Super-infection by Bacillus thuringiensis H34 or 3a3b call 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) i s 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 com bined infection Bt-IAV in a mouse model of pneumonia. Bacteria used for sup er-infections were Bt serotype H34 isolated from human infection and the in secticidal strain 3a3b obtained from a commercial source. Virus strain was A/Scotland/20/74 (H3N2) adapted to BALB/c mice by serial lung passage. Comb ined infection with 4% of the viral lethal dose 50% (LD50) 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) s pores. The insecticidal strain Bt 3a3b was less pathogenic than Bt H34. A d ose of 10(4) spores associated with 4% of IAV LD50 killed 50% of the mice. This inoculum must be compared with the doses usually sprayed in agricultur e: 10(11) spores m(-2). Total protection against superinfection was obtaine d when mice were treated with amantadine. Even if only a few cases of Bt hu man infection have been reported. these results suggest a possible risk rot workers spraying Bt-based biopesticides during flu outbreaks. (C) 2000 Fed eration of European Microbiological Societies, Published by Elsevier Scienc e B.V. All rights reserved.