Leimbach Andreas, Poehlein Anja, Vollmers John, Görlich Dennis, Daniel Rolf, Dobrindt Ulrich
Institute of Hygiene, University of Münster, Mendelstrasse 7, 48149, Münster, Germany.
Department of Genomic and Applied Microbiology, Göttingen Genomics Laboratory, Institute of Microbiology and Genetics, Georg-August-University of Göttingen, Göttingen, Germany.
BMC Genomics. 2017 May 8;18(1):359. doi: 10.1186/s12864-017-3739-x.
Escherichia coli bovine mastitis is a disease of significant economic importance in the dairy industry. Molecular characterization of mastitis-associated E. coli (MAEC) did not result in the identification of common traits. Nevertheless, a mammary pathogenic E. coli (MPEC) pathotype has been proposed suggesting virulence traits that differentiate MAEC from commensal E. coli. The present study was designed to investigate the MPEC pathotype hypothesis by comparing the genomes of MAEC and commensal bovine E. coli.
We sequenced the genomes of eight E. coli isolated from bovine mastitis cases and six fecal commensal isolates from udder-healthy cows. We analyzed the phylogenetic history of bovine E. coli genomes by supplementing this strain panel with eleven bovine-associated E. coli from public databases. The majority of the isolates originate from phylogroups A and B1, but neither MAEC nor commensal strains could be unambiguously distinguished by phylogenetic lineage. The gene content of both MAEC and commensal strains is highly diverse and dominated by their phylogenetic background. Although individual strains carry some typical E. coli virulence-associated genes, no traits important for pathogenicity could be specifically attributed to MAEC. Instead, both commensal strains and MAEC have very few gene families enriched in either pathotype. Only the aerobactin siderophore gene cluster was enriched in commensal E. coli within our strain panel.
This is the first characterization of a phylogenetically diverse strain panel including several MAEC and commensal isolates. With our comparative genomics approach we could not confirm previous studies that argue for a positive selection of specific traits enabling MAEC to elicit bovine mastitis. Instead, MAEC are facultative and opportunistic pathogens recruited from the highly diverse bovine gastrointestinal microbiota. Virulence-associated genes implicated in mastitis are a by-product of commensalism with the primary function to enhance fitness in the bovine gastrointestinal tract. Therefore, we put the definition of the MPEC pathotype into question and suggest to designate corresponding isolates as MAEC.
大肠杆菌性牛乳腺炎是乳制品行业中具有重大经济意义的疾病。对乳腺炎相关大肠杆菌（MAEC）的分子特征分析并未发现共同特征。然而，已提出一种乳腺致病性大肠杆菌（MPEC）致病型，表明了使MAEC与共生大肠杆菌区分开来的毒力特征。本研究旨在通过比较MAEC和共生牛大肠杆菌的基因组来研究MPEC致病型假说。
我们对从牛乳腺炎病例中分离出的8株大肠杆菌以及从乳房健康奶牛中分离出的6株粪便共生菌株的基因组进行了测序。通过将该菌株组与来自公共数据库的11株牛相关大肠杆菌进行补充，我们分析了牛大肠杆菌基因组的系统发育历史。大多数分离株源自A和B1系统发育群，但MAEC和共生菌株都不能通过系统发育谱系明确区分。MAEC和共生菌株的基因含量都高度多样，且受其系统发育背景主导。尽管个别菌株携带一些典型的大肠杆菌毒力相关基因，但没有任何对致病性重要的特征可明确归因于MAEC。相反，共生菌株和MAEC在任一致病型中富集的基因家族都很少。在我们的菌株组中，只有气杆菌素铁载体基因簇在共生大肠杆菌中富集。
这是对包括多个MAEC和共生分离株在内的系统发育多样菌株组的首次特征描述。通过我们的比较基因组学方法，我们无法证实先前那些主张对使MAEC引发牛乳腺炎的特定特征进行正向选择的研究。相反，MAEC是从高度多样的牛胃肠道微生物群中招募的兼性和机会性病原体。与乳腺炎相关的毒力相关基因是共生的副产物，其主要功能是增强在牛胃肠道中的适应性。因此，我们对MPEC致病型的定义提出质疑，并建议将相应的分离株指定为MAEC。
