A rare waterborne outbreak of <i>Bacillus paranthracis</i> in Shandong province, China, 2020: epidemiologic survey, genomic insights, and virulence characteristics

<i>Bacillus paranthracis</i>, a Gram-positive conditional pathogen of <i>Bacillus cereus</i> group species, is capable of causing foodborne and waterborne illnesses, leading to intestinal diseases in humans characterized by diarrhoea and vomiting. However, documented cases of <i>B. paranthracis</i> infection outbreaks are rare in the world, and the genomic background of outbreak strains is seldom characterized. This study retrospectively analyzed strains obtained from an outbreak in schools, as well as from water systems in peri-urban areas, China, in 2020. In total, 28 <i>B. cereus</i> group isolates were retrieved, comprising 6 from stool samples and 22 from water samples. Epidemiological and phylogenetic investigations indicated that the <i>B. paranthracis</i> isolate from drinking water as the causative agent of the outbreak. The genomic comparison revealed a high degree of consistency among 8 outbreak-related strains in terms of antimicrobial resistance gene profiles, virulence gene profiles, genomic content, and multilocus sequence typing (MLST). The strains related to the outbreak show highly similar genomic ring diagrams and close phylogenetic relationships. Additionally, this study shed light on the pathogenic potential and complexity of <i>B. cereus</i> group through its diversity in virulence genes and mice infection model. The findings highlight the usefulness of <i>B. paranthracis</i> genomes in understanding genetic diversity within specific environments and in tracing the source of pathogens during outbreak situations, thereby enabling targeted infection control interventions.

副炭疽芽孢杆菌（Bacillus paranthracis）是蜡样芽孢杆菌群（Bacillus cereus group）的革兰氏阳性条件致病菌，可引发食源性与水源性疾病，导致人类出现以腹泻、呕吐为主要特征的肠道病症。然而，全球范围内已报道的副炭疽芽孢杆菌感染暴发病例极为罕见，且暴发菌株的基因组背景鲜有解析。本研究针对2020年中国城郊地区某学校暴发疫情及周边水系分离得到的菌株展开回顾性分析，共回收28株蜡样芽孢杆菌群分离株，其中6株来源于粪便样本，22株来源于水样。流行病学与系统发育调查结果显示，饮用水中分离得到的副炭疽芽孢杆菌菌株为此次暴发的致病原。基因组比较分析显示，8株暴发相关菌株在抗菌素耐药基因谱、毒力基因谱、基因组构成及多位点序列分型（multilocus sequence typing，MLST）方面均呈现高度一致性，此类暴发相关菌株的基因组环形图谱高度相似，系统发育关系紧密。此外，本研究通过毒力基因多样性分析与小鼠感染模型，揭示了蜡样芽孢杆菌群的致病潜力与复杂性。本研究结果凸显了副炭疽芽孢杆菌基因组在解析特定环境内的遗传多样性，以及暴发情境下的病原溯源中的应用价值，可为针对性感染防控干预提供支撑。