Neisseria perflava isolated from a clinical sample reduces influenza virus replication in respiratory cells

Various bacteria are present in the oral cavity and constitute the oral microbiota. Although the oral microbiota has been analyzed using next-generation sequencing, few studies have investigated whether specific commensal bacteria directly affect immune responses to infections. Here, we focused on Neisseria species present in the oral cavity and investigated their effects on respiratory cells infected with several viruses.

Six Neisseria species were isolated from human saliva. The epithelial cell lines were stimulated with bacterial culture supernatants before viral infection. Changes in the viral susceptibility were assessed.

Culture supernatants of two Neisseria species were found to affect cells susceptible to influenza viral infection and suppress influenza viral replication. The mechanism underlying the suppression of N. perflava was further investigated. This activity was observed in the 10-30 kDa protein range fractionated by ultrafiltration. Although viral replication was suppressed by stimulation with bacterial proteins, the infection efficiency of the virus and cytokine production were unaffected. Replication of SARS-CoV-2 and human rhinovirus were also suppressed.

Viral infection was performed after supernatant stimulation, suggesting that exposure to oral bacteria directly affects viral infection in the surrounding cells. This effect has been observed for several viruses. Viral genome replication in cells may be suppressed by enhanced expression of viral replication suppression genes. Further analyses are required to elucidate the detailed underlying mechanisms.