Antibiotic resistance is now considered a catastrophic threat to public health, as more and more deadly, drug-resistant bacteria have been appearing in our cities, farms, and hospitals. In theory, the solution should be simple: We need to stop overusing antibiotics. Yet nearly every attempt to do so thus far has failed.

Now California is taking drastic measures. This weekend, Gov. Jerry Brown signed the toughest restrictions yet on antibiotic use in the United States, banning the state's livestock producers from using certain antibiotics for routine disease prevention and growth promotion.

"The science is clear that the overuse of antibiotics in livestock has contributed to the spread of antibiotic resistance and the undermining of decades of lifesaving advances in medicine," Brown said in a statement.

This is, potentially, a huge deal. Across the United States, more than 70 percent of medically important antibiotics are sold for use in animals, so curbing overuse in this area has always been a priority for public health reformers. And California's new law could push other states to follow suit.

Why curbing antibiotic use on farms is so controversial

The science behind drug resistance is straightforward enough. The more we use antibiotics to kill off disease-causing bacteria, the more likely those bacteria are to evolve resistance, developing random mutations to outwit our drugs. And overuse has become a huge problem. In the United States alone, antibiotic-resistant infections are now associated with 23,000 deaths and 2 million illnesses every year. By 2050, a report out of the UK suggested drug-resistant infections will kill more people than cancer.

So public health experts have been looking for places to curtail misuse. It would help, for instance, if doctors stopped prescribing unnecessary antibiotics. But another key place to look is on farms, where the vast majority of medically important antibiotics are sold.

Antibiotics on farms are typically used in three ways: to treat sick animals, to prevent infections, and to fatten up animals. The first use is uncontroversial: Everyone agrees that it's okay to use antibiotics to treat animals that come down with disease. But public health experts have criticized the latter two uses. They argue that many livestock producers needlessly overuse antibiotics to prevent infections and promote growth — essentially relying on them as an alternative to hygiene and good nutrition. These are considered "nontherapeutic" uses.

And experts argue this overuse has real consequences. Both the Food and Drug Administration and the Centers for Disease Control and Prevention have told Congress that there's a link between the routine, nontherapeutic use of antibiotics on farms and the superbug crisis in humans.

So far, however, policymakers have struggled to solve the issue. Farmers and ranchers have resisted broad restrictions on antibiotic use. After all, they say, it's not always easy to distinguish between therapeutic and nontherapeutic uses. What if you have 1,000 chickens and 100 get sick? Is it okay to give drugs to all of them? A blanket ban might miss some important nuances. These groups (along with agricultural and pharmaceutical lobbying groups) also argue that any restrictions could raise costs for livestock producers.

The federal government has also struggled with this issue. Last year, the FDA set rules that asked livestock producers to voluntarily phase out the use of antibiotics to boost animal growth (a practice that has been dwindling anyway). But the FDA didn't place any restrictions on using antibiotics for disease prevention, which critics argued was a huge loophole. What's more, the FDA rules added little oversight in how the drugs were used day to day.

California passed the nation's strictest law on antibiotic use

Now California is stepping in. The California bill, which was drafted by Sen. Jerry Hill, is designed to fill in some of these regulatory gaps and further reduce the use of certain antibiotics on farms.

Effective January 1 2018, SB 27 requires livestock producers to get prescriptions for any medically important antibiotics (i.e., those used by humans) that they want to give to livestock. That means it'll be more difficult for farmers to buy these drugs over the counter — which is where they get most of their drugs right now. So this provision is meant to add some oversight.

The law also bans livestock producers from using antibiotics to promote growth or for ambiguous "prevention" purposes. The idea is that veterinarians would only write prescriptions for antibiotics if they were used specifically to treat sick animals or prevent a disease where there's a clear risk. So if, say, a rancher had hundreds of cattle and a few came down with an infection, he'd be able to give antibiotics to the rest as a preventive measure if the vet thought that made sense. But he couldn't give them daily antibiotics "just in case," as a matter of routine.

The California law also establishes an antibiotic stewardship program and requires the state Department of Food and Agriculture to bolster oversight by creating a tracking system on antibiotic usage and resistance.

California's antibiotic approach could spread to other states

California is one of the nation's top producers of livestock, so the law will have a major immediate impact. But it could also have a ripple effect elsewhere, since a number of other states are considering similar legislation. Crucially, California could demonstrate that responsible antibiotic use, guided by veterinarians, is a workable middle ground on the prevention question.

According to Bloomberg, only seven lawmakers voted against the bill and industry associations in the state didn't put up much of a fight. "I think the bill is basically doing something that we in California have been doing all along, which is phasing out antibiotic use," Bill Mattos, president of the California Poultry Federation, told Bloomberg. "It's something that the industry is living with."

Perhaps the most immediate impact of SB 27 would be drawing more attention to the problem of antibiotic resistance. The state law is part of a trend toward cutting back on antibiotics in response to consumer concern. In April, Tyson Foods — the biggest chicken seller in the US — announced plans to eliminate the use of human antibiotics in its flocks by 2017. The pledge came two months after McDonald's announced that within two years it will stop buying chicken that has been reared on antibiotics meant for humans.

Rep. Louise Slaughter (D-NY), the only microbiologist in Congress, applauded the bill. "This bill will make real change in agricultural antibiotic use and curb the growth in antibiotic resistance that threatens the health of our country," she wrote in a letter to Gov. Brown.

Avinash Kar, a senior attorney with the Natural Resources Defense Council’s Health Program, called SB 27 a "game changer": "By reining in the misuse of these miracle drugs, it helps ensure that life-saving antibiotics will be effective when we need them most."