Study: Antibiotic-free poultry meat less likely to harbor multidrug-resistant Salmonella

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An analysis by researchers in Pennsylvania found that meat from conventionally raised poultry harbored nearly twice as much multidrug-resistant Salmonella as meat from antibiotic-free poultry, according to a study reported today at IDWeek 2019.

The findings come from a study conducted by scientists with the Pennsylvania Department of Health, Penn State College of Medicine, and the Food and Drug Administration (FDA) that examined non-typhoidal Salmonella cultures from nearly 3,500 samples of chicken and turkey bought from randomly selected stores in Pennsylvania from 2008 through 2017. Analysis of the Salmonella cultures found that 55% of those from conventionally raised poultry meat were resistant to three or more antibiotic classes, compared with 28% of the cultures from the antibiotic-free poultry meat.

Salmonella is a leading cause of foodborne illness, affecting more than 1.2 million Americans each year. While most cases are self-limiting, some salmonellosis cases require antibiotics and hospitalization. Drug-resistant Salmonella is harder to treat and can cause more severe and sometimes deadly infections.

"Antibiotic-resistant Salmonella is a public health concern," presenting author Xin Yin, MPH, a doctoral student at Penn State College of Medicine, said in a press conference. "The key message here for consumers or the average person is that, when you go to grocery stores to purchase poultry meat, you should read the antibiotic-related production claims, and think about the rate of antibiotic-resistant Salmonella before you make the purchase." 

In a related study conducted by some of the same scientists, nearly a third of Salmonella cultures from poultry, ground beef, and pork samples randomly bought from stores in Pennsylvania from 2015 through 2017 were resistant to three or more antibiotic classes. And a quarter of cultures from human Salmonella infections collected during the same period were resistant to three or more antibiotic classes.

More multidrug-resistance in conventional meat

In the first study, Yin and his colleagues analyzed 3,481 chicken and turkey samples collected for the National Antibiotic Resistance Monitoring System (NARMS), a program that tracks antibiotic resistance in Salmonella and other potentially harmful bacteria that are commonly transmitted through food. Poultry packages with "organic," "antibiotic-free," or "no antibiotics ever" labels were considered antibiotic-free for the purposes of the study. These labels refer to antibiotics that are also used in human medicine. 

Yin said they focused on poultry because it's an important vehicle for the transmission of drug-resistant enteric pathogens (bacteria that cause gastrointestinal illness), and because Americans eat a lot of chicken. 

"In 2019, the projected consumption of poultry meat [per capita] is around 50 kilograms," Yin said. He also noted that antibiotics have been widely used to treat illness in poultry.

Overall, the researchers found Salmonella strains in 10.2% of the conventionally raised poultry meat samples (280 of 2,733), compared with 5.3% of the antibiotic-free chicken and turkey samples (40 of 748). Review of antimicrobial susceptibility and whole-genome sequencing data from 320 of the Salmonella isolates found resistance to three or more antibiotics in 55% of the isolates from conventionally raised meat (154 of 280), compared with 27.5% of the isolates in the meat labelled antibiotic-free (11 of 40).

Further analysis found that 24.3% of isolates from the conventional poultry meat (68 of 280) contained the extended-spectrum beta-lactamase (ESBL) gene blaCMY-2, which confers resistance to beta-lactam antibiotics.

Yin said it's noteworthy that a similar study conducted 10 years ago, using far fewer samples, found no statistically significant difference in the amount of antibiotic-resistant Salmonella found in conventional and antibiotic-free poultry-meat samples. He also noted that a broader analysis of the NARMS database from 2008 through 2017 confirmed the findings. 

Resistance compared in meat and human isolates

In the second study, led by Nkuchia M'ikanatha, DrPH, MPH, a surveillance epidemiologist with the Pennsylvania Department of Health, the researchers set out to compare antibiotic resistance levels in Salmonella isolates from retail meat samples with resistance levels from clinical isolates collected from patients with Salmonella infections. They used pulsed-field gel electrophoresis—a method of DNA fingerprinting used to investigate foodborne disease outbreaks—to match the animal and human isolates. A total of 96 Salmonella isolates collected from 2,520 poultry, ground beef, and pork samples were compared with 109 clinical Salmonella isolates.

In the Salmonella isolates from the meat samples, 29.2% (28 of 96) were resistant to three or more antibiotic classes and 17.7% (17 of 96) were resistant to five or more classes. Four of the isolates contained genes conferring resistance to eight antibiotic classes. The researchers also observed in the meat isolates that resistance to ceftriaxone, which is frequently used to treat severe Salmonella infections, rose from 12% in 2015 to 27% in 2016, then declined to 14.7% in 2017.

In the clinical isolates, resistance to three or more antibiotic classes was found in 25.7% (28 of 109), and resistance to five or more antibiotics was found in 11.0% (12 of 109). Two of the clinical isolates carried genes that confer resistance to eight antibiotic classes, and ceftriaxone resistance rose from 0% in 2015 to 12.5% in 2016 and 24.3% in 2017.

"The genes associated with high resistance are especially concerning, because Salmonella can share them with other bacteria, such as E coli, and cause other multidrug-resistant infections, not just salmonellosis," said M'ikanatha, who worked on both studies. 

M'ikanatha added that the findings are significant because clinicians need to be aware that not all Salmonella cases are caused by "garden variety" Salmonella.

"When you have a Salmonella infection where the clinician decides that antimicrobial therapy is indicated, it's very important to look at the susceptibility test results, and determine whether the drug they intend to use is going to be susceptible," he said.

Federico Perez, MD, an assistant professor of medicine at Case Western Reserve University and a moderator at the press conference, said the findings also highlight the important links between antibiotic use in animals and human health.

"As clinicians, it is crucial to know the prevalence of antibiotic resistance in Salmonella that affects our patients. Furthermore, for those interested in the science of antibiotics resistance, characterization of resistance determinants in Salmonella illuminates how antibiotic use in animals raised for human consumption impacts human health, thus informing the concept of One Health," he said.

See also:

Oct 3 IDWeek 2019 press release

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