Surface ultrastructure of SARS coronavirus revealed by atomic force microscopy

Abstract

Atomic force microscopy has been used to probe the surface nanostructures of severe acute respiratory syndrome coronavirus (SARS-CoV). Single crown-like virion was directly visualized and quantitative measurements of the dimensions for the structural proteins were provided. A corona of large, distinctive spikes in the envelope was measured after treatment with hydroxyoctanoic acid. High-resolution images revealed that the surface of each single SARS-CoV was surrounded with at least 15 spherical spikes having a diameter of 7.29 +/- 0.73 nm, which is in close agreement with that of S glycoproteins earlier predicted through the genomes of SARS-CoV. This study represents the first direct characterization of the surface ultrastructures of SARS-CoV particles at the nanometre scale and offers new prospects for mapping viral surface properties.

Figure 1

Topographic AFM images of the native SARS‐CoV particle on mica. The scanning areas are 10 × 10 µm² and 2 × 2 µm² for the low‐ (A) and high‐resolution (B) images respectively. An image acquired by zooming into the boxed areas is displayed in B. The high‐resolution image and corresponding cursor profile (a) clearly reveal the presence of SARS‐CoV particles.

Figure 2

AFM images of the SARS‐CoV treated with hydroxyoctanoic acid. Height image (top row) and phase image (bottom row). Images of entire viruses are shown in left column (total scanning area 10 × 10 µm²). Images acquired by zooming into the boxed are displayed in the second and third columns (total scanning area 2 × 2 µm²) respectively. A new tip was used for the images in the third columns.

Figure 3

Topographic AFM images of different single SARS‐CoV virions before (the first column) and after (the second to fifth column) treatment with hydroxyoctanoic acid. Images acquired by zooming into the boxed areas in Fig. 2A are displayed in the B–E (two‐dimensional) and B′–E′ (three‐dimensional). Scale bar = 100 nm for each image in the first row. The corresponding cursor profiles (the third row) provide quantitative measurements of the dimensions for spike proteins.

Figure 4

AFM images of a SARS‐CoV particle after treatment with protease. Height image and phase image of an individual virion is shown in A and B respectively. Scale bar = 50 nm for A and B.

Figure 5

(A) Two‐ and (B) three‐dimensional AFM images and contour map (C) of a single SARS‐CoV virion. Scale bar = 100 nm in A and C. The corresponding cursor profiles (middle and bottom row) provide quantitative measurements of the dimensions for the spike proteins (1–15) displayed in C.

Figure 6

Images acquired by zooming into the boxed areas in Fig. 2C are displayed in A (height image) and B (phase image). Scale bar = 200 nm. In the height image (A) a section line is placed through two small spherical particles. The resulting profiles (a and b) are shown in the right panel. The corresponding cursor profiles (a and b) provide quantitative measurements of the dimensions for the particles released from virion itself.