Summary: To see whether the initial damage caused by penicillin involved the osmotic barrier, certain characters of were examined at intervals during the first 2 hr. after addition of penicillin to growing cultures. In the 30 min. following addition of penicillin the cell appeared to expose nearly all its reserve penicillin-binding component (PBC), the penicillin uptake being double that which occurred without growth and the reserve PBC disappearing. The amount of PBC in the cell and its rate of exposure to penicillin in excess of normal synthesis (rate of turnover?) were both very small. Between 30 and 60 min. the uptake of Na, Mg, K, P and increase of total dry matter ceased abruptly. Continued increase in dry wt., while P, Na, Mg and K decreased slightly, resulted in the penicillintreated cells becoming relatively deficient in these elements. Synthesis of lipid phosphorus and some, but not all, large molecule phosphates still continued, and Fe and Co uptake were not affected. After 60-80 min. water entered and solutes began to leave the cell, and synthesis of large molecule P ceased. The primary site of action of penicillin is probably not concerned with gross assimilation of Na, K, Mg, P, Fe, Co, or any substance contributing more than 10% of the dry wt., or with gross synthesis of lipid P. It may, however, involve a reaction turning over 3000 times while the cell mass is doubled.


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