Abstract
Streptococcus uberis is a common cause of clinical and subclinical mastitis in dairy cattle. Several virulence mechanisms have been proposed to contribute to the species’ ability to cause disease. Here, virulence characteristics were compared between S. uberis strains FSL Z1-048, which consistently caused clinical mastitis in a challenge model, and FSL Z1-124, which consistently failed to cause disease in the same model, to ascertain whether in vitro virulence characteristics were related to clinical outcome. Macrophages derived from bovine blood monocytes failed to kill FSL Z1-048 whilst reducing survival of FSL Z1-124 by 42.5%. Conversely, blood derived polymorphonuclear cells caused more reduction (67.1 vs. 44.2%, respectively) in the survival of FSL Z1-048 than in survival of FSL Z1-124. After 3 h of coincubation with bovine mammary epithelial cell line BME-UV1, 1000-fold higher adherence was observed for FSL Z1-048 compared to FSL Z1-124, despite presence of a frame shift mutation in the sua gene of FSL Z1-048 that resulted in predicted truncation of the S. uberis Adhesion Molecule (SUAM) protein. In contrast, FSL Z1-124 showed higher ability than FSL Z1-048 to invade BME-UV1 cells. Finally, observed biofilm formation by FSL Z1-124 was significantly greater than for FSL Z1-048. In summary, for several hypothetical virulence characteristics, virulence phenotype in vitro did not match disease phenotype in vivo. Evasion of macrophage killing and adhesion to mammary epithelial cells were the only in vitro traits associated with virulence in vivo, making them attractive targets for further research into pathogenesis and control of S. uberis mastitis.
Highlights
Streptococcus uberis is one of the most common causes of clinical and subclinical mastitis in dairy cattle [1, 2]
Macrophage killing assay The ability of macrophages derived from blood monocytes to kill S. uberis FSL Z1-048 and FSL Z1-124 was tested
Bacteria recovered after 2 h were 124 ± 33% of the bacteria incubated in the medium only, suggesting a replication in presence of macrophages (Figure 1A)
Summary
Streptococcus uberis is one of the most common causes of clinical and subclinical mastitis in dairy cattle [1, 2]. Macrophages represent a major component of the cell population in the healthy lactating mammary gland and the first line of defense against pathogens [13,14,15,16]. Their general function is to phagocytise and kill pathogens. After intramammary challenge with S. uberis strain O140J, microscopic analysis demonstrated presence of the bacteria within the cytoplasm of mammary macrophages. This suggests that macrophages are capable of phagocytising S. uberis and may play a role in its clearance from the mammary gland, actual killing of S. uberis O140J was not observed during in vitro experiments [17, Tassi et al Vet Res (2015) 46:123
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