A virus has a girth of only a few thousand atoms –so small, they can infect bacteria. These microscopic entities are protected by a protein shell or capsid, which grows and changes shape as the virus matures. Seen by Atomic Force Microscopy, young HK97 viruses – similar to the cold sore virus Herpes simplex – are pentagon-shaped (top left). They mature into football-like structures (bottom left). Arranged in a regular pattern of hexagons and pentagons, shell protein connections or cross-links increase with age. CGI (yellow and blue hexagons) highlights the changing structure (right-hand images). To test viral vulnerability researchers prodded capsids with tiny probes. While the more mature HK97 virus has chainmail-like armour its extra cross-links add surprisingly little strength. However, they do provide durability – the younger capsids disintegrate after relatively few pokes. Identifying weak points in the viral life cycle will inform the design of effective drugs.
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