Electron micrographs of human and avian influenza viruses with high and low pathogenicity

Background: An outbreak of highly pathogenic avian influenza (HPAI) H5N1 virus in Thailand was first reported in 2004. To date, electron micrographs demonstrating the morphology of HPAI H5N1 virus particle are quite limited. Objective: To demonstrate the morphology of HPAI H5N1 virus particles, avian influenza viruses with low pathogenicity, seasonal influenza viruses, and H5N1 structural components in infected cells. The M amino acid residues that might affect the viral morphology were also analyzed. Methods: Electron micrographs of negatively-stained virus particles and positively-stained thin sections of the HPAI H5N1 virus infected cells were visualized under a transmission electron microscope. M amino acid sequences of the study viruses were retrieved from the GenBank database and aligned with those of reference strains with known morphology and residues that are unique for the morphological type of the virus particles. Results: Morphologically, three forms of influenza virus particles, spherical, regular, and irregular rods, and long filamentous particles, were demonstrated. However, the spherical form was the most predominant morphological type and accounted for more than 80% of the virus populations examined. In addition, the viral entry and exit steps including incomplete particles in infected Madin-Darby canine kidney cells were visualized. Our analyses did not find any M amino acid residues that might influence the viral morphology. Conclusion: Of all virus isolates studied, we demonstrated that the spherical particles were the major population observed regardless of virus subtype, host of origin, virus virulence, or passage history. Our study suggested that the morphology of influenza virus particles released, might not be strongly influenced by M gene polymorphism.