A simplified microwell pseudoreplica for the detection of viruses by electron microscopy and immunoelectron microscopy

Detection limit of negative staining electron microscopy for the diagnosis of bioterrorism-related micro-organisms

Aims:  To determine the detection limit of diagnostic negative staining electron microscopy for the diagnosis of pathogens that could be used for bioterrorism.

Methods and Results:  Suspensions of vaccinia poxvirus and endospores of Bacillus subtilis were used at defined concentrations as a model for poxviruses and spores of anthrax (Bacillus anthracis), both of which are pathogens that could be used for bioterrorist attacks. Negative staining electron microscopy was performed directly or after sedimentation of these suspensions on to the sample supports using airfuge ultracentrifugation. For both virus and spores, the detection limit using direct adsorption of a 10‐μl sample volume onto the sample support was 10⁶ particles per ml. Using airfuge ultracentrifugation with a sample volume of 80 μl, the detection limit could be reduced to 10⁵ particles per ml for spores and to 5 × 10⁴ particles per ml for poxviruses. The influence on particle detection of incubation time, washing and adsorption procedures was investigated.

Conclusions:  The reproducibility and sensitivity of the method were acceptable, particularly considering the small sample volume and low particle number applied onto the sample support.

Significance and Impact of the Study:  Diagnostic negative staining electron microscopy is used for the diagnosis of pathogens in emergency situations because it allows a rapid examination of all particulate matter down to the nanometre scale. This study provides precise detection limit for the method, an important factor for the validation and improvement of the technique.

Use of protein A in the serum-in-agar diffusion method in immune electron microscopy for detection of virus particles in cell culture

A modified technique using protein A in the serum-in-agar (SIA) method for immune electron microscopy (IEM) was presented. Grids coated with staphylococcal protein A were floated on samples mounted on agar containing 2% antiserum and incubated at 37 C, for 60 min. After washing and staining, the grids were observed in an electron microscope. The effects of protein A on virus detection were evaluated using poliovirus and bovine rotavirus infected cell culture fluids. The results showed that the technique using protein A (PA-SIA) had at least 10-fold higher sensitivity for virus detection than the original SIA. The optimal concentration of protein A was 1 to 10 micrograms/ml for coating the grids to trap virus particles. The PA-SIA method was also compared with immunosorbent electron microscopy (ISEM). The former showed higher or at least the same sensitivity and some advantages in detecting antigen-antibody reaction than the latter method. These results indicate that our PA-SIA method may be superior to other IEM techniques presented previously for the detection and identification of viruses.

Enumeration of enterovirus particles by scanning electron microscopy

Enumeration of virus particles requires relatively concentrated and uniformly dispersed virus preparations, which is difficult to achieve by the usual methods of negative staining and transmission electron microscopy. We have developed an electrophoretic method that concentrates enteroviruses onto a polycarbonate membrane for examination by high-resolution scanning electron microscopy. The electrophoretic apparatus comprises three chambers in electrical series, each containing 3.5 ml of dilute buffer. The center chamber is inoculated with virus. A 15-nm porosity membrane, which does not pass virus, separates the center from the side chambers. A constant current is applied, and chilled buffer is pumped past the electrodes for 2 h. The virus suspension is recovered, and changes in titer (or radioactivity if labeled virus is used) due to electrophoresis are measured. Buffer pH, relative to the viral isoelectric points, determines the direction of virus migration. Particle counts are calculated from the mean of 25 randomly chosen fields photographed at 35-60,000 X magnification and related to titers measured by plaque assay.