Hesta and the Benedictine Sisters of Boerne are part of an investor movement using the stock market to lobby the world's biggest fast-food chain.
They want the company to make burgers from animal products farmed with fewer antibiotics.
The use of antibiotics in meat production has emerged as a new front of shareholder activism, similar to the lobbying that fossil fuel companies have been facing over climate targets.
However, the investor group's push is divisive, with McDonald's hitting back and scientists raising questions about some of the policies being put forward.
The unlikely antibiotic allies are resolute in their bid to bring about structural change, despite the criticism.
"We intend to stay at the table," Sister Susan Mika, a representative of Benedictine Sisters of Boerne, told the ABC.
So how did a group of Texan nuns and a major super fund end up in a cross-country battle with an American takeaway staple?
The Catholic nuns taking on McDonald's
The Benedictine Sisters of Boerne have a 32-year history of lobbying companies on issues such as water access and foreign workers' rights.
Their activism was spearheaded by Sister Mika and others, quickly growing into a coalition of 26 monasteries across the United States and Mexico.
However, the sisters didn't get interested in antibiotics reduction until about 10 years ago, when global attention was turning to antimicrobial resistance (AMR).
AMR occurs when bugs that cause infectious disease can't be killed by the medicines designed to quash them, such as antibiotics.
It has given rise to so-called "superbugs", with bacterial AMR contributing to the deaths of about 4.95 million people in 2019, according to data published in the Lancet.
The problem is particularly acute in poorer countries, and it also affects farm animals when they face disease outbreaks. Even plants are facing superbugs.
"It's really a significant problem," said Branwen Morgan, the lead at the Antimicrobial Resistance Mission at Australia's national science body, the CSIRO.
"And there's no time to wait. We all have a shared responsibility, trying to work out how we mitigate the impacts of AMR, which affects food security, biosecurity and economic security."
It is those economic impacts that piqued Hesta's interest.
"So antimicrobial resistance is a systemic threat to both global health and our ability to deliver strong long-term returns to our members," the fund's head of responsible investment, Kim Farrant, told the ABC.
Ms Farrant cites modelling by the World Bank, which previously estimated that AMR – if left unchecked – could wipe out 3.8 per cent of the world's gross domestic product by 2050.
The bank's high-impact scenario found global trade would drop, especially in agriculture as animals get sick, and that healthcare costs globally would soar.
"It's the same impact as the global financial crisis," Ms Farrant says of the World Bank's forecast.
Hesta, which is a specialist superannuation fund for Australian healthcare workers, has also done surveys with its 1 million members about this topic, and found 80 per cent of them were aware of it.
Members told the fund they were seeing diseases that were no longer treatable, and they were having to give patients multiple types of antibiotics and keep them in hospital longer.
"And they had views on what Hesta should be doing as a fund to help address this issue," Ms Farrant says.
So what can a fund do about AMR?
While AMR is a natural phenomenon, the World Health Organisation (WHO) links the "overuse and misuse" of antibiotics in people, plants and animals to the exacerbation of this trend.
"In Australia, we're really fortunate that we have quite strong regulation and practices [around antibiotic use in animals and humans]. But this is very much a global issue," Hesta's Kim Farrant says.
And this global approach has turned McDonald's into a target.
Fairr, which is a global investor alliance working on risks in the world's food sector, identified the fast-food chain as the top company to lobby over this issue.
That's due to its sheer size and ability to influence a massive network of agricultural suppliers, Fairr said.
Fairr also highlighted other food companies, including KFC's parent company Yum! Brands, The Cheesecake Shop, Domino's Pizza, Starbucks, Wendy's and the owner of Burger King, Restaurant Brands International.
All of these entities are listed on the world's biggest stock market on Wall Street, and are beholden to engaging with their shareholders as public companies.
Hesta owns McDonald's stock — specifically 0.014 per cent of its market value — through an international fund, as well as shares in Yum! Brand's and US food giants Tyson and Hormel.
It officially joined Fairr in 2020 to fight AMR before going on to lodge shareholder resolutions two years later.
These are actions that are lodged by entities that own stocks in a company and are voted on by all other shareholders, and are a big part of the shareholder movement that is also fighting climate risks.
Hesta's first AMR resolution was against Hormel, the American owner of tinned meat, Spam.
The Hormel Foods resolution urged the company to conduct a study into the public health costs from the use of antibiotics in its supply chain.
The vote received 14 per cent in favour, excluding management's 47 per cent ownership.
The next year, Hesta lodged more resolutions, this time against three companies: Hormel, Tyson, and McDonald's.
What do experts say about antibiotics resistance?
The action against McDonald's was done by Hesta in tandem with an advocacy group that is prominent on AMR, The Shareholder Commons, and another investment fund, Amundi.
Their joint resolution on the NYSE called on McDonald's to comply with WHO guidelines on the use of antibiotics in food-producing animals.
WHO has long recommended that antibiotics deemed medically important for human use be significantly restricted in food-producing animals.
That includes the "complete restriction of these antibiotics for growth promotion and disease prevention without diagnosis".
"Healthy animals should only receive antibiotics to prevent disease if it has been diagnosed in other animals in the same flock, herd, or fish population," the WHO recommended last decade.
However, scientists said implementing these standards was easier said than done.
Nations have varying approaches to antibiotic use and distinct interpretations of what is classified as medically important.
That is despite WHO having its own global list, and experts recognising that lists vary from country to country, when national health priorities are considered.
Outside of WHO, which is primarily concerned with human health, there are also advice lists from other specialist bodies, such as the World Organisation for Animal Health.
In Australia, for instance, regulators don't use the phrase "medically important" but they do have rules around what can and can't be used in agricultural settings.
"We have a relatively unique position in Australia," the CSIRO's Branwen Morgan said.
"And that's because we [have] very strict regulation around the use of antimicrobials in our primary industries."
Dr Morgan said those pushing new policies had to therefore be "very careful of unintended negative consequences".
"Guidelines for antimicrobial use are very context dependent and locally dependent," she said.
"We wouldn't want companies in the animal or food industry sector to stop using antimicrobials. Businesses with unhealthy animals aren't good businesses and economically viable businesses."
She also said the most pressing issues in Australia were not the use of antibiotics on farms, but the overuse of them in humans and in the treatment of waste, which could spread into the environment.
On a global scale, Dr Morgan said helping low socio-economic countries implement stronger rules on the use of antibiotics was a more important issue.
McDonald's hits back at Hesta's proposal
In response to Hesta's resolution, McDonald's argued it was already doing plenty to eradicate unnecessary antibiotics in its supply chain, with policies that echoed WHO guidelines.
The fast-food company released a policy in 2017 to eradicate medically important antibiotics of the highest priority from its chicken supply chain in "top markets".
It also pledged to stop the use of antibiotics entirely in chicken for the purposes of growth promotion and for disease prevention in flocks.
However, this year, McDonald's said some markets still had not achieved these milestones, with the company hoping to fulfil its promise in 2027.
It has not made similar pledges on pork or beef.
"Our intention is to help drive positive behavioural change and transparency," it said in May.
"As well as enable comprehensive assessment of antibiotic use across our in-scope supply chains and industries in the future.
"To this end, we are engaged in global partnerships across our supply chains to gather antibiotic use data, which helps inform the development, implementation, and evolution of our responsible protein-specific antibiotic use policies applicable for in-scope markets.
"Because each supply chain is unique, we must work in a targeted way to meet our commitments."
McDonald's was contacted for comment about Hesta's and the Texan nuns' proposals but referred the ABC to its shareholder statements and declined to say anything further.
In previous statements, it has called Hesta's push "duplicative" and "unnecessary", and recommended that all other shareholders vote against it. The resolution got 18 per cent support, excluding company management.
The Shareholder Commons, which was the lead on the action, disputes McDonald's claim that their action was unnecessary and says the fast-food chain still isn't following WHO guidelines.
Hesta's Kim Farrant said the fund was satisfied with the results of its efforts, having also lobbied pharmaceutical companies to make changes.
"Engagement is never a one-meeting sort of process," she said.
"It occurs over a number of years and is intended to drive awareness and action.
"We do hope to see further support from a range of investors as they also recognise this as a systemic risk."
As part of their bid to create more awareness, Hesta moved its attention to lodging a similar AMR vote against the owner of KFC, Yum! Brands during Wall Street's shareholder meeting season in May.
They received a similar result, while McDonald's was instead tackled by the sisters.
How the sisters have gone further than Hesta
In Texas, the sisters have been lobbying McDonald's for far longer than Hesta.
In their 10 years of activism against the $194 billion company, they have achieved something that the fund has found difficult: getting meetings with McDonald's representatives.
All because they hold just $US2,000 in McDonald's shares.
"We own the stock, and so we said that we would step up and be the leader in order to raise questions with the company," Sister Mika said.
"If you're owning shares in any company, you are an owner of the company. That gives you power.
"And that's how we look at this. It's a ministry to us."
"When we first started raising questions [10 years ago] the company said they would start with chicken, which they did, and they have taken it out of the chicken across the US [and many other markets].
"So we're still working on the pork and the beef."
The sisters also started lodging shareholder resolutions against McDonald's a few years ago, with their latest lodged just a few months ago to tackle those meats.
It called for McDonald's to phase out the use of all medically important antibiotics "for disease prevention purposes" in pork and beef products.
"The basic premise of what we're talking about here is that there are antibiotics for animals and there's antibiotics for humans," Sister Mika says.
The sisters' vote in May 2024 got just 15.16 per cent in favour, and another slap down from McDonald's, which vigorously defended its existing policies.
"Our board unanimously recommends that you vote against this proposal," it noted.
"Overall reduction remains an intended outcome, while allowing for the necessary treatment of sick animals, aligned with herd veterinarian direction."
McDonald's didn't respond to multiple requests for comment from the ABC.
However, the pressure on companies about antibiotics isn't going away.
Last year, despite Australia being seen as having tight policies on agricultural use, supermarket giant Woolworths put out a brief policy on the use of antibiotics in its supply chain.
Its rival Coles says it has one too, but that it is confidential.
Meanwhile, other companies overseas have walked back on pledges after finding them too difficult to implement, including the US food giant Tyson, which backflipped on an antibiotics-free chicken pledge.
"Consumers have a very powerful voice," the CSIRO's Dr Branwen says.
"And sometimes they may not realise that what they're doing and calling for a particular action is actually going to have detrimental effects."
Despite her reluctance to support any specific shareholder resolution, Dr Branwen thinks Hesta and the sisters have a role to play in fighting a global problem.
"Shareholder action is a really powerful mechanism to make the broader community aware of the antimicrobial resistance problem globally," she said.
"A key way that investors and insurers can help to focus on the AMR problem at large is by considering appropriate use and what that means."
That includes measures to reduce the need for antibiotics, she said, such as developing better vaccines and giving farm animals better ventilation, so they don't get sick to begin with.
Back in Texas, the sisters are about to enter their 10th year of engaging with McDonald's, and Sister Mika said they would be continuing that relationship as long as possible.
"I think most companies want to improve," she said.
"And so we're in it for the long haul. We intend to stay at the table for as long as possible."