Pathological changes of highly pathogenic Bacillus cereus on Pelodiscus sinensis

Area of Science:

• Aquatic Animal Health
• Microbiology
• Pathogen Identification

Background:

• An outbreak of a disease with high mortality occurred in Chinese Softshell Turtle (Pelodiscus sinensis) farms in Hubei Province.
• This outbreak necessitated the identification of the causative agent and its pathogenic mechanisms.

Purpose of the Study:

• To isolate and identify the pathogen responsible for the mass mortality in Pelodiscus sinensis.
• To characterize the pathogenicity of the isolated strain and its effects on turtle tissues and microbiota.
• To determine antibiotic susceptibility for potential treatment strategies.

Main Methods:

• Isolation and identification of the bacterial pathogen from diseased turtles.
• Molecular characterization of virulence genes (hemolysin BL, NHE, Enterotoxin FM).
• Pathogenicity testing (LD50 determination) and tissue distribution analysis in Pelodiscus sinensis.
• Histopathological examination of infected organs.
• Analysis of intestinal microbiota composition using 16S rRNA sequencing.
• Antimicrobial susceptibility testing.

Main Results:

• A highly pathogenic Bacillus cereus strain (Y271) was isolated, possessing genes for Hemolysin BL, Non-hemolytic enterotoxin (NHE), and Enterotoxin FM.
• The isolated strain exhibited high pathogenicity against Pelodiscus sinensis (LD50 = 6.80 × 10^3 CFU/g), with significant proliferation in spleen, liver, and kidney.
• Histopathology revealed severe tissue damage, necrosis, and inflammation in multiple organs, particularly the spleen, liver, kidney, and intestine.
• Intestinal microbiota analysis showed altered composition in diseased turtles, with increased Firmicutes, Fusobacterium, Actinomyces, Allobaculum, Rothia, Aeromonas, and Clostridium, and a reduced number of unique taxa.
• The Y271 strain was sensitive to florfenicol, enrofloxacin, neomycin, and doxycycline.

Conclusions:

• Bacillus cereus is identified as the etiological agent responsible for the mass mortality event in Pelodiscus sinensis.
• The study highlights the significant pathological impact of Bacillus cereus infection on turtle organs and intestinal microbiome.
• The findings underscore the potential risks of Bacillus cereus outbreaks in aquaculture and provide insights into antibiotic treatment options.