Epidemiological typing of bovine streptococci by pulsed-field gel electrophoresis.

Pulsed-field gel electrophoresis (PFGE) was used to investigate the epidemiology of streptococcal mastitis in dairy cattle. The most prevalent streptococcal species, Streptococcus uberis (60-80% of streptococcal isolates), was highly heterogeneous, with different cows only rarely sharing the same pulsotype. S. agalactiae was rarely encountered, however all eight isolates from one farm generated identical PFGE profiles, which differed from those of all other isolates examined, confirming cow-to-cow transmission. Fifty-two isolates of S. dysgalactiae from 27 cows on 5 farms generated 6 different profiles. However, on individual farms, only one or two pulsotypes usually predominated. This species is generally regarded as an environmental pathogen but our data suggest that cow-to-cow transmission of S. dysgalactiae may occur. In spite of the variation in PFGE profiles of isolates from different cows, persistent infections in individual cows were usually caused by the same pulsotype of S. uberis or S. dysgalactiae.

Virulence of Staphylococcus epidermidis in a mouse model: significance of extracellular slime.

The ability to produce large quantities of biofilm on solid surfaces in vitro is believed to distinguish potentially pathogenic strains of Staphylococcus epidermidis from commensals. Biofilm consists of staphylococcal cells encased in a matrix of extracellular polysaccharide (also referred to as slime), firmly adherent to each other and to the underlying surface structure. The association of slime with colonization of catheter surfaces in vivo has been examined extensively. Less attention has been paid to the contribution of slime to infections that occur in the absence of an inserted device. In a mouse model of subcutaneous infection without an implanted device 10 S. epidermidis strains (5 slime-positive, 5 slime-negative) produced abscesses; thus a foreign body is not essential for the expression of virulence by S. epidermidis. Biofilm-positive strains produced significantly more abscesses, that persisted longer than biofilm-negative strains. In these chronic infections, large numbers of staphylococci were associated with macrophages and viable staphylococci were cultured from specimens of pus collected at autopsy. Thus slime or components of slime appear to delay the clearance of S. epidermidis from host tissues, possibly by interfering with intracellular killing mechanisms. However, differences in the capacity to produce abscesses, within both the slime-positive and slime-negative groups, indicate that other factors also contribute to the virulence of S. epidermidis.