Super‐infection by Bacillus thuringiensis H34 or 3a3b can lead to death in mice infected with the influenza A virus

Bacterial super‐infections are the main cause of complication and mortality after influenza virus (IAV) infection. Since Bacillus thuringiensis (Bt) is considered non‐pathogenic for humans and is widely sprayed in urban areas, the aim of this work was to evaluate the potential pathogenicity of a combined infection Bt‐IAV in a mouse model of pneumonia. Bacteria used for super‐infections were Bt serotype H34 isolated from human infection and the insecticidal strain 3a3b obtained from a commercial source. Virus strain was A/Scotland/20/74 (H3N2) adapted to BALB/c mice by serial lung passage. Combined infection with 4% of the viral lethal dose 50% (LD 50 ) and 10 2 spores of Bt H34 killed 40% of the mice. Mortality rates increased up to 55% and 100% when combined infections were done with respectively 10 4 and 10 7 spores. The insecticidal strain Bt 3a3b was less pathogenic than Bt H34. A dose of 10 4 spores associated with 4% of IAV LD 50 killed 50% of the mice. This inoculum must be compared with the doses usually sprayed in agriculture: 10 11 spores m −2 . Total protection against super‐infection was obtained when mice were treated with amantadine. Even if only a few cases of Bt human infection have been reported, these results suggest a possible risk for workers spraying Bt‐based biopesticides during flu outbreaks.