Intranasal administration of live Lactobacillus species facilitates protection against influenza virus infection in mice

► Lactobacillus species facilitates protection against influenza virus infection in mice. ► Intranasal administration bestowed higher protective efficacy than the oral route. ► Live bacteria conferred higher protection against influenza than dead bacteria. ► There were huge differences in protective effects of various Lactobacillus strains on influenza virus infection. Influenza virus infections continue to be a significant public health problem. For improved therapies and preventive measures against influenza, there has been an increased tendency in modern medicine involving the use of probiotics. In this study, we compared the protective efficacy of various live and dead Lactobacillus species against challenge with influenza virus in mice according to the administration route and dose. In addition, to understand the underlying mechanism behind this clinical protective effect, we performed immunologic assays including examination of IgA levels and cytokine profiles in the lung. The survival rate of mice receiving intranasal administration of Lactobacillus was higher than after oral administration, and administration of live bacteria was more protective than of dead bacteria. The lung levels of interleukin (IL)-12 and IgA were significantly increased (P