Capsule-negative Staphylococcus aureus induces chronic experimental mastitis in mice.

TUCHSCHERR LPN; BUZZOLA FR; ALVAREZ LP; CACCURI RP; SORDELLI DO

INFECTION AND IMMUNITY

ASM Press

Lugar: Washington DC, USA; Año: 2005 vol. 73 p. 7932 - 7937

0019-9567

Staphylococcus aureus capsular polysaccharides (CP) have been shown to enhance staphylococcal virulence in numerous animal models of infection. Although serotypes 5 CP (CP5) and 8 (CP8) predominate among S. aureus isolates from humans, most staphylococcal isolates from bovines with mastitis in Argentina are capsule-negative. This study was designed to evaluate the effects of CP5 and CP8 expression on the pathogenesis of experimental murine mastitis. Lactating mice were challenged by the intramammary route with one of three isogenic S. aureus strains producing CP5, CP8, or no capsule. Significantly greater numbers of the acapsular mutant were recovered from the infected glands 12 days after bacterial challenge compared with the encapsulated strains. Histopathological analyses revealed greater polymorphonuclear and mononuclear leukocyte infiltration and congestion in the mammary glands of mice infected with the encapsulated strains compared with the acapsular mutant, and the serotype 5 strain elicited more inflammation than the serotype 8 strain. In vitro experiments revealed that the acapsular S. aureus strain was internalized by MAC-T bovine epithelial cells in significantly greater numbers than the CP5- or CP8-producing strains. Taken together, the results suggest that S. aureus lacking a capsule were able to persist in the murine mammary gland, whereas encapsulated strains elicited more inflammation and were eliminated faster.  Loss of CP5 or CP8 expression may enhance the persistence of staphylococci in the mammary gland of chronically infected hosts.