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.