THE technique of inoculation of staphylococci into the mammary gland of the mouse (Chandler, 1970) has been used to demonstrate that strains of Staphy Zococcus aureus differ in their virulence as assessed by the clinical response, the number of bacteria recovered from the infected glands and the histopathological lesions induced (Anderson, 197 1, 1972). The inoculation of cell-free staphylococcal " toxin " i.e., total extracellular products into the mammary gland of the mouse causes death, and the inoculation of diluted " toxin " produces a coagulative necrosis consistent with the action of a-toxin (Anderson and Mason, 1974).In the present study a comparison was made of the effect of cell-free toxic preparations and live staphylococci derived from two strains. One strain was known to be of high virulence and the other of low virulence in the mammary gland of mice and cattle (Anderson, 1971 ; Reiter et al., 1971).
MATERIALS AND METHODSMice. Lactating mice of the BSVS strain were used between the 3rd and 6th day after parturition; the offspring were removed from their mother 1 hour before the inoculations were given.Both strains, originally isolated from clinical cases of mastitis in cattle, were coagulase positive and produced a-and p-haemolysins on 5% (v/v) ox-blood agar. Strain BB is more virulent than strain Mexi in cattle (Reiter et al., 1971), and these strains (named C2 and 22 respectively) exhibited similar relative virulence in mice (Anderson, 1971).Biochemical tests. Gelatinase activity of each strain was determined in stab cultures in nutrient gelatin (12% w/v gelatin, Oxoid Ltd, Code CM135a) incubated at 37°C for 7 days.Cultures were cooled to 4°C for 30 min. each day and examined for liquefaction.Each strain was inoculated into litmus milk (Cruikshank, Duguid and Swain,1965, p. 817), incubated at 37"C, and examined at daily intervals for 7 days for acid coagulation, dye reduction and peptonisation. Strain BB liquefied gelatin and in litmus milk produced an acid clot with dye reduction but no peptonisation. Strain Mexi failed to liquefy gelatin and produced no reaction in litmus milk. Freeze-dried cultures of the staphylococci were reconstituted by overnight incubation in Hartley's broth (Cruikshank, Duguid and Swain, Staphylococci. Strains BB and Mexi of staphylococcus were used.Production and assay of " toxin".