Bombyx mori as a model for Niallia circulans pathogenicity.

Increasing incidences of resistance to antibiotics by pathogenic bacteria is a worldwide concern and isolation of antibiotic-resistant strains of Niallia circulans (formerly known as Bacillus circulans), an opportunistic human pathogen, has been reported. Due to their lack of ethical constraints as well as their cost-effective rearing, invertebrates have been commonly used to study infection by bacteria pathogenic to humans. In this study, we demonstrate that a foodborne strain of N. circulans kills larvae of the silkworm, Bombyx mori within 48 h after hemolymph injection. The infected larvae turned black with an increase in the phenoloxidase (PO) activity in the hemolymph. Midgut injection of N. circulans resulted in the killing of larvae within 96 h. A significant increase in bacterial load was observed in the hemolymph 12 h after infection. The viable hemocyte number decreased to 48% within 12 h of injection. RT-qPCR analysis revealed that upon hemolymph infection with N. circulans the expression of the antimicrobial peptide (AMP) genes, Bmdefensin-B and Bmgloverin-3, were upregulated 2.5- and 1.8-fold, respectively, whereas 1.6-fold upregulation was observed for BmToll-2 in the larval fat body. Therapeutic effects of antibiotics like tetracycline, imipenem, ceftriaxone, ampicillin, and clindamycin were observed against N. circulans in the Bombyx larvae with varying efficacies. Results from this study suggest that larvae of B. mori can be used as infection models for screening therapeutics that are effective against N. circulans.

Klebsiella pneumoniae pathogenicity in silk moth larvae infection model.

The emergence of antibiotic resistant bacteria is a major health concern worldwide in recent years. The objective of this study is to establish the larvae of the silk moth (commonly known as silkworm), Bombyx mori as an infection model to study antibacterial effect of antibiotics against Klebsiella pneumoniae. In this study, the pathogenicity of a K. pneumoniae strain isolated from food to silkworm larvae was examined. Within 72 h of bacterial injection, all silkworm larvae were killed in a dose-dependent manner with their body color turning into black due to increased melanization. Bacterial numbers in the larval hemolymph (blood) significantly increased after 9 h of infection with a decrease in viable circulatory hemocytes in hemolymph. When presented with bacteria laden leaves, larvae did not eat but injection of bacteria directly into the midgut killed larvae within 12 h with a higher load required in comparison to that required for the killing by hemolymph injection. Administration of four different antibiotics into larval hemolymph showed therapeutic effect at different doses with varying efficacies against hemolymph-injected K. pneumoniae. These results indicate that the silkworm larvae can be used as an infection model not only to study the pathogenicity of K. pneumoniae but also to perform rapid screening for the identification of antibiotics effective against multidrug-resistant strains of K. pneumoniae.