Scanning electron probe x-ray microanalysis of elemental distributions in freeze-dried cryosections of Bacillus coagulans spores

Rapid and reliable detection of bacterial endospores in environmental samples by diagnostic electron microscopy combined with X-ray microanalysis

Diagnostic negative staining electron microscopy is a front-line method for the rapid investigation of environmental and clinical samples in emergency situations caused by bioterrorism or outbreaks of an infectious disease. Spores of anthrax are one of the diagnostic targets in case of bioterrorism, because they have been used as a bio-weapon in the past and their production and transmission are rather simple. With negative staining electron microscopy bacterial spores can be identified based on their morphology at the single cell level. However, because of their particular density, no internal structures are visible which sometimes makes it difficult to distinguish spores from particles with a similar size and shape that are frequently present in environmental samples. Spores contain a high concentration of calcium ions besides other elements, which may allow a proper discrimination of spores from other suspicious particles. To investigate this hypothesis, negative staining electron microscopy, using either transmission or scanning electron microscopes, was combined with energy dispersive X-ray microanalysis, which reveals the element content of individual nanoparticles. A peak pattern consisting of calcium, sulphur and phosphorus was found as a typical signature within the X-ray spectrum of spores in various Clostridium and Bacillus species, including all strains of anthrax (Bacillus anthracis) tested. Moreover, spores could be reliably identified by this combined approach in environmental samples, like household products, soil or various presumed bioterrorist samples. In summary, the use of X-ray spectroscopy, either directly in the transmission electron microscope, or in a correlative approach by using scanning electron microscopy, improves the emergency diagnostics of suspicious environmental samples.

Spatially resolved characterization of water and ion incorporation in Bacillus spores

We present the first direct visualization and quantification of water and ion uptake into the core of individual dormant Bacillus thuringiensis subsp. israelensis (B. thuringiensis subsp. israelensis) endospores. Isotopic and elemental gradients in the B. thuringiensis subsp. israelensis spores show the permeation and incorporation of deuterium in deuterated water (D(2)O) and solvated ions throughout individual spores, including the spore core. Under hydrated conditions, incorporation into a spore occurs on a time scale of minutes, with subsequent uptake of the permeating species continuing over a period of days. The distribution of available adsorption sites is shown to vary with the permeating species. Adsorption sites for Li(+), Cs(+), and Cl(-) are more abundant within the spore outer structures (exosporium, coat, and cortex) relative to the core, while F(-) adsorption sites are more abundant in the core. The results presented here demonstrate that elemental abundance and distribution in dormant spores are influenced by the ambient environment. As such, this study highlights the importance of understanding how microbial elemental and isotopic signatures can be altered postproduction, including during sample preparation for analysis, and therefore, this study is immediately relevant to the use of elemental and isotopic markers in environmental microbiology and microbial forensics.

The silicon layer supports acid resistance of Bacillus cereus spores

Silicon (Si) is considered to be a "quasiessential" element for most living organisms. However, silicate uptake in bacteria and its physiological functions have remained obscure. We observed that Si is deposited in a spore coat layer of nanometer-sized particles in Bacillus cereus and that the Si layer enhances acid resistance. The novel acid resistance of the spore mediated by Si encapsulation was also observed in other Bacillus strains, representing a general adaptation enhancing survival under acidic conditions.

Protozoal digestion of coat-defective Bacillus subtilis spores produces "rinds" composed of insoluble coat protein

The Bacillus subtilis spore coat is a multilayer, proteinaceous structure that consists of more than 50 proteins. Located on the surface of the spore, the coat provides resistance to potentially toxic molecules as well as to predation by the protozoan Tetrahymena thermophila. When coat-defective spores are fed to Tetrahymena, the spores are readily digested. However, a residue termed a "rind" that looks like coat material remains. As observed with a phase-contrast microscope, the rinds are spherical or hemispherical structures that appear to be devoid of internal contents. Atomic force microscopy and chemical analyses showed that (i) the rinds are composed of insoluble protein largely derived from both outer and inner spore coat layers, (ii) the amorphous layer of the outer coat is largely responsible for providing spore resistance to protozoal digestion, and (iii) the rinds and intact spores do not contain significant levels of silicon.

Differentiation of spores of Bacillus subtilis grown in different media by elemental characterization using time-of-flight secondary ion mass spectrometry

We demonstrate the use of time-of-flight secondary ion mass spectrometry (TOF-SIMS) in a forensics application to distinguish Bacillus subtilis spores grown in various media based on the elemental signatures of the spores. Triplicate cultures grown in each of four different media were analyzed to obtain TOF-SIMS signatures comprised of 16 elemental intensities. Analysis of variance was unable to distinguish growth medium types based on 40Ca-normalized signatures of any single normalized element. Principal component analysis proved successful in separating the spores into groups consistent with the media in which they were prepared. Confusion matrices constructed using nearest-neighbor classification of the PCA scores confirmed the predictive utility of TOF-SIMS elemental signatures in identifying sporulation medium. Theoretical calculations based on the number and density of spores in an analysis area indicate an analytical sample size of about 1 ng, making this technique an attractive method for bioforensics applications.

Improvement of selective removal of heavy metals in cyanobacteria by NaOH treatment

In the freshwater cyanobacterium, Tolypothrix tenuis, treatment with 0.1 M NaOH increased its Cd-selective adsorption ability in the presence of Ca(2+) or Mg(2+). The selective adsorption was also achieved by other alkaline treatments. Energy-distributed spectroscopy analysis revealed that Cd(2+) was found mainly on the surface of non-treated cells, whereas it was distributed throughout the cell after NaOH treatment. The alkaline treatment was effective in increasing the selective adsorption ability of the cyanobacterium for other bivalent heavy metals such as Cu(2+), Pb(2+) and Zn(2+). The treatment was also applicable to Anabaena variabilis and Microcystis aeruginosa, which are typical cyanobacteria causing algal blooms. The main binding site of Cd(2+) in NaOH-treated cells is assumed to be the carboxyl groups because the binding ability of the cells was diminished by the esterification of carboxyl groups. These results suggest that alkaline treatment of cyanobacteria is a useful technique for producing biosorbents having highly specific binding abilities for heavy metals.

Investigation of bacterial spore structure by high resolution solid-state nuclear magnetic resonance spectroscopy and transmission electron microscopy

High resolution solid-state nuclear magnetic resonance spectroscopy (NMR) in combination with transmission electron microscopy (TEM) of spores of Bacillus cereus, an outer coatless mutant B. subtilis 322, an inner coatless mutant B. subtilis 325 and of germinated spores of B. subtilis CMCC 604 were carried out. Structural differences in the coats, mainly protein of spores were reflected by NMR spectra which indicated also differences in molecular mobility of carbohydrates which was partially attributed to the cortex. Dipicolinic acid (DPA) of spores of B. cereus displayed a high degree of solid state order and may be crystalline. Heat activation was studied on spores of B. subtilis 357 lux + and revealed a structural change when analysed by TEM but this was not associated with increases in molecular mobility since no effects were measured by NMR.

Bacillus subtilis spore coat

In response to starvation, bacilli and clostridia undergo a specialized program of development that results in the production of a highly resistant dormant cell type known as the spore. A proteinacious shell, called the coat, encases the spore and plays a major role in spore survival. The coat is composed of over 25 polypeptide species, organized into several morphologically distinct layers. The mechanisms that guide coat assembly have been largely unknown until recently. We now know that proper formation of the coat relies on the genetic program that guides the synthesis of spore components during development as well as on morphogenetic proteins dedicated to coat assembly. Over 20 structural and morphogenetic genes have been cloned. In this review, we consider the contributions of the known coat and morphogenetic proteins to coat function and assembly. We present a model that describes how morphogenetic proteins direct coat assembly to the specific subcellular site of the nascent spore surface and how they establish the coat layers. We also discuss the importance of posttranslational processing of coat proteins in coat morphogenesis. Finally, we review some of the major outstanding questions in the field.

Freeze-substitution studies of bacteria

Typically, models of bacterial structure combine biochemical data obtained from bulk analyses of cell populations with electron microscopic observation of individual cells. Recent development of a battery of cryotechniques specific for biological electron microscopy have begun to supercede routine procedures such as conventional thin sectioning. One of these cryotechniques, freeze-substitution, combines the advantages of ultrarapid freezing with standard microtomy methods. This technique is particularly well suited to the examination of bacterial structure and has yielded additional ultrastructural information consistent with biochemical data but often challenging models of cell structure obtained from conventional microscopical methods. In addition to retaining more accurately the spatial distribution of cell components, freeze-substitution has been successfully combined with immunochemical labelling techniques and has enabled identification and localization of specific molecules both within the cell and on the cell surface. In this review, I describe current ideas on bacterial ultrastructure, modified in accordance with new data obtained from recent freeze-substitution studies.

Surface layers of bacteria

Since bacteria are so small, microscopy has traditionally been used to study them as individual cells. To this end, electron microscopy has been a most powerful tool for studying bacterial surfaces; the viewing of macromolecular arrangements of some surfaces is now possible. This review compares older conventional electron-microscopic methods with new cryotechniques currently available and the results each has produced. Emphasis is not placed on the methodology but, rather, on the importance of the results in terms of our perception of the makeup and function of bacterial surfaces and their interaction with the surrounding environment.

Biological calcium absorption edge imaging using monochromatic synchrotron radiation

Soft X-ray contact absorption edge images of unfixed, unstained biological specimens were made using monochromatic synchrotron radiation. X-ray contact replicas of unfixed, hydrated biological specimens at the nitrogen absorption edge and above and below the CaLIII absorption edge were compared to comparative conventional morphological and elemental high-resolution imaging methods (scanning and transmission electron microscopy, TEM-histochemistry and TEM-X-ray microanalysis). Soft X-ray absorption edge images made above the calcium absorption edge clearly revealed morphological detail and identified regions ladened with calcium as verified by TEM histochemistry of identical spores. Similarly, nitrogen absorption edge images identified residual nitrogenous material in the spore resuspension medium, and non-viable spores with nitrogen loss due to protoplast disaggregation.

High-resolution solid-state 13C nuclear magnetic resonance of bacterial spores: identification of the alpha-carbon signal of dipicolinic acid

Natural-abundance solid-state 13C nuclear magnetic resonance spectra were obtained for bacterial spores for the first time by using the technique of cross-polarization magic-angle-spinning nuclear magnetic resonance spectroscopy. A resonance at about 150 ppm, detectable in spore samples having a Mn content of less than 0.05%, was consistent with an identification as the alpha-carbon signal of calcium dipicolinate; this signal was missing from a spore sample treated with acid to release dipicolinate and from a spore coat preparation. Carbohydrate peaks were particularly intense in spores and coat preparations of Bacillus macerans. Signals ascribable to beta-hydroxybutyrate were prominent in a B. cereus sample.

Frontiers in electron probe microanalysis: application to cell physiology

The application of electron probe microanalysis techniques, using X-ray and electron energy loss instruments, to problems in cell physiology is reviewed. The details of the special methodological requirements for the analysis of cryosections at high spatial resolution in an analytical electron microscope are discussed together with a comprehensive review of data obtained on major organ systems and cell types.

Electron probe analysis, X-ray mapping, and electron energy-loss spectroscopy of calcium, magnesium, and monovalent ions in log-phase and in dividing Escherichia coli B cells

The elemental composition of individual cells of rapidly frozen and cryosectioned Escherichia coli B was measured with electron optical microanalytic methods. The Ca content was high (26.2 mmol/kg) in a 10-nm-wide region of the cell envelope. Amounts of cytoplasmic Ca in actively dividing cells were significantly higher (32.6 mmol/kg [dry weight]) than in the log-phase (1.5 mmol/kg) cells. Cellular Mg was 205 mmol/kg (dry weight) and it was uniformly distributed throughout the cell. Cells washed in distilled water before freezing lost monovalent ions (Na, Cl, and K), but the membrane-bound Ca and cellular Mg were not reduced, indicating that cellular Mg and membrane Ca are more tightly bound.

Mechanism of chemical manipulation of the heat resistance of Clostridium perfringens spores

The mechanism of chemical manipulation of the heat resistance of Clostridium perfringens type A spores was studied. Spores were converted to various ionic forms to base-exchange technique and these spores were heated at 95 degrees C. Of the four ionic forms, i.e. Ca2+, Na+, H+ and native, only hydrogen spores appeared to have been rapidly inactivated at this temperature, when survivors were enumerated on the ordinary plating medium. However, the recovery of the survivors was improved when the plating medium was supplemented with lysozyme, and more dramatically when the heated spores were pretreated with alkali followed by plating in the medium containing lysozyme. In contrast to crucial damage to germination, in particular to spore lytic enzyme, no appreciable amount of DPA was released from the heat-damaged H-spores. These results suggest that a germination system is involved in the thermal inactivation of the ionic forms of spores, and that exchangeable cation load plays a role in protection from thermal damage of the germination system within the spore. An enhancement of thermal stability of spore lytic enzyme in the presence of a high concentration of NaCl was consistent with the hypothesis.

Electron probe and electron energy loss analysis in biology

Methods, applications and limitations of quantitative electron probe analysis, X-ray mapping, electron energy loss analysis and energy filtered imaging are described, with emphasis on the analysis of thin (less than 200nm) cryosections. Energy dispersion electron probe analysis can measure reliably 5 to 10mM/Kg of biologically prevalent elements in 50nm diameter areas of 100 to 150 nm thick cryo sections during 100-300 sec counts. The minimal detectable mass (MDM) with a conventional thermionic electron source is approximately 10(-19)g Fe (100 sec count) and can be reduced to 10(-20)g through the use of a field emission gun (FEG). A spatial resolution of 8.7nm is demonstrated in two-dimensional Fourier transforms of Mo X-ray maps of stained catalase crystals. Significant biological results of quantitative electron probe analysis include the measurement of total Ca released from the Mg and K taken up by the sarcoplasmic reticulum during muscle contraction, and the demonstration that mitochondria do not contribute to the physiological regulation of cytoplasmic free Ca levels in cardiac, vascular smooth and striated muscle. Electron energy loss analysis (EELS) promises a significant improvement in sensitivity for the measurement of Ca; based on statistical errors of the measurement, 250 microM/Kg Ca should be measureable with EELS in 250 sec. through the Ca L-edge loss. The use of a doubly corrected magnetic sector spectrometer as a transmission electron microscope imaging filter outside the microscope vacuum is illustrated, and the resolution of the iron core (7.5nm) and surrounding organic shell of single ferritin molecules is demonstrated in, respectively, iron M and carbon K loss images.