SUMMARYThe progress of secretion of a-toxin and total extracellular protein by Staphylococcus aureus (Wood 46), grown aerobically at 37 "C, in a 3% (w/v) tryptone soya broth medium supplemented with vitamins was followed. Exoprotein was secreted at a high rate by intact bacteria during the exponential phase (to 9 h) and into the post-exponential phase. After 18 h, when exoprotein accounted for 33% of the total protein in the culture, no further exoprotein was secreted although the bacterial density continued to increase at a low rate beyond this time. During the phase of active secretion, a-toxin represented a constant proportion of total exoprotein, the differential rate of synthesis of which increased by a factor of four after the end of exponential growth. Concomitant with the increase in the differential rate of exoprotein formation there was a fourfold increase in the intracellular concentration of RNA precursor material.

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A comparison has been made of total exoprotein formation by Staphylococcus aureus (Wood 46) in a tryptone soya broth (TSB) medium and a casein hydrolysate-yeast extract-containing (CCY) medium in which exponential growth occurred with doubling times of 3.0 and 1.3 h, respectively. It was found that the differential rate of exoprotein formation was biphasic in each case, increasing after the cessation of exponential growth by a factor of 4 in TSB and 7 in CCY medium. Although this relative change was greater in CCY medium, the maximum value of the differential rate was less than 40% of that achieved in TSB medium. It was further shown that throughout the growth cycle, and in both cultures, a-toxin accounted for the same proportion of total extracellular protein.

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