Escherichia coliandStaphylococcus aureusElicit Differential Innate Immune Responses following Intramammary Infection

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Abstract
Staphylococcus aureusandEscherichia coliare among the most prevalent species of gram-positive and gram-negative bacteria, respectively, that induce clinical mastitis. The innate immune system comprises the immediate host defense mechanisms to protect against infection and contributes to the initial detection of and proinflammatory response to infectious pathogens. The objective of the present study was to characterize the different innate immune responses to experimental intramammary infection withE. coliandS. aureusduring clinical mastitis. The cytokine response and changes in the levels of soluble CD14 (sCD14) and lipopolysaccharide-binding protein (LBP), two proteins that contribute to host recognition of bacterial cell wall products, were studied. Intramammary infection with eitherE. coliorS. aureuselicited systemic changes, including decreased milk output, a febrile response, and induction of the acute-phase synthesis of LBP. Infection with either bacterium resulted in increased levels of interleukin 1β (IL-1β), gamma interferon, IL-12, sCD14, and LBP in milk. High levels of the complement cleavage product C5a and the anti-inflammatory cytokine IL-10 were detected at several time points followingE. coliinfection, whereasS. aureusinfection elicited a slight but detectable increase in these mediators at a single time point. Increases in IL-8 and tumor necrosis factor alpha were observed only in quarters infected withE. coli. Together, these data demonstrate the variability of the host innate immune response toE. coliandS. aureusand suggest that the limited cytokine response toS. aureusmay contribute to the well-known ability of the bacterium to establish chronic intramammary infection.