PHASE VARIATION OF SLIME PRODUCTION IN STAPHYLOCOCCUS-AUREUS - IMPLICATIONS IN COLONIZATION AND VIRULENCE

Two methods commonly used for slime detection in coagulase-negative st aphylococci (tube biofilm formation and colony morphology in Congo red agar) were used to study 144 ruminant mastitis Staphylococcus aureus strains. Slime production was detected in 21 strains. A majority of ce lls (85%) in slime-producing (SP) strains and a minority of cells (5%) in non-slime-producing (NSP) strains showed a condensed exopolysaccha ride matrix (slime) surrounding the bacterial cell wall, as revealed b y electron microscopy and immunofluorescence. In vivo slime production was also detected immunohistochemically after experimental infection of the mammary gland in sheep. Upon repeated subcultures in Congo red agar, NSP variants were obtained from four ovine and four bovine SP st rains at a frequency ranging from 0.5 x 10(-4) to 10(-4). Because SP v ariants could not be obtained from NSP strains within this range or at a higher frequency, they were obtained by the tube biofilm formation (requiring repeated subculturing of NSP strains in tryptic soy broth c ontaining 2% glucose for subsequent recovery of colonies adherent to t he walls of the culture tubes). In experimental challenge, the SP vari ant showed a significantly higher colonization capacity than did the N SP variant of the same strain used (P < 0.001). However, the NSP varia nt had a higher virulence than did the SP variant (P < 0.001). These r esults may help to explain the different roles of S. aureus slime prod uction cell types (SP and NSP) coexisting in disease.