Salmonella enterica serovar Derby isolated from eggs show genomic and phenotypic traits that may be linked to inability to produce human infection.

Salmonella enterica serovar Derby causes foodborne disease (FBD) outbreaks worldwide, mainly from contaminated pork but also from chickens. During a major epidemic of FBD in Uruguay due to S. Enteritidis from contaminated poultry, we conducted a large survey of commercially available eggs, where we isolated many S. Enteritidis strains but surprisingly also a much larger number (ratio 5:1) of S. Derby strains. At that time, reporting of suspected Salmonella-related human infection was mandatory in the country but no single case of S. Derby infection was detected in that period suggesting that the S. Derby egg strains were impaired for human infection. We sequenced fourteen of these egg isolates as well as fifteen isolates from pork or human infection, that were isolated in Uruguay before and after that period. All sequenced strains had the same sequence type ST40, but phylogenomic analysis showed that they clustered in three different groups. All egg isolates clustered together as a monophyletic group and showed marked differences in gene content with the strains in the other two clusters. Differences included absence of genes involved in acid tolerance, heme-uptake, and type VI secretion system, as well as the absence of a C terminal fragment of SpeF protein. We also identified differences in the mobile genetic element contents, the composition of plasmids, insertion sequences, transposons, and phages between egg isolates and human/pork isolates. Mice challenge experiments showed that while mice infected with human/pork isolates died between 1-7 days p.i., all animals infected with the egg strains survived the challenge. Altogether our results suggest that loss of gene functions, and the absence of plasmids in egg isolates may explain why these S. Derby were not capable of producing human infection despite being the main serovar recovered from eggs countrywide.

肠炎沙门氏菌德比血清型（Salmonella enterica serovar Derby）可引发全球范围内的食源性疾病（Foodborne Disease, FBD）暴发，其传播媒介主要为受污染的猪肉，亦可来自鸡肉。在乌拉圭暴发由受污染家禽携带的肠炎沙门氏菌肠炎血清型（Salmonella enterica serovar Enteritidis）引发的大规模食源性疾病疫情期间，我们针对市售鸡蛋开展了一项大规模调查。此次调查中，我们分离到多株肠炎沙门氏菌肠炎血清型菌株，但令人意外的是，德比血清型菌株的数量高达前者的5倍。当时该国强制上报疑似沙门氏菌感染病例，但此期间未检测到任何德比血清型感染病例，这提示从鸡蛋中分离的德比血清型菌株存在人类感染缺陷。我们对14株鸡蛋分离株以及同期前后在乌拉圭分离的15株猪肉来源或人类感染来源分离株进行了测序。所有测序菌株均属于同一序列型ST40（Sequence Type ST40），但系统发育基因组学分析显示，它们可聚类为三个不同的类群。所有鸡蛋分离株共同构成一个单系群（monophyletic group），且与另外两个类群的菌株在基因组成上存在显著差异。这些差异包括：参与酸耐受（acid tolerance）、血红素摄取（heme-uptake）以及VI型分泌系统（Type VI Secretion System）的基因缺失，同时还缺失了SpeF蛋白（SpeF protein）的C端片段。我们还发现，鸡蛋分离株与人类/猪肉分离株在移动遗传元件（mobile genetic element）组成、质粒（plasmids）、插入序列（insertion sequences）、转座子（transposons）以及噬菌体（phages）的构成上均存在差异。小鼠攻毒实验（mice challenge experiments）结果显示，感染人类/猪肉分离株的小鼠在感染后（post infection, p.i.）1至7天内死亡，而所有感染鸡蛋菌株的小鼠均存活至实验结束。综合以上结果，我们推测鸡蛋分离株的基因功能缺失以及质粒缺失，可解释为何尽管德比血清型是该国鸡蛋中分离到的主要血清型，却无法引发人类感染。