ST. PETERSBURG — In June 1952, Lloyd Conover stood alone in a Brooklyn laboratory, watching a chemical reaction unfold before him.

The young chemist for Pfizer had been toying with an idea considered impossible at the time: What if he could modify two existing molecules, each with antibiotic properties, to create a more powerful antibiotic?

The experiment yielded Tetracycline, a breakthrough in the history of antibiotics and for many years one of the most prescribed, treating acne and ear infections, malaria and bacterial pneumonia.

The drug was also important for the implications of its discovery. Conventional wisdom had held antibiotics could only be cultivated from nature, such as the mold that became penicillin, not modified in the lab. After Tetracycline, scientists created generations of drugs by altering molecules, widening the array of illnesses they could treat while minimizing side effects.

Dr. Conover, a careful and methodical man who remained modest about his discovery, died Saturday at Westminster Suncoast retirement community in St. Petersburg. He was 93.

He was also my stepfather. In December 2005, Dr. Conover married my mother, Katharine Meacham. Both were widowed and in their 80s when they met.

He started at Pfizer in 1950, during a surge of interest by the company in developing new antibiotics. At the time and despite being around since 1849, Pfizer had made a name for itself more for its mastery of chemical processes, such as the manufacture of citric acid, than inventions. The company got its start in antibiotics by producing more than half of the penicillin for Allied forces during World War II.

In the late 1940s, after collecting 135,000 soil samples from around the globe, the company discovered chlortetracycline and oxytetracycline, better known by their brand names Aureomycin and Terramycin. These molecules, among the first of the "broad spectrum antibiotics," secreted substances that inhibited the growth of a range of bacteria.

The drugs came with side effects. But their discovery was significant. By 1952, Dr. Conover and a half-dozen other researchers had discovered their molecular structure, a find Dr. Conover attributed to the leadership of Robert Woodward, a Nobel prize-winning chemist who led the team.

"All the experience chemists had had, to the extent that we knew about it, had been that whatever chemists did was bad," Dr. Conover said in a 2009 interview. "It either destroyed the activity or it made the substance more toxic. And it was the conventional wisdom that what these wonderful molecules made by living organisms just can't be improved on."

Breaking down Aureomycin and Terramycin to their atoms was a kind of re-engineering, a way to better understand how they stopped bacteria from replicating, allowing the body's immune system to kill off the remaining bacteria.

"There was no thought when we started this that there was anything we could do through this chemical work that could lead to antibiotics, even though in the end that is what happened," Dr. Conover said.

Lloyd Hillyard Conover was born in 1923 in Orange City, N.J., the son of a lawyer and an artist. He grew up in Summit, N.J., where he hiked in the woods and amused himself in inventive ways.

As a child, he created a cannonball by melting lead and pouring it into a bored-out tennis ball. He discovered chemistry in high school and entered Amherst College in 1941, intending to become a chemist.

"I had never encountered a subject in school that I had found so intrinsically interesting and fascinating, and because it was those things so easy to remember," Dr. Conover said. "I didn't have to study it, I just read it."

In college he rekindled a high school romance with Virginia Kirk, a relationship interrupted by World War II. Dr. Conover served in the Navy in the Pacific. On his way home from the war, he sent Virginia a telegram proposing marriage, which she accepted.

In 1950 he earned a doctorate in chemistry at the University of Rochester, and joined Pfizer the same year. His research team celebrated when they uncovered the molecular structure of Aureomycin and Terramycin, but Dr. Conover wasn't finished. He couldn't stop thinking about how chemically similar the two organisms were, differing only by two atoms, neither of which seemed necessary for biological activity.

"I would have to say one day, which was unique in my whole career, I really had a 'Eureka' moment," Dr. Conover said.

What would happen, he wondered, if he replaced the chlorine atom in Aureomycin and the oxygen atom in Terramycin with hydrogen?

"That molecule that was then unknown, I said to myself, ought to be just as good and maybe better than either Terramycin or Aureomycin," Dr. Conover said.

The attempt at bonding worked on the first try. Not only was Tetracycline chemically stable, it would prove more versatile in treating infections and had fewer side effects.

"At the start of an industrial research career, I couldn't have lucked out much more than that," he said.

Dr. Conover received a patent for the drug, then successfully weathered 27 years of legal challenges from competitors. He stayed at Pfizer his entire career, where he was responsible for about 300 patents, retiring in 1984 as a senior vice president.

He fathered four children in Waterford, Conn. A stickler for safety, he tied ropes to their second-floor bedposts and taught them to rappel down the side of the house in case of fire. He also conscripted them to plant Christmas trees, which they sold every year.

"We were told that this was going to help fund our college educations, and that it was work we needed to do," said Heather Conover, 64, his daughter.

Virginia died of cancer in 1988. Dr. Conover married Marie Solomons, who died 10 years later in 2003, also of cancer.

In 2004, Marie's grandson and his fiance, who is my niece, decided they knew someone Dr. Conover should meet. They pitched the matchmaking idea to Katharine Meacham, my mother, two years after Robert Meacham had died.

Dr. Conover and my mother married in 2005, in a joint ceremony with their grandchildren, who were also getting married.

For the past 12 years I have known him as a kind and meticulous man, a romantic who loved theater and opera. He never shopped for clothes but wore whatever my mother got him. He would cut open toothpaste tubes so as not to waste any, then fly his entire extended family to his flat in Folkestone, England, for relaxation and theater in London.

He read poetry aloud to my mother, including during his past year, in a long-term care unit at Westminster Suncoast. Friday night, my sister Kathy and my mother sang songs with him by his bedside. The last night of his life, he mouthed the words to Irving Berlin's Always.

Contact Andrew Meacham at ameacham@tampabay.com or (727) 892-2248. Follow @torch437.