“No one has even looked at these strains in the United States,” says Smith. Doctors, she says, often don't determine what strain of MRSA is causing an infection, so it is possible that the bug has been stealthily migrating between farms and hospitals for years.
Smith's next step was to see if there were other ways MRSA might make it off the farm. At a grocery store on the outskirts of Iowa City, Smith pulls a shopping cart from a metal corral, takes a sterile swab from her purse, wipes the cart's handle and deposits the swab in a plastic sheath. Then she heads to the meat case. “I think the average consumer doesn't think of this as a risk,” Smith says, picking up a shrink-wrapped tray of bright red steaks. Her swabs have told a different story.
Beginning in January 2012, Smith and her research assistant, Dipendra Thapaliya, spent a year collecting weekly swabs and meat samples from local grocery stores, including this one. They found S. aureus on nearly every type of surface. Five per cent of grocery carts carried MRSA. Of meat samples, 30% harboured S. aureus, 11% had S. aureus resistant to multiple antibiotics and 3% carried MRSA. The data, which have not been published, also showed that pork products had some of the highest levels of MRSA, whereas meat labelled 'antibiotic free' had little or none. This mirrors what Smith and her colleagues found in samples from farms across the state.
On the menu
Now Smith is conducting detailed genetic analyses of the samples to identify MRSA subtypes and where they might have originated. She and her colleagues have found ST398 in the grocery-store samples. But to their surprise, they also found ST5, which is generally found in hospitals and in 'community' infections with no obvious link to farms or hospitals. Last year, Smith and Timothy Frana, a microbiologist and veterinarian at Iowa State University in Ames, found that veterinary students who carried MRSA in their noses after visiting pig farms picked up mainly ST5, most of which was resistant to tetracycline. The presence of ST5 among livestock suggests that S. aureus strains may move easily between pigs and people, and may become resistant on farms. “It's the most interesting finding from our study,” says Frana.
In lab tests, Smith and her colleagues have found that 30% of the S. aureus harboured in meat is resistant to tetracycline. Given Levy's data from the 1970s, this is not surprising, says Smith. But researchers, including Heaney and his group at Johns Hopkins, are finding that a bacterium's drug-resistance profile can give information about where the bug came from that sequence type and other gene-based categorizations may not. In a study comparing workers from different farms, tetracycline-resistant MRSA showed up only in workers from farms where antibiotics were used.
In the last phase of their research, Smith and her colleagues will try to determine whether MRSA is trafficking between farms, households and clinics (see 'From the farm and back again'). They are taking nasal samples from 1,300 people and swabbing doorknobs, kitchen sinks and even family pets in 96 households around Iowa City. The researchers are comparing the S. aureus strains they find in these samples with the MRSA strains they have found in grocery stores and farms and with strains local doctors find in infected patients. If a strain shows up in all the locations, the researchers will sequence the whole genomes of individual isolates to retrace their movement and evolution. The results have the potential to create the first complete link between farms and clinical cases of MRSA.