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Abstract
Streptococcus uberis is a common cause of intramammary infection and mastitis in dairy cattle. Unlike other mammary pathogens, S. uberis evades detection by mammary epithelial cells, and the host–pathogen interactions during early colonisation are poorly understood. Intramammary challenge of dairy cows with S. uberis (strain 0140J) or isogenic mutants lacking the surface-anchored serine protease, SUB1154, demonstrated that virulence was dependent on the presence and correct location of this protein. Unlike the wild-type strain, the mutant lacking SUB1154 failed to elicit IL-1β from ex vivo CD14+ cells obtained from milk (bovine mammary macrophages, BMM), but this response was reinstated by complementation with recombinant SUB1154; the protein in isolation elicited no response. Production of IL-1β was ablated in the presence of various inhibitors, indicating dependency on internalisation and activation of NLRP3 and caspase-1, consistent with inflammasome activation. Similar transcriptomic changes were detected in ex vivo BMM in response to the wild-type or the SUB1154 deletion mutant, consistent with S. uberis priming BMM, enabling the SUB1154 protein to activate inflammasome maturation in a transcriptionally independent manner. These data can be reconciled in a novel model of pathogenesis in which, paradoxically, early colonisation is dependent on the innate response to the initial infection.
Highlights
Streptococcus uberis is a widespread cause of intramammary infection in dairy cattle and a frequent cause of bovine mastitis worldwide
Combining in vivo and in vitro study, we investigated the early pathogenesis of S. uberis as it colonises the bovine mammary gland. in vivo, we confirmed earlier observations [32] that SUB1154 was required for virulence and optimal colonisation of the mammary gland
We demonstrated that cell surface anchoring of the SUB1154 protein was required for full virulence. This latter observation is consistent with a previous investigation of a srtA mutant of this strain, in which, to the strain expressing the truncated SUB1154 in this study, it was able to colonise and induce a cellular infiltration in the absence of clinical signs of disease [32]. These data reflect the necessity of SUB1154 and its association with the bacterial cell as a virulence factor, which is further underpinned by the ubiquitous presence of full-length sub1154 sequences containing the SrtA anchor motif in mammary isolates of S. uberis [54,55]
Summary
Streptococcus uberis is a widespread cause of intramammary infection in dairy cattle and a frequent cause of bovine mastitis worldwide. Treatment of bovine mastitis is a common reason for the administration of antibiotics within dairy production, representing approximately 30% of all antibiotic use attributed to cattle [1,2]. Mastitis reduces milk yields by 600–1200 kg per affected animal per year [3], creating inefficiencies resulting in the need for a greater number of farmed animals. Ruminants are a common source of greenhouse gases, with estimates placing CO2 e emissions between 1.03 and 1.3 kg CO2 e L−1 (milk) [4] inefficiency due to mastitis in the UK alone can be considered to Pathogens 2020, 9, 997; doi:10.3390/pathogens9120997 www.mdpi.com/journal/pathogens.
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