Ionic liquid-based observation technique for nonconductive materials in the scanning electron microscope: Application to the characterization of a rare earth ore

Microscopy Methods for Biofilm Imaging: Focus on SEM and VP-SEM Pros and Cons

Simple Summary

Bacterial biofilms cause infections that are often resistant to antibiotic treatments. Research about the formation and elimination of biofilms cannot be undertaken without detailed imaging techniques. In this review, traditional and cutting-edge microscopy methods to study biofilm structure, ultrastructure, and 3-D architecture, with particular emphasis on conventional scanning electron microscopy and variable pressure scanning electron microscopy, are addressed, with the respective advantages and disadvantages. When ultrastructural characterization of biofilm matrix and its embedded bacterial cells is needed, as in studies on the effects of drug treatments on biofilm, scanning electron microscopy with customized protocols such as the osmium tetroxide (OsO₄), ruthenium red (RR), tannic acid (TA), and ionic liquid (IL) must be preferred over other methods for the following: unparalleled image quality, magnification and resolution, minimal sample loss, and actual sample structure preservation. The first step to make a morphological assessment of the effect of the various pharmacological treatments on clinical biofilms is the production of images that faithfully reflect the structure of the sample. The extraction of quantitative parameters from images, possible using specific software, will allow for the scanning electron microscopy morphological evaluation to no longer be considered as an accessory technique, but a quantitative method to all effects.

Abstract

Several imaging methodologies have been used in biofilm studies, contributing to deepening the knowledge on their structure. This review illustrates the most widely used microscopy techniques in biofilm investigations, focusing on traditional and innovative scanning electron microscopy techniques such as scanning electron microscopy (SEM), variable pressure SEM (VP-SEM), environmental SEM (ESEM), and the more recent ambiental SEM (ASEM), ending with the cutting edge Cryo-SEM and focused ion beam SEM (FIB SEM), highlighting the pros and cons of several methods with particular emphasis on conventional SEM and VP-SEM. As each technique has its own advantages and disadvantages, the choice of the most appropriate method must be done carefully, based on the specific aim of the study. The evaluation of the drug effects on biofilm requires imaging methods that show the most detailed ultrastructural features of the biofilm. In this kind of research, the use of scanning electron microscopy with customized protocols such as osmium tetroxide (OsO₄), ruthenium red (RR), tannic acid (TA) staining, and ionic liquid (IL) treatment is unrivalled for its image quality, magnification, resolution, minimal sample loss, and actual sample structure preservation. The combined use of innovative SEM protocols and 3-D image analysis software will allow for quantitative data from SEM images to be extracted; in this way, data from images of samples that have undergone different antibiofilm treatments can be compared.

Characterization of Scardovia wiggsiae Biofilm by Original Scanning Electron Microscopy Protocol

Early childhood caries (ECC) is a severe manifestation of carious pathology with rapid and disruptive progression. The ECC microbiota includes a wide variety of bacterial species, among which is an anaerobic newly named species, Scardovia wiggsiae, a previously unidentified Bifidobacterium. Our aim was to provide the first ultrastructural characterization of S. wiggsiae and its biofilm by scanning electron microscopy (SEM) using a protocol that faithfully preserved the biofilm architecture and allowed an investigation at very high magnifications (order of nanometers) and with the appropriate resolution. To accomplish this task, we analyzed Streptococcus mutans’ biofilm by conventional SEM and VP-SEM protocols, in addition, we developed an original procedure, named OsO₄-RR-TA-IL, which avoids dehydration, drying and sputter coating. This innovative protocol allowed high-resolution and high-magnification imaging (from 10000× to 35000×) in high-vacuum and high-voltage conditions. After comparing three methods, we chose OsO₄-RR-TA-IL to investigate S. wiggsiae. It appeared as a fusiform elongated bacterium, without surface specialization, arranged in clusters and submerged in a rich biofilm matrix, which showed a well-developed micro-canalicular system. Our results provide the basis for the development of innovative strategies to quantify the effects of different treatments, in order to establish the best option to counteract ECC in pediatric patients.

Determining E1 and E2 values for yttrium aluminum garnet ceramics using the Duane-Hunt limit

A major challenge when performing scanning electron microscopy and X-ray analysis on many ceramic materials is their electrical insulation properties, which leads to buildup of the surface charge and reduced contrast in the secondary electron image. A new procedure was established to quantitatively determine the neutral state values, E₁ and E₂ , of yttrium aluminum garnet (YAG) ceramics using the Duane-Hunt limit (E_DHL ) of Bremsstrahlung, in order to eliminate this charge effect. Thirty-eight E_DHL values were linearly fitted with the last portion of X-ray spectra acquired under the incident energy, E₀ , from 0.35 to 5.0 kV. According to the distribution of E_DHL , two piecewise linear fitting was first employed with a breakpoint of 1.0 kV. Consequently, two intersection points of 0.54 and 2.48 kV, which correspond to E₁ and E₂ for YAG ceramics, were directly determined using a theoretical curve (E_DHL  = E₀ ). As a result, the high-resolution images of the YAG ceramic grain structure were successfully obtained using the calculated E₁ and E₂ values.

Chromosome interior observation by focused ion beam/scanning electron microscopy (FIB/SEM) using ionic liquid technique

Attempts to elucidate chromosome structure have long remained elusive. Electron microscopy is useful for chromosome structure research because of its high resolution and magnification. However, biological samples such as chromosomes need to be subjected to various preparation steps, including dehydration, drying, and metal/carbon coating, which may induce shrinkage and artifacts. The ionic liquid technique has recently been developed and it enables sample preparation without dehydration, drying, or coating, providing a sample that is closer to the native condition. Concurrently, focused ion beam/scanning electron microscopy (FIB/SEM) has been developed, allowing the investigation and direct analysis of chromosome interiors. In this study, we investigated chromosome interiors by FIB/SEM using plant and human chromosomes prepared by the ionic liquid technique. As a result, two types of chromosomes, with and without cavities, were visualized, both for barley and human chromosomes prepared by critical point drying. However, chromosome interiors were revealed only as a solid structure, lacking cavities, when prepared by the ionic liquid technique. Our results suggest that the existence and size of cavities depend on the preparation procedures. We conclude that combination of the ionic liquid technique and FIB/SEM is a powerful tool for chromosome study.