What acid was added to make strain Q-176 produce the highly potent penicillin G?

Phenylacetic acid was added to strain Q-176 to make it produce the highly potent penicillin G.

Q-176 was created at the University of Wisconsin by exposing the earlier strain X-1612 to ultraviolet radiation. Although Q-176 produced more total penicillin than its predecessor, it mainly produced penicillin K, which was less desirable for the manufacturing program.

Researchers found that the mould’s product could be influenced by supplying a suitable chemical side-chain precursor. Adding phenylacetic acid redirected Q-176 toward penicillin G, also called benzylpenicillin. Penicillin G became the principal commercial form because of its strong antibacterial activity and usefulness in medical treatment.

This is not the same as adding an acid simply to adjust the culture’s pH. Phenylacetic acid acted as a precursor that influenced which penicillin molecule the mould synthesized. The episode illustrates why industrial penicillin development required both better fungal strains and carefully designed nutrient media: mutation increased productivity, while chemical feeding helped determine the product’s identity.