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Foster Farms and the Complexity of Antibiotic Resistance

With the official case count in the Foster Farms chicken Salmonella outbreak rising again on Wednesday, many reports have highlighted the fact that at least six of the seven Salmonella strains associated with the outbreak are resistant to antibiotics.

What may not be clear to consumers is what exactly it means for the strains to be antibiotic-resistant. Are the strains resistant to all antibiotics, or just some? Will common antibiotics still treat the most severe Salmonella infections?

The short answer is that the strains of Salmonella Heidelberg involved in the outbreak are resistant to a number of antibiotics, but not several commonly used to treat severe Salmonella infections. However, the issue of antibiotic resistance is complex, and it often takes time for epidemiologists and clinicians to piece together a detailed understanding of each strain’s full spectrum of resistance.

It also doesn’t help when a federal government shutdown furloughs the specialists working to uncover those resistances, said Dr. Christopher Braden, director of the Division of Foodborne, Waterborne and Environmental Diseases at the U.S. Centers for Disease Control and Prevention.

In the case of the most recent Foster Farms Salmonella outbreak, epidemiologists gathered Salmonella samples from three distinct sources: humans sickened in the outbreak, retail packages of Foster Farms chicken, and environmental samples taken from the implicated Foster Farms production facilities by inspectors with the USDA’s Food Safety and Inspection Service.

Due in part to delays caused by the government shutdown, one of the seven strains involved in the outbreak has not yet been subjected to any antimicrobial resistance testing, Braden said. But each of the other six have shown resistance to at least some antibiotics, and four of those strains include samples classified as multi-drug resistant, meaning they resist treatment from at least three different classes of antibiotics.

To date, samples collected from outbreak victims were resistant to combinations of the following antibiotics: ampicillin, chloramphenicol, gentamicin, kanamycin, streptomycin, sulfisoxazole, and tetracycline.

Antibiotics are sorted into classes based on a number of factors, including the function by which they kill bacteria. The Foster Farms outbreak strains all seem to be susceptible to fluoroquinolones and cephalosporins, two classes of drugs commonly used to treat the most severe Salmonella infections.

Deciding when to use antibiotics

A physician’s decision to use antibiotics is not made lightly. The vast majority of Salmonella infections don’t require antibiotics to treat, Braden said.

Physicians should only resort to antibiotics for Salmonellosis when the bacteria has infected a patient’s bloodstream, he added. In the Foster Farms outbreak, around 14 percent of Salmonella isolates have come from patients’ blood.

In such cases, the patient’s clinic would be performing antibiotic-resistance testing on the isolate. But most clinical resistance tests take two days to produce results, and so, in the meantime, physicians immediately begin treating patients with the antibiotic most likely to be effective.

“I can’t say what every physician would pick,” Braden said. “Not knowing immediately if the strain is drug-resistant or not, I would probably pick a cephalosporin.”

Braden said Salmonella strains resistant to fluoroquinolones or cephalosporins are very rare, and there has been no recorded case of a Salmonella strain that was resistant to all antibiotics.

If, for some reason, the physician’s first choice doesn’t prove effective, they may move on to testing another drug or await the results of the resistance-testing on the patient’s Salmonella isolate.

A physician’s first or second choice will most likely be effective in treating the infection, Braden said, but it would not be impossible for the strain to resist two seemingly wise choices without having the results of the resistance test.

Resistant strains more virulent

For reasons medical professionals still do not completely understand, drug-resistant strains seem to cause more serious infections in patients whether antibiotic treatment is appropriate or not.

Whereas the average Salmonella outbreak has a hospitalization rate of around 20 percent, the Foster Farms outbreak has hospitalized 40 percent of patients, a proportion likely attributable in part to the high rate of antibiotic-resistance among the outbreak strains.

“It’s not something that’s understood intuitively,” Braden said. “For some reason, they’re sicker than patients who have infections from strains that aren’t antibiotic-resistant.”

While this outbreak has been linked to three Foster Farms processing plants in central California, one of the seven outbreak strains matches the strain responsible for a Salmonella outbreak earlier this year that derived from a Foster Farms plant in Washington State.

What is curious about that strain, according to Braden, is that in the Washington State outbreak the strain was resistant to cephalosporins, while the same strain isolated from the California outbreak was not.

The discrepancy between the two resistance profiles of the same strain has Braden worried that CDC researchers are missing a piece of the antibiotic-resistance puzzle in these outbreaks.

Part of that problem resulted from CDC being required to sideline its National Antimicrobial Resistance Monitoring System during the government shutdown.

“The shutdown and the furloughs really threw a wrench in this investigation,” Braden said. “I can see we still don’t have some of the data we should have simply because we weren’t functioning for a while.”

© Food Safety News
  • John Mark Carter

    There’s a new piece of information out regarding the unusually high virulence of some of these new antibiotic resistant strains. We know well that antibiotic resistance is commonly acquired from other bacteria via exchange of fairly large segments of DNA, such as plasmids and phage. Many of these new strains share a Multi-Drug Resistance gene that is linked to some new virulence factors as well as some well-described virulence factors. So it seems both traits arrived on the same piece of DNA.