of detection of approximately 500 spores when tested with Bacillus globigii (Bg), a commonly used simulant for Bacillus anthracis (Ba), with a total analysis time of only 5 min. Additionally, the system performed well when tested with real postal dust samples spiked with Bg in the presence of other common contaminants. This new approach is highly customizable towards a large number of relevant toxic chemicals, environmental factors, and analytes of relevance to clinical chemistry applications.     

Application of carbohydrate microarray technology for the detection of Burkholderia pseudomallei, Bacillus anthracis and Francisella tularensis antibodies

We developed a microarray platform by immobilizing bacterial 'signature' carbohydrates onto epoxide modified glass slides. The carbohydrate microarray platform was probed with sera from non-melioidosis and melioidosis (Burkholderia pseudomallei) individuals. The platform was also probed with sera from rabbits vaccinated with Bacillus anthracis spores and Francisella tularensis bacteria. By employing this microarray platform, we were able to detect and differentiate B. pseudomallei, B. anthracis and F. tularensis antibodies in infected patients, and infected or vaccinated animals. These antibodies were absent in the sera of na?ve test subjects. The advantages of the carbohydrate microarray technology over the traditional indirect hemagglutination and microagglutination tests for the serodiagnosis of melioidosis and tularemia are discussed. Furthermore, this array is a multiplex carbohydrate microarray for the detection of all three biothreat bacterial infections including melioidosis, anthrax and tularemia with one, multivalent device. The implication is that this technology could be expanded to include a wide array of infectious and biothreat agents.     

Comparison of direct and indirect immunoradiometric assays (IRMA) for Bacillus anthracis spores immobilised on multispot microscope slides

A solid phase immunoradiometric assay (IRMA) is described in which Bacillus anthracis spores were heat fixed to the wells of glass multispot microscope slides. Assays for spores of B. anthracis Vollum and Sterne strains with ³H labels were evaluated in the direct and indirect versions. Neither signal nor signal-to-noise characteristics of indirect assays were greatly improved by the use of immunopurified antibody (IPAB) or IgG anti-bacterial reagents rather than antiserum. However, the specificity of the direct and indirect assays for B. anthracis strains and B. cereus NCTC 8035 was altered by immunopurification of the anti-bacterial reagent. Although the signal-to-noise ratio was sometimes higher in indirect than in direct assays, signal values were usually no better. Evidence was produced that the overall ratio of the indirect: direct antibody molecules bound by preparations of B. anthracis spores rarely exceeded two but the antibody-molecular ratio for antigens on extracellular material in spore preparations was much higher than the ratio for antigens on the spores themselves.     

Immuno-detection of anthrose containing tetrasaccharide in the exosporium of Bacillus anthracis and Bacillus cereus strains

Aims:  Bacillus anthracis strains of various origins were analysed with the view to describe intrinsic and persistent structural components of the Bacillus collagen-like protein of anthracis glycoprotein associated anthrose containing tetrasaccharide in the exosporium.
Methods and Results:  The tetrasaccharide consists of three rhamnose residues and an unique monosaccharide – anthrose. As anthrose was not found in spores of related strains of bacteria, we envisioned the detection of B. anthracis spores based on antibodies against anthrose-containing polysaccharides. Carbohydrate–protein conjugates containing the synthetic tetrasaccharide, an anthrose–rhamnose disaccharide or anthrose alone were employed to immunize mice. All three formulations were immunogenic and elicited IgG responses with different fine specificities. All sera and monoclonal antibodies derived from tetrasaccharide immunized mice cross-reacted not only with spore lysates of a panel of virulent B. anthracis strains, but also with some of the B. cereus strains tested.
Conclusions:  Our results demonstrate that antibodies to synthetic carbohydrates are useful tools for epitope analyses of complex carbohydrate antigens and for the detection of particular target structures in biological specimens.
Significance and Impact of the Study:  Although not strictly specific for B. anthracis spores, antibodies against the tetrasaccharide may have potential as immuno-capturing components for a highly sensitive spore detection system.     

UV resistance of Bacillus anthracis spores revisited: validation of Bacillus subtilis spores as UV surrogates for spores of B. anthracis Sterne

Recent bioterrorism concerns have prompted renewed efforts towards understanding the biology of bacterial spore resistance to radiation with a special emphasis on the spores of Bacillus anthracis. A review of the literature revealed that B. anthracis Sterne spores may be three to four times more resistant to 254-nm-wavelength UV than are spores of commonly used indicator strains of Bacillus subtilis. To test this notion, B. anthracis Sterne spores were purified and their UV inactivation kinetics were determined in parallel with those of the spores of two indicator strains of B. subtilis, strains WN624 and ATCC 6633. When prepared and assayed under identical conditions, the spores of all three strains exhibited essentially identical UV inactivation kinetics. The data indicate that standard UV treatments that are effective against B. subtilis spores are likely also sufficient to inactivate B. anthracis spores and that the spores of standard B. subtilis strains could reliably be used as a biodosimetry model for the UV inactivation of B. anthracis spores.     

Detection of Bacillus anthracis from spores and cells by loop-mediated isothermal amplification without sample preparation

Loop-mediated isothermal amplification (LAMP) is a technique capable of rapidly amplifying specific nucleic acid sequences without specialized thermal cycling equipment. In addition, several detection methods that include dye fluorescence, gel electrophoresis, turbidity and colorimetric change, can be used to measure or otherwise detect target amplification. To date, publications have described the requirement for some form of sample nucleic acid extraction (boiling, lysis, DNA purification, etc.) prior to initiating a LAMP reaction. We demonstrate here, the first LAMP positive results obtained from vegetative cells and spores of Bacillus anthracis without nucleic acid extraction. Our data show that the simple addition of cells or spores to the reaction mixture, followed by heating at 63°C is all that is required to reproducibly amplify and detect target plasmid and chromosomal DNA via colorimetric change. The use of three primer sets targeting both plasmids and the chromosome of B. anthracis allows for the rapid discrimination of non-pathogenic bacteria from pathogenic bacteria within 30 min of sampling. Our results indicate that direct testing of B. anthracis spores and cells via LAMP assay will greatly simplify and shorten the detection process by eliminating nucleic acid purification. These results may allow more rapid detection of DNA from pathogenic organisms present in field and environmental samples.     

Species-specific peptide ligands for the detection of Bacillus anthracis spores

Currently available detectors for spores of Bacillus anthracis, the causative agent of anthrax, are inadequate for frontline use and general monitoring. There is a critical need for simple, rugged, and inexpensive detectors capable of accurate and direct identification of B. anthracis spores. Necessary components in such detectors are stable ligands that bind tightly and specifically to target spores. By screening a phage display peptide library, we identified a family of peptides, with the consensus sequence TYPXPXR, that bind selectively to B. anthracis spores. We extended this work by identifying a peptide variant, ATYPLPIR, with enhanced ability to bind to B. anthracis spores and an additional peptide, SLLPGLP, that preferentially binds to spores of species phylogenetically similar to, but distinct from, B. anthracis. These two peptides were used in tandem in simple assays to rapidly and unambiguously identify B. anthracis spores. We envision that these peptides can be used as sensors in economical and portable B. anthracis spore detectors that are essentially free of false-positive signals due to other environmental Bacillus spores.     

Effect of Bacillus anthracis lethal toxin on human peripheral blood mononuclear cells

Lethal toxin (LeTx) plays a central role in anthrax pathogenesis, however a cytotoxicity of LeTx has been difficult to demonstrate in vitro. No cytolytic effect has been reported for human cells, in contrast to murine cell lines, indicating that cell lysis can not be considered as a marker of LeTx activity. We have recently shown that murine macrophage-like RAW 264.7 cells treated with LeTx or infected with anthrax spores underwent changes typical of apoptotic death. Here we demonstrate that cells from human peripheral blood display a proapoptotic behavior similar to murine cells. TUNEL assay detected a nucleosomal degradation typical of apoptosis in peripheral blood mononuclear cells (PBMC) treated with LeTx. Membrane staining with apoptotic dyes was detected in macrophages derived from monocytes in presence of LeTx. The toxin inhibited production of proinflammatory cytokines in PBMC stimulated with a preparation of Bacillus anthracis cell wall. Infection of PBMC with anthrax spores led to the appearance of a large population of cells stained positively for apoptosis, with a reduced capacity to eliminate spores and vegetative bacteria. The aminopeptidase inhibitor, bestatin, capable of protecting cells from LeTx, restored a bactericidal activity of infected cells. These findings may be explained by LeTx expression within phagocytes and support an important role of LeTx as an early intracellular virulence factor contributing to bacterial dissemination and disease progression.     

Identification of capsule-forming Bacillus anthracis spores with the PCR and a novel dual-probe hybridization format.

Anthrax is a fatal infection of humans and livestock that is caused by the gram-positive bacterium Bacillus anthracis. The virulent strains of B. anthracis are encapsulated and toxigenic. In this paper we describe the development of a PCR technique for identifying spores of B. anthracis. Two 20-mer oligonucleotide primers specific for the capB region of 60-MDa plasmid pXO2 were used for amplification. The amplification products were detected by using biotin- and fluorescein-labeled probes in a novel dual-probe hybridization format. Using the combination of PCR amplification and dual-probe hybridization, we detected two copies of the bacterial genome. Because the PCR assay could detect a minimum of 100 unprocessed spores per PCR mixture, we attempted to facilitate the release of DNA by comparing the effect of limited spore germination with the effect of mechanical spore disruption prior to PCR amplification. The two methods were equally effective and allowed us to identify single spores of B. anthracis in PCR mixtures.     

Application of the real-time PCR for the detection of airborne microbial pathogens in reference to the anthrax spores

To establish the rapid detection method of airborne bacterial spores, we examined Bacillus anthracis spores by real-time PCR. One hundred liters of air were trapped on a filter of an air monitor device. After it was suspended in PBS, spores of B. anthracis were artificially added. The suspension was also heated at 95 degrees C for 15 min and used for real-time PCR using anthrax-specific primers. A single cell of B. anthracis was detected by real-time PCR within 1 h. Our results provide evidence that anthrax spores from the atmosphere can be detected rapidly, suggesting that real-time PCR provides a flexible and powerful tool to prevent epidemics.