Abstract
Mastitis remains a major infection of dairy cows and an important issue for the dairy farmers, and Escherichia coli (E. coli) bovine mastitis is a disease of significant economic importance in the dairy industry. Our study identified six isolates belong to phylogroup B2 from 69 bovine mastitis E. coli strains. Except for one serotype O1 strain, all group B2 isolates were identified into serotype O2 and showed significantly higher mortality in the mouse infection than other phylogroups' strains. Genomic analyses and further tests were performed to examine the role of secretion systems, fimbriae, and toxins during the systemic infection of O2:K1 strain BCE049. Two integral T6SS loci and three predicted effectors clusters were found to assemble the functional T6SS complex and deliver diverse toxic effectors to modulate bacterial virulence in the mouse infection model. A total of four T4SS loci were harbored in the BCE049 genome, three of them are encoded in different plasmids, respectively, whereas the last one locates within the bacterial chromosome at FQU84_16715 to FQU84_16760, and was significantly involved in the bacterial pathogenicity. Numerous predicted pilus biosynthesis gene loci were found in the BCE049 genome, whereas most of them lost long fragments encoding key genes for the pili assembly. Unexpectedly, a type IV pilus gene locus locating at FQU84_01405 to FQU84_01335 in the plasmid 2, was found to be required for the full virulence of mastitis strain BCE049. It should be noted that a genetic neighborhood inserted with diverse genes is encoded by the plasmid 1, which harbors three prominent toxins including β-hemolysin, cytotoxic necrotizing factor 2 and cytolethal distending toxin type III. Consequent studies verified that these toxins significantly contributed to the bacterial pathogenicity. These findings provide a molecular blueprint for understanding the underlying mechanisms employed by the bovine mastitis E. coli to colonize in host and cause systemic infection.
Highlights
Mastitis is an inflammation of the cow udder mostly triggered by the invasion of pathogenic bacteria
Genomic analyses of O2:K1 strain BCE049 found four T6SS-related, one T4SS-related, two type 1 fimbria–like, and one type P fimbria– like loci are encoded within the chromosome, and three T4SSrelated, one type IV pili, one hemolysin A–like, one CNF2–like, and one cytolethal distending toxin type III–like loci are encoded within the plasmid 1, 2, or 3
Our present study demonstrated that T6SS1, T6SS2, chromosome-encoding T4SS, type IV pili, hemolysin A, and CNF2 are required for the full virulence of strain BCE049 in the mouse infection model
Summary
Mastitis is an inflammation of the cow udder mostly triggered by the invasion of pathogenic bacteria. Escherichia coli (E. coli), one of the main pathogens involved in cases of bovine mastitis, is responsible for significant losses on the dairy farms [1,2,3,4,5]. E. coli mastitis results in a subclinical. The subclinical mastitis is mostly like an infection caused by the environmental opportunistic bacteria [10], but systemic infection may be a typical symptom of the extraintestinal pathogenic E. coli (ExPEC) disease. These two different infection phenotypes whether caused by different E. coli pathotypes should be further explored
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