Final answer:
The genus primarily responsible for over half of all natural antibiotics and semisynthetic drugs is Streptomyces. When a natural antimicrobial compound is chemically modified, the resultant drug is categorized as a semisynthetic drug.
Step-by-step explanation:
More than half of all antibiotics and semisynthetic drugs originate from the genus Streptomyces. This particular group of soil bacteria is renowned for producing a diverse array of antimicrobial compounds. Scientists, such as Selman Waksman, have extensively studied Streptomyces due to their capacity to synthesise various antimicrobials, for which Waksman was awarded the Nobel Prize in Physiology and Medicine in 1952.
Regarding the discovery and production of new antimicrobial drugs, when a scientist modifies a naturally occurring compound by making changes such as the addition of a hydroxyl group to a chemical side chain, this process results in what is known as a semisynthetic drug. Semisynthetic drugs expand the range of microbial targets, for instance allowing an antimicrobial that originally only killed gram-negative bacteria to also be effective against gram-positive bacteria. This amplification of properties provides a broader spectrum of antimicrobial activity.