Submitted to: Mastitis Council Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 12/19/2005
Publication Date: 1/23/2006
Citation: Bannerman, D.D. 2006. Natural defenses: Different responses for different bacteria. Mastitis Council(NMC)45th Annual Meeting Proceedings. pp.88-96. Interpretive Summary:
Technical Abstract: Successful establishment and persistence of infection is mediated by both intrinsic properties of the pathogen itself and the nature of the host's response to the pathogen. The innate immune systems represents the first line of active defense against invading pathogens once they have penetrated the physical barriers of the skin and other tissues such as the streak canal of the teat. Elements of the innate immune system represent ancient and highly conserved mechanisms by which the host responds and protects itself against pathogens. Further, the innate immune system is poised to immediately respond to the earliest stages of infection and recognize pathogens that have not been previously encountered. The outcome of intramammary infection, consisting of either host eradication of the pathogen or the establishment of persistent chronic infection, is often governed by events in the immediate hours and days following initial infection. Thus, the innate branch of the immune system, which elicits the immediate responses to infection, is the primary host determinant of the outcome of intramammary infection. S. aureus and E. coli are among the most prevalent Gram-positive and Gram-negative bacteria, respectively, to cause intramammary infections in dairy cattle. Striking differences exist in the clinical course and outcome of mastitis caused by these two pathogens. Intramammary infection by E. coli is acute in nature and in mid- to late-lacating cows these infections generally clear within a few days. S. aureus causes intramammary infections that are often subclinical and establishes chronic infections that can persist for the life of the animal. Difference in the severity and outcome of mastitis that accompany intramammary infection by E. coli and S. aureus may be attributed to the differential host immune responses that each bacteria elicits. Several studies have recently been undertaken to characterize the innate immune response during intramammary infection and to delineate whether pathogen-dependent differences exist in the host response. In a recent study, we have compared the innate immune response elicited by intramammary infection with E. coli and S. aureus. Interestingly, we found striking differences in the production of IL-8, TNF-alpha, and IL-10 elcited by E. coli and S. aureus. The surprising finding of a complete lack of induction of either IL-8 or TNF-alpha in S. aureus-infected quarters is consistent with that of a previous study. Since both studies used different strains of S. aureus, the finding appears to be generalizable to S. aureus and not due to strain variation. Because the ability of bacteria to establish infection is mediated, in part, by the ability of the host to respond to the invading organism, the lack of induction of the pro-inflammatory cytokines, TNF-alpha and IL-8, may contribute to the ability of S. aureus to establish a chronic infection in the bovine mammary gland. Together, these data suggest that differences in the host immune response influence resolution of infection and that interventions that alter this response may have potential therapeutic benefit for the treatment of intramammary infections in cattle.