Identification and pathogenicity evaluation of Bacillus cereus isolated from dairy goats in China.

Abstract

Bacillus cereus (B. cereus) eye infections in dairy goats cause conjunctivitis and keratitis, threatening animal welfare and productivity. To isolate B. cereus, evaluate its pathogenicity, and characterize its biological properties, we employed morphological observation, biochemical identification, 16S rRNA gene sequencing, specific gene detection, growth curve analysis, antimicrobial susceptibility testing, virulence gene screening, and histopathological examination of experimentally infected mice. The isolated strain was confirmed as B. cereus and designated LY. On Luria-Bertani (LB) agar, the isolate formed circular, convex, waxy-white colonies; on sheep blood agar, it exhibited non-hemolytic, light gray, ground-glass-like colonies. It was Gram-positive, with slender rods, subterminal spores, and occurring singly or in chains. Biochemical tests showed fermentation of glucose, maltose, fructose, and xylose, but not mannitol or sucrose; it was catalase-positive, urease-negative, HS-negative, nitrate reduction-negative, and methyl red (MR)-positive. Growth analysis revealed optimal growth at pH 7.0 and 37 °C, with growth observed between pH 7.0-9.0 and 32-42 °C. Antimicrobial susceptibility testing revealed resistance to 11 antibiotics, including penicillin, vancomycin, and polymyxin B, and susceptibility to 14, such as amikacin, gentamicin, and kanamycin. PCR confirmed the presence of virulence genes plcR, hblC, hblD, nheA, nheB, nheC, and entFM, indicating high pathogenic potential. Histopathology of infected mice revealed severe hemorrhage in parenchymal organs (e.g., liver, kidney) and the gastrointestinal tract (e.g., stomach, intestine). In conclusion, LY, a B. cereus strain with multiple virulence genes, strong pathogenicity, and multidrug resistance, was isolated, indicating potential pathogenic risks to dairy goat herds.