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Scientists know that bacteria in the respiratory tract, such as specific Staphylococcus aureus strains, can boost influenza infectivity by supplying a protease that cleaves the viral spike protein hemagglutinin, a step that facilitates viral host entry. The bacterium Porphyromonas gingivalis causes periodontal disease and has been associated with severe cases of influenza, but scientists have not determined whether this connection involves hemagglutinin cleavage by a specific enzyme. In a recent from the Journal of Biological Chemistry, Noriaki Kamio and a team at Nihon University in Japan investigated the link between P. gingivalis and influenza infectivity.

The authors used immunofluorescence microscopy and a canine kidney cell line to determine that an element in the supernatant of P. gingivalis culture enhanced cell-to-cell spread of influenza, similar to cells treated with trypsin protease. The researchers hypothesized that the supernatant contains a protease that can cleave viral hemagglutinin. They suspected that the gingipain cysteine proteases Rgp and Kgp produced by P. gingivalis could perform this function.

They tested the involvement of each protease by adding individual inhibitors of Rgp and Kgp to the supernatant and found that only the Rgp inhibitor blocked hemagglutinin cleavage. In addition, the authors showed that supernatant from a P. gingivalis strain lacking Rgp could not cleave hemagglutinin. They also performed a plaque assay to measure viral titers and found that the viral concentration was lower when cells were incubated with the bacterial strain missing Rgp. Therefore, they concluded that Rgp likely cleaves hemagglutinin and enhances viral replication.

The authors suggested that future studies using human respiratory cells and animal models will help unravel details of how P. gingivalis strengthens influenza infectivity. These results may help scientists pinpoint proteins in other bacteria that may promote respiratory viral spread.