Abstract
Mastitis is a serious medical condition of dairy cattle. Here, we evaluated whether the degree of virulence of mastitis pathogens in a mouse model can be linked to the inflammatory response that they provoke. Clinical isolates of Staphylococcus aureus (S. aureus) (strain 556 and 392) and Escherichia coli (E. coli) (676 and 127), and laboratory control strains [8325-4 (S. aureus) and MG1655 (E. coli)], were injected i.p. into mice, followed by the assessment of clinical scores and inflammatory parameters. As judged by clinical scoring, E. coli 127 exhibited the largest degree of virulence among the strains. All bacterial strains induced neutrophil recruitment. However, whereas E. coli 127 induced high peritoneal levels of CXCL1, G-CSF, and CCL2, strikingly lower levels of these were induced by the less virulent bacterial strains. High concentrations of these compounds were also seen in blood samples taken from animals infected with E. coli 127, suggesting systemic inflammation. Moreover, the levels of CXCL1 and G-CSF, both in the peritoneal fluid and in plasma, correlated with clinical score. Together, these findings suggest that highly virulent clinical mastitis isolates produce a distinct cytokine profile that shows a close correlation with the severity of the bacterial infection.
Highlights
Mastitis is the most costly disease in the dairy industry worldwide, incurring economic losses in terms of reduced milk yield, poorer milk quality, and treatment costs [1, 2]
The clinical isolates used in this study were all derived from the milk of cows suffering from acute clinical mastitis
The E. coli clinical isolate 676 carried genes encoding cytotoxic necrotizing factor, cytolethal distending toxin, as well as the F17 fimbriae. This trio of virulence factors encoded on the same plasmid is characteristic of type 2 necrotoxic E. coli (NTEC-2), which has been reported to frequently colonize the gastrointestinal tract of healthy ruminants [reviewed by De Rycke et al [20]], and been shown to cause both diarrhea and invasion of the blood stream in newborn calves [21], dependent only on factors encoded on the Vir plasmid [22]
Summary
Mastitis is the most costly disease in the dairy industry worldwide, incurring economic losses in terms of reduced milk yield, poorer milk quality, and treatment costs [1, 2]. Mastitis is defined as an inflammation of the mammary gland, typically the response to an intramammary infection [3, 4]. A mastitis case is categorized as either clinical or subclinical. Cows with clinical mastitis present visually recognizable symptoms, such as milk abnormalities and visible changes in the udder. Developing cases are termed acute clinical mastitis. Subclinical mastitis is marked by the absence of macroscopically visual symptoms, though milk abnormalities or udder changes may occur transiently in a minority of cases. Subclinical cases that persist for at least 2 months are termed chronic. Intramammary infections are typically of bacterial origin and the responsible pathogens are broadly categorized as either contagious (spread between udder quarters or cows) or environmental (opportunistic pathogens) [5, 6]. Staphylococci, streptococci, and coliform bacteria constitute the majority of etiological mastitis agents [7, 8]
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