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What is MRSA?

MRSA is an acronym that described a particular bacterium: methicillin-resistant Staphylococcus aureus. There’s a lot of information packed into that. Staphylococcus aureus — most people just say “staph” — is a member of a huge, common family of bacteria that live in and on humans through most of our lives. “Methicillin” was the first drug invented to improve on penicillin, when bacteria started to develop penicillin resistance in the 1950s. Methicillin itself isn’t used any more, but many of the drugs that doctors use every day were modeled on its chemical structure. So when we say “MRSA,” what we’re really describing is a very common bacterium that cannot be treated by most of the antibiotics usually used in medicine.

How much MRSA is there?

It’s hard to know, because no one counts it comprehensively — but every survey that is done shows that there is more than we realize. About 19,000 Americans die of its most serious forms in a year, and about 369,000 are hospitalized. At least 4 million Americans are unknowing carriers of the bug; it may not make them sick, but they may pass it to others. It sends at least 7 million Americans, probably many more, to the doctor or emergency room every year. When it occurs in a hospital patient, it adds days to their hospital stay and thousands of dollars to their final bill. Overall, MRSA may cost the healthcare system several billion extra dollars each year.

Who is at risk for MRSA?

Originally, medicine thought that the only people at risk were people who were already in hospitals: the elderly, people with compromised immune systems from AIDS or cancer, patients undergoing surgery, and anyone with a lot of lines — IVs, catheters — piercing their skin. That’s because MRSA was, at first, just a hospital bug. But in the 1990s, MRSA infections started to show up outside hospitals, in children and then adults who had no connection with the health care system. Now it is clear that there are two overlapping epidemics, in hospitals and in the wider world, caused by related but genetically distinct strains. The “community” strain, as the non-hospital one is called, seems to strike wherever people are in close quarters, have skin-to-skin contact, don’t have perfect hygiene, or already have cuts or abraded skin. If you think for a moment, that might describe soldiers in military housing, or prisoners in a correctional institution — but it also describes kids playing school football, and women in a gym.

If MRSA is staph that’s resistant to antibiotics, why can’t we just come up with new drugs?

The reason that so many antibiotics have no effect on MRSA is because those antibiotics have a similar chemical structure, based on a four-cornered arrangement of atoms called the “beta-lactam ring.” To be effective against MRSA, a new antibiotic would have to have a totally new chemical structure. But developing a totally new drug is an expensive enterprise with an uncertain financial return; as a result, fewer drug companies are developing new antibiotics, turning instead to the more stable markets of lifestyle drugs and drugs for chronic diseases. And when a new antibiotic is developed, MRSA rapidly gains resistance to it — making the drug useless, keeping the company from realizing the gain on its investment, and consequently discouraging drug development by other firms. Some researchers fear that, sooner or later, MRSA will force medicine to return to a “pre-antibiotic” era.

Does hospital cleanliness play a role in MRSA?

Sadly, yes. It’s amazing to realize that something as simple as washing hands regularly is a challenge for health care workers, but it’s true: Study after study shows that the rate of hand-washing hovers around 50% unless hospitals work on it consistently and hard. In addition, there’s a ferocious debate in health care over the best way to control hospital infections. Researchers have demonstrated you can control transmission in a hospital by checking patients at admission to see if they are unknowingly carrying bugs such as MRSA — many European countries do this. But others contend it’s too expensive, doesn’t detect all problematic bacteria, or is too vulnerable to lobbying by the companies that make the tests. And federal health authorities haven’t provided any leadership to settle the dispute.

What about the role of animals?

At least 70 percent of antibiotics used in the United States every year — some estimates say more than 80 percent — are given to food animals, not to people. Livestock get antibiotics if they are sick, which is totally appropriate. But they also get antibiotics if they are well, but belong to a herd or flock that has a sick member. And many get smaller-than-treatment, or sub-therapeutic, doses because those doses have the odd effect of making animals put on weight. Giving antibiotics when no disease is present is strongly discouraged in humans — that’s why pediatricians prefer not to prescribe immediately when a child has an ear infection — and it’s a bad idea in animals also. It encourages resistant bacteria to develop in the animals themselves, and also in the manure pits that are present on very large farms. Those bacteria leave farms in groundwater, in dust in the air, and via farmworkers themselves.

And it’s clear that MRSA is being affected by this: The third epidemic of MRSA is a strain called ST398 that arose in livestock in the last decade. It was first found in pigs in the Netherlands, and then in pig-farm workers and their families, and then in hospital personnel. It’s now been found in most of the European Union, in Canada, and in the United States, in pigs and farm workers, and in retail meat. The genetics make clear that this strain arose in animals, and the resistance factors do too: ST398 is resistant to drugs that are used as veterinary antibiotics.