David E. Hoffman is a contributing editor to The Post.

Donald Kennedy, a Stanford University biology professor, had been commissioner of the Food and Drug Administration for less than two months in 1977 when he plunged into a difficult scientific and political debate that remains unresolved today. Kennedy wanted to cut back on the widespread use of antibiotics on farms to make animals grow faster and prevent disease.

In particular, Kennedy sought to curb the farm use of tetracyclines. His reasoning was that the antibiotics could create resistant bacteria that would no longer be susceptible to the drugs — a potential threat to human health. Kennedy warned that bacteria move back and forth between people and animals, carrying genetic material for resistance in both directions. He predicted that antibiotic resistance would rise in the “linked ecosystem” shared by all.

Kennedy’s proposal ran into a wall of opposition. The Texas Farm Bureau warned of “a devastating effect on animal agriculture.” The Mississippi Pork Producers Association said it would cause “a tremendous economic blow to our industry.” The National Broiler Council said it would set an “ultimately disastrous precedent.” The National Turkey Federation said it was based on “flimsy scientific evidence.”

Then came the final verdict: Congress told Kennedy to stand down. His proposal was shelved, largely at the behest of the farmers and their powerful champion in the House, Rep. Jamie L. Whitten (D-Miss.).

I heard this story while researching a documentary film on this subject, “The Trouble With Antibiotics,” which is scheduled to air Tuesday on the PBS series “Frontline.” I kept coming back to a nagging question: Was Kennedy right nearly four decades ago?

After Kennedy’s initiative was blocked, study after study examined the issue as antibiotics continued to pour into agriculture. Today, tetracyclines are still used extensively: According to 2012 data just released by the FDA, tetracyclines accounted for 41 percent of the 31 million pounds of antibiotics sold for farm animals in the United States.

Farmers, ranchers and those who make drugs for animals have long insisted that agriculture is not the major culprit in antibiotic resistance. They say antibiotics such as tetracyclines aren’t important for human health, and the benefits for animals outweigh the risks for people. It is certainly true that tetracyclines are not at the forefront of human medicine, although they have some minor uses. But what if there isn’t such a wall between the resistant bacteria in people and animals? What if they share the same ecosystem?

A study published last year about the use of tetracyclines on cattle raises this question anew. The research, by a team that included H. Morgan Scott of Texas A&M University and Guy Loneragan of Texas Tech University, showed that the use of a tetracycline led to “co-selection,” a process in which the antibiotic expanded the population of bacteria that are resistant to other antibiotics as well. In their experiment, the tetracycline expanded resistance to a cephalosporin, a class of antibiotic that is highly valued in human medicine.

This dynamic is what Kennedy was worried about in 1977.

After years of inaction, the FDA last year asked the animal drug-makers to voluntarily stop producing antibiotics for growth promotion over the next three years, and all agreed. The FDA also said it would require antibiotic use to take place under supervision of a veterinarian — something Kennedy also wanted a long time ago. But the FDA will continue to authorize the widespread use of antibiotics on healthy farm animals as a means to prevent disease. How much of a reduction can be expected? The data on antibiotic use on U.S. farms is skimpy, and no one really knows. The FDA commissioner, Margaret A. Hamburg, told me that she hopes the new approach will get results — and that more needs to be done.

Much is at stake. Antibiotic-resistant infections kill an estimated 23,000 Americans each year and sicken 2 million people. Resistance is growing to some of our most effective antibiotics. In September, the President’s Council of Advisors on Science and Technology declared the problem of antibiotic resistance has become “dire,” requires “urgent attention,” is growing at an “alarming rate” and threatens medicine, economic growth, public health, agriculture and national security.

If they are correct, it would seem smart and prudent to be vigilant on every front where resistance might be coming from, both human and animal.

On that, Kennedy was clearly right: We share the same ecosystem.