Influenza virus particles are usually spherical and about 100 nm in diameter (80–120 nm), although virions may dis play great variation in size (Figure 1).
Fig1. Influenza virus. A: Electron micrograph of influenza virus A/Hong Kong/1/68(H3N2). Note the pleomorphic shapes and glycoprotein projections covering particle surfaces (315,000×). (Courtesy of FA Murphy and EL Palmer.) B: Schematic view of influenza. Virus particles have segmented genomes consisting of seven or eight different RNA molecules, each coated by capsid proteins and forming helical nucleocapsids. Viral glycoproteins (hemagglutinin and neuraminidase) protrude as spikes through the lipid envelope. (Reproduced with permission from Willey JM, Sherwood LM, Woolverton CJ: Prescott, Harley, and Klein’s Microbiology, 7th ed. McGraw Hill, 2008. © McGraw-Hill Education.)
The single-stranded, negative-sense RNA genomes of influenza A and B viruses occur as eight separate segments; influenza C viruses contain seven segments of RNA, lacking a neuraminidase gene. Sizes and protein-coding assignments are known for all the segments (Table 1). Most of the segments code for a single protein. The first 12–13 nucleotides at each end of each genomic segment are conserved among all eight RNA segments; these sequences are important in viral transcription.
Table1. Coding Assignments of Influenza Virus A RNA Segments
Influenza virus particles contain nine different structural proteins. The nucleoprotein (NP) associates with the viral RNA to form a ribonucleoprotein (RNP) structure 9 nm in diameter that assumes a helical configuration and forms the viral nucleocapsid. Three large proteins (PB1, PB2, and PA) are bound to the viral RNP and are responsible for RNA transcription and replication. The matrix (M1 ) protein, which forms a shell underneath the viral lipid envelope, is important in particle morphogenesis and is a major component of the virion (∼40% of viral protein).
A lipid envelope derived from the cell surrounds the virus particle. Two virus-encoded glycoproteins, hemagglutinin (HA) and neuraminidase (NA), are inserted into the envelope and are exposed as spikes about 10 nm long on the surface of the particle. These two surface glycoproteins determine antigenic variation of influenza viruses and host immunity. The HA represents about 25% of viral protein and the NA about 5%. The M2 ion channel protein and the NS2 protein are also present in the envelope but at only a few copies per particle.
Because of the segmented nature of the genome, when a cell is coinfected by two different viruses of a given type, mixtures of parental gene segments may be assembled into progeny virions. This phenomenon, called genetic reassortment, may result in sudden changes in viral surface antigens—a property that explains the epidemiologic features of influenza and poses significant problems for vaccine development.
Influenza viruses are relatively hardy in vitro and may be stored at 0–4°C for weeks without loss of viability. Lipid solvents, protein denaturants, formaldehyde, and irradiation destroy infectivity. Both infectivity and hemagglutination are more resistant to inactivation at alkaline pH than at acid pH.
