Quantitative evaluation of Mycobacterium abscessus clinical isolate virulence using a silkworm infection model

Mycobacterium abscessus causes chronic skin infections, lung diseases, and systemic or disseminated infections. Here we investigated whether the virulence of M. abscessus clinical isolates could be evaluated by calculating the median lethal dose (LD50) in a silkworm infection model. M. abscessus subsp. abscessus cells were injected into the silkworm hemolymph. When reared at 37{degrees}C, the silkworms died within 2 days post-infection with M. abscessus subsp. abscessus. Viable cell numbers of M. abscessus subsp. abscessus increased in the hemolymph of silkworms injected with M. abscessus subsp. abscessus. Silkworms were not killed by injections with heat-killed M. abscessus subsp. abscessus cells. The administration of clarithromycin, an antibacterial drug used to treat the infection, prolonged the survival time of silkworms injected with M. abscessus subsp. abscessus. The LD50 values of 7 clinical isolates were determined using the silkworm infection model and differed by up to 9-fold. The Mb-17 isolate, which was identified as a pathogenic strain in the silkworm infection model, induced more detachment of human THP-1-derived macrophages during infection than the Mb-10 isolate. These findings suggest that the silkworm M. abscessus infection model with can be used to quantitatively evaluate the virulence of M. abscessus clinical isolates in a short time period.