The variation of highly pathogenic avian influenza H5N1 virus results in gradually increased virulence in poultry, and human cases continue to accumulate. The neuraminidase (NA) stalk region of influenza virus varies considerably and may associate with its virulence. The NA stalk region of all N1 subtype influenza A viruses can be divided into six different stalk-motifs, H5N1/2004-like (NA-wt), WSN-like, H5N1/97-like, PR/8-like, H7N1/99-like and H5N1/96-like. The NA-wt is a special NA stalk-motif which was first observed in H5N1 influenza virus in 2000, with a 20-amino acid deletion in the 49th to 68th positions of the stalk region. Here we show that there is a gradual increase of the special NA stalk-motif in H5N1 isolates from 2000 to 2007, and notably, the special stalk-motif is observed in all 173 H5N1 human isolates from 2004 to 2007. The recombinant H5N1 virus with the special stalk-motif possesses the highest virulence and pathogenicity in chicken and mice, while the recombinant viruses with the other stalk-motifs display attenuated phenotype. This indicates that the special stalk-motif has contributed to the high virulence and pathogenicity of H5N1 isolates since 2000. The gradually increasing emergence of the special NA stalk-motif in H5N1 isolates, especially in human isolates, deserves attention by all.

During 2004-2006 swine influenza virus surveillance, two strains of H3N8 influenza viruses were isolated from pigs in central China. Sequence and phylogenetic analyses of eight gene segments revealed that the two swine isolates were of equine origin and most closely related to European equine H3N8 influenza viruses from the early 1990s. Comparison of hemagglutinin (HA) amino acid sequences showed several important substitutions. One substitution caused the loss of a potential glycosylation site, and two substitutions, located at the cleavage site and adjacent to the receptor-binding pocket, respectively, had been reported previously in canine H3 HAs. This expansion of host range of equine H3N8 influenza viruses with mutations in the HA protein might raise the possibility of transmission of these viruses to humans.

SUMMARY For successful infection, bacteriophages must overcome multiple barriers to transport the genome and proteins across the bacterial cell envelope. We use cryo-electron tomography to study infection initiation of phage P22 in Salmonella enterica sv. Typhimurium, revealing how a channel forms to allow genome translocation into the cytoplasm. Our results show free phages initially attaching obliquely to the cell through interactions between the O antigen and two of the six tailspikes; the tail needle also abuts the cell surface. The virion then orients to the perpendicular and the needle penetrates the outer membrane. The needle is released and the internal head protein gp7* is ejected and assembles into an extra-cellular channel extending from the gp10 baseplate to the cell surface. A second protein, gp20, is ejected and assembles into a structure that extends the extra-cellular channel across the outer membrane into the periplasm. Insertion of the third ejected protein gp16 into the cytoplasmic membrane likely completes the overall trans-envelope channel into the cytoplasm. Construction of a trans-envelope channel is an essential step during infection by all short-tailed phages of Gram-negative bacteria because such virions cannot directly deliver their genome into the cell cytoplasm.

Bacteriophage SP6 exhibits dual-host adsorption specificity. The SP6 tailspikes are recognized as important in host range determination but the mechanisms underlying dual host specificity are unknown. Cryo-electron tomography and sub-tomogram classification were used to analyze the SP6 virion with a particular focus on the interaction of tailspikes with host membranes. The SP6 tail is surrounded by six V-shaped structures that interconnect in forming a hand-over-hand hexameric garland. Each V-shaped structure consists of two trimeric tailspike proteins: gp46 and gp47, connected through the adaptor protein gp37. SP6 infection of Salmonella enterica serovars Typhimurium and Newport results in distinguishable changes in tailspike orientation, providing the first direct demonstration how tailspikes can confer dual host adsorption specificity. SP6 also infects S. Typhimurium strains lacking O antigen; in these infections tailspikes have no apparent specific role and the phage tail must therefore interact with a distinct receptor to allow infection.