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BackgroundThe existence of a genetic basis for host responses to bacterial intramammary infections has been widely documented, but the underlying mechanisms and the genes are still largely unknown. Previously, two divergent lines of sheep selected for high/low milk somatic cell scores have been shown to be respectively susceptible and resistant to intramammary infections by Staphylococcus spp. Transcriptional profiling with an 15K ovine-specific microarray of the milk somatic cells of susceptible and resistant sheep infected successively by S. epidermidis and S. aureus was performed in order to enhance our understanding of the molecular and cellular events associated with mastitis resistance.ResultsThe bacteriological titre was lower in the resistant than in the susceptible animals in the 48 hours following inoculation, although milk somatic cell concentration was similar. Gene expression was analysed in milk somatic cells, mainly represented by neutrophils, collected 12 hours post-challenge. A high number of differentially expressed genes between the two challenges indicated that more T cells are recruited upon inoculation by S. aureus than S. epidermidis. A total of 52 genes were significantly differentially expressed between the resistant and susceptible animals. Further Gene Ontology analysis indicated that differentially expressed genes were associated with immune and inflammatory responses, leukocyte adhesion, cell migration, and signal transduction. Close biological relationships could be established between most genes using gene network analysis. Furthermore, gene expression suggests that the cell turn-over, as a consequence of apoptosis/granulopoiesis, may be enhanced in the resistant line when compared to the susceptible line.ConclusionsGene profiling in resistant and susceptible lines has provided good candidates for mapping the biological pathways and genes underlying genetically determined resistance and susceptibility towards Staphylococcus infections, and opens new fields for further investigation.

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Staphylococcus aureus is a prevalent pathogen for mastitis in dairy ruminants and is responsible for both clinical and subclinical mastitis. Mammary epithelial cells (MEC) represent not only a physical barrier against bacterial invasion but are also active players of the innate immune response permitting infection clearance. To decipher their functions in general and in animals showing different levels of genetic predisposition to Staphylococcus in particular, MEC from ewes undergoing a divergent selection on milk somatic cell count were stimulated by S. aureus . MEC response was also studied according to the stimulation condition with live bacteria or culture supernatant. The early MEC response was studied during a 5 h time course by microarray to identify differentially expressed genes with regard to the host genetic background and as a function of the conditions of stimulation. In both conditions of stimulation, metabolic processes were altered, the apoptosis-associated pathways were considerably modified, and inflammatory and immune responses were enhanced with the upregulation of il1a, il1b, and tnfa and several chemokines known to enhance neutrophil ( cxcl8) or mononuclear leukocyte ( ccl20) recruitment. Genes associated with oxidative stress were increased after live bacteria stimulation, whereas immune response-related genes were higher after supernatant stimulation in the early phase. Only 20 genes were differentially expressed between Staphylococcus spp-mastitis resistant and susceptible animals without any clearly defined role on the control of infection. To conclude, this suggests that MEC may not represent the cell type at the origin of the difference of mastitis susceptibility, at least as demonstrated in our genetic model. Supernatant or heat-killed S. aureus produce biological effects that are essentially different from those induced by live bacteria.

The highest prevalence of MRSA was found in the nasal cavity of incoming pigs in three commercial pig slaughter and pork processing plants. A reduction in MRSA prevalence occurred along the processing chain, and pork products from these plants showed significantly lower MRSA than the initial steps of slaughter and processing, suggesting a reduction in MRSA during the slaughter process with minimal cross-contamination.