Identification and Characterization of Bacillus cereus SW7-1 in Bombyx mori (Lepidoptera: Bombycidae)

Area of Science:

• Microbiology
• Animal Pathology
• Biotechnology

Background:

• Bacterial diseases in silkworms lead to substantial economic losses in the sericulture industry.
• Identifying and characterizing these pathogens is crucial for disease management and prevention.

Purpose of the Study:

• To isolate and identify the bacterial pathogen responsible for disease outbreaks in silkworms.
• To characterize the isolated strain's biological and biochemical properties.
• To assess its pathogenicity and antibiotic susceptibility.

Main Methods:

• Isolation of the pathogenic bacterial strain (SW7-1) from diseased silkworms.
• Bacteriological characterization, including colony morphology and Gram staining.
• 16S rRNA gene sequencing for precise identification.
• Biochemical tests, enzyme activity assays (casease, lipase, amylase), and hemolytic activity assessment.
• Pathogenicity testing through re-feeding experiments and determination of median lethal concentration (LC50).
• Antibiotic susceptibility testing.

Main Results:

• The isolated strain SW7-1 exhibited characteristic colony morphology and was Gram-positive.
• 16S rRNA gene sequencing confirmed SW7-1 as Bacillus cereus (>99% identity).
• The strain demonstrated hemolytic activity and produced extracellular enzymes (casease, lipase, amylase).
• SW7-1 caused septicemia-like symptoms and high mortality in healthy silkworms (LC50 = 5.45 × 10^4 cfu/ml).
• High susceptibility was observed for gentamycin, streptomycin, erythromycin, norfloxacin, and ofloxacin; moderate susceptibility to tetracycline and rifampicin.

Conclusions:

• The isolated strain SW7-1 is identified as Bacillus cereus, a significant pathogen of silkworms.
• This finding highlights the economic impact of B. cereus in sericulture.
• The study suggests potential zoonotic implications, as human infections are possible.