Salmonella

Janice Haney Carr/CDC

Salmonella: Under very high magnification, this digitally-colored scanning electron microscopic image from the CDC reveals the presence of Salmonella typhimurium bacterium. Recently researchers in Pennsylvania have seen antibiotic-free or organic poultry to be half as likely to be contaminated with drug-resistant salmonella.

WASHINGTON — Meat from conventionally raised poultry was twice as likely to contain multidrug-resistant salmonella as poultry labeled antibiotic-free or organic, according to a first-of-its-kind study that was presented at the recent IDWeek 2019.

A related study of a variety of meat purchased from randomly selected stores found nearly one-third of salmonella in contaminated samples was resistant to three or more antibiotics and more than one in six were resistant to five or more antibiotics. Researchers also looked at data from people who had developed salmonella infection and determined their antibiotic resistance rates were only slightly lower.

Eating salmonella-contaminated meat that is not properly handled or cooked is one of the most common causes of the infection. Severe salmonella infection requires treatment with antibiotics, which is especially challenging when the bug is highly resistant to drugs. 

Antibiotics often are used in animal food production to prevent and treat infection as well as to promote growth. However, overuse and misuse of antibiotics in humans and animals leads to antibiotic resistance, meaning they are less effective when they are needed to treat illness. The U.S. Food and Drug Administration (FDA) noted limiting the use of medically important antibiotics in animals is vital to assuring human and animal health.

“These findings underscore how important it is to use antibiotics judiciously, not only in human medicine but in food-producing animals,” said Dr. Nkuchia M. M’ikanatha, an author on both studies and an epidemiologist with the National Antimicrobial Resistance Monitoring System (NARMS), Harrisburg, Pa. “We found a significant percentage of salmonella bacteria in meat that causes infection in humans were resistant to three or more antibiotics, meaning treatments may not work for some patients who really need them.”

Those at increased risk for severe salmonella infections include people with pre-existing conditions such as heart disease and adults 50 and older whose immune systems are suppressed, he said.

In the first study, researchers reviewed data from chicken and turkey randomly purchased in Pennsylvania between 2008-2017 and found 280 of the 2,733 (10.2 percent) samples of conventionally raised poultry were contaminated with non-typhoidal salmonella compared to 40 of the 748 (5.3 percent) poultry samples labeled antibiotic-free. Further, they determined that 154 of 280 (55 percent) salmonella cultures from conventionally raised poultry were resistant to three of more antibiotics compared to 11 of 40 (28 percent) salmonella cultures from poultry raised without antibiotics.

They also found 68 of 280 (24.3 percent) of the cultures from conventional poultry and 3 of 40 (7.5 percent) of the cultures from antibiotic-free poultry contained a gene that makes salmonella difficult to treat with the only class of antibiotics recommended for use in children.

“Although contamination of retail poultry was found in both conventionally raised and antibiotic-free samples, our results show that Salmonella in poultry produced without antibiotics — based on packaging claims — were significantly less resistant to antibiotics compared with poultry raised using conventional methods,” said Xin Yin, lead author of the study and doctoral candidate at Pennsylvania State College of Medicine, Hershey, Pa. “Consumers should read production labels and make informed choices based on the evidence about the risk of poultry contamination with drug-resistant salmonella.”

In the second study, researchers determined 96 of 2,520 poultry, ground beef and pork chop samples (3.8 percent) purchased during 2015-2017 from randomly selected stores in Pennsylvania contained non-typhoidal salmonella (in this study, researchers did not assess whether the animals were raised with or without antibiotics). They tested the salmonella-contaminated meat samples and 109 cultures from human salmonella infections collected during the same period for susceptibility to 15 antibiotics.

In the salmonella cultures from meat:

  • 28 (29.2 percent) were resistant to three or more antibiotic classes.
  • 17 (17.7 percent) were resistant to five or more antibiotic classes.
  • Resistance to ceftriaxone — the antibiotic most often used to treat serious salmonella infection — was found in 3 in 25 (12 percent) in 2015, 10 in 37 (27 percent) in 2016 and 5 of 34 (14.7 percent) in 2017.
  • 4 contained genes that were resistant to eight antibiotic classes.

In the salmonella cultures from humans:

  • 28 (25.7 percent) were resistant to more than three antibiotic classes.
  • 12 (11 percent) were resistant to more than five antibiotic classes.
  • None were resistant to ceftriaxone in 2015, 6 of 48 (12.5 percent) were resistant in 2016 and 9 of 37 (24.3 percent) were resistant in 2017.
  • 2 carried genes that were resistant to eight antibiotic classes.

“The genes associated with high resistance are especially concerning because salmonella can share them with other bacteria and cause other multidrug-resistant infections, not just salmonellosis.” said M’ikanatha.

In addition to Yin and Dr. M’ikanatha, co-authors of the first study are: Lisa Dettinger, Melinda Johnston, William Eckroth, Brigitte Husband, James Tait, Epiphanie Nyirabahizi, and Heather Tate.

In addition to M’ikanatha and Yin, co-authors of the second study are: Kelly E. Kline, Sameh W. Boktor, Lisa Dettinger, Deepanker Tewari, and Heather Tate.

IDWeek is the annual meeting of the Infectious Diseases Society of America, the Society for Healthcare Epidemiology of America, the HIV Medicine Association and the Pediatric Infectious Diseases Society.

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