Location of Staphylococcus aureus within the experimentally infected bovine udder and the expression of capsular polysaccharide type 5 in situ

The objective of this study was to locate Staphylococcus aureus in the bovi ne udder and to investigate the expression of capsular polysaccharide type 5 (CP5) in situ in both the early and chronic stages of experimental intram ammary S. aureus infections. Bovine udder tissue was obtained in early and chronic stages of intramammary infection; i.e., 24 to 96 h and 122 d after experimental intramammary infection with S. aureus Newbould 305. The presen ce and location of S. aureus was investigated by Gram staining of tissue se ctions. The expression of CP5 by S. aureus in situ was investigated by immu nochemical staining of tissue sections with specific antibodies against CP5 . Both in the early and chronic stages of infection, S. aureus was located within the lumen of alveoli or lactiferous ducts, in association with the e pithelium, and within phagocytic cells. The staphylococci were mainly obser ved in clusters and often in the presence of polymorphonuclear neutrophils. Expression of CP5 by S. aureus was observed both in the early and chronic stages of infection. In general, CP5-positive S. aureus were located in alv eoli and in association with the mammary epithelium. In the chronic infecti on, CP5-positive S. aureus were also located deep in the interstitial tissu e. These results indicate that-both in early and chronic stages of experime ntal S. aureus mastitis-colonization of the mammary epithelia and invasion into the interstitial tissue occurs and that CP5 is expressed by S. aureus Newbould 305 in situ. The invasion of S. aureus in the interstitial tissue and the expression of CP5 probably help the bacteria to withstand the host defense mechanisms.