Staphylococcus aureus exopolysaccharide in vivo demonstrated by immunomagnetic separation and electron microscopy.

Staphylococcus aureus strains were separated from mastitis milk samples without cultivation by using monodisperse magnetic polymer particles coated with polyclonal antiserum against an encapsulated S. aureus strain. Exopolysaccharide was verified by transmission electron microscopy and the serum soft-agar culture technique. Capsular polysaccharide was found on virtually all clinical isolates. Surface protein A and S. aureus-specific cell wall components were masked when the strains were cultured on an exopolysaccharide-promoting medium. Masking of surface determinants was dependent on their concentration on the bacterial surface as well as on exopolysaccharide abundance. The polysaccharide layer on in vivo bacteria was reduced markedly after just one transfer from milk to blood agar plates but was reexpressed after culturing was done on a capsule-generating medium.