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Salmonella Vaccine Contributed to Rise of Antibiotic-Resistant Bacteria

Researchers find that the <i>Salmonella</i> vaccine for poultry contributed to the rise of antibiotic-resistant bacteria in the UK. Credit: Mary Parker, Quadram Institute.

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Researchers from the University of São Paulo, Brazil and the Quadram Institute Bioscience, UK have conducted an investigation into the evolution of Salmonella bacteria in Brazilian poultry. The study identified that introducing a Salmonella vaccine for poultry – in addition to increasing use of antibiotics by Brazilian farmers – has led to strains evolving that are antibiotic-resistant. The results are published in PLoS Genetics.

Salmonella enterica – A common cause of food poisoning

An estimated 1.35 million Salmonella enterica infections (salmonellosis) occur in the US each year, resulting in 26,500 hospitalizations and 420 deaths, according to the Center for Disease Control and Prevention. Hallmark symptoms of infection include diarrhea, stomach cramps and a fever, typically presenting between 6 hours to 6 days post-infection.

What is Salmonella enterica?

Salmonella enterica (S. enterica) is a specific species of Salmonella. It comprises six subspecies and several thousand serovars (or serotypes) that can cause illnesses ranging from mild to severe. Serovar refers to a group of closely related microorganisms that can be distinguished by specific antigens presented on their cell surface. 

There are several ways in which a human can become infected with S. enterica, such as eating contaminated food, poor food handling, touching infected surfaces and interacting with infected pets or other animals.

Foods that can be a source of Salmonella include:

  • Raw meats, such as poultry and seafood
  • Raw, or undercooked eggs
  • Dairy products that have not been pasteurized
  • Fruits and vegetables that are contaminated

Depending on where we live in the world, it may not be able to source certain foods locally, requiring importation from other countries. Brazil, for example, is the world’s largest chicken meat exporter.

Different serovars of Salmonella can be found across different countries. A research team led by Professor Andrea Micke Moreno of the University of São Paulo and Professor Alison Mather, group leader at the Quadram Institute Bioscience and a food standards agency fellow, conducted a genomics study to explore whether certain strains of the bacterium found in Brazil contribute to food poisoning cases that occur in the UK. 

Moreno and colleagues used next-generation sequencing (NGS) to compare the genomes of 183 Salmonella obtained from chickens in Brazil and 357 Salmonella genomes obtained from humans, poultry and imported Brazilian poultry products in the UK. In addition, they analyzed over 1,259 genomes of the two main types of Salmonella that are found in Brazil using a publicly available database, Entero, to explore how these strains may have evolved.

Genomic detective work

In 2003, Brazil introduced a Salmonella vaccine for poultry. Through their evolutionary analysis the research team identified that, at a similar time as the immunization protocol was introduced, lineages of the two main types of Salmonella now found in Brazil – Salmonella Heidelberg and Salmonella Minnesota – developed.

These serovars of Salmonella express specific genes that enable their resistance to three types of antibiotics: sulphonamides, tetracyclines and beta-lactams. Through their genomic analysis of humans, poultry and imported Brazilian products in the UK, the research team report that these serovars were “the most prevalent serovars in Brazil and in meat products imported from Brazil into the UK”.

Interestingly, the antibiotic-resistant forms of the bacteria were not linked to many cases of Salmonella infection occurring in humans and did not seem to have spread to domesticated UK chickens. “Long-term surveillance data collected in the UK showed no increase in the incidence of Salmonella Heidelberg or Salmonella Minnesota in human cases of clinical disease in the UK following the increase of these two serovars in Brazilian poultry,” the authors write in PLoS Genetics. “These findings indicate that even should Salmonella from imported Brazilian poultry products reach UK consumers, they are very unlikely to be causing disease.”

The researchers conclude that introduction of the Salmonella Enteriditis vaccine in 2003 – combined with increasing use of antimicrobials in Brazil – may have resulted in a replacement of Salmonellae in Brazilian poultry flocks with serovars that are more drug resistant, but less associated with disease in humans in the UK.

Moreno and colleagues refer to their methodology as “genomic detective work”.

“We have tracked how changes in chicken rearing in Brazil have changed the profile of Salmonella bacteria found circulating within the poultry industry,” says Mathers in a news release announcing the publication. “While this poses no immediate health risk to importing countries like the UK, the bacteria were resistant to antimicrobial drugs, and this highlights the importance of taking a ‘One Health’ approach that sees the connections between the health of people, animals and the environment, especially when assessing global food supply chains,” she emphasizes.

The authors note that unfortunately, they were unable to access official data on human Salmonella infections in Brazil to explore whether the serovars have impacted Brazilian public health.

Reference: Alikhan N-F, Moreno L, Castellanos L, et al. Dynamics of Salmonella enterica and antimicrobial resistance in the Brazilian poultry industry and global impacts on public health. PLoS Genetics. 2022. doi: 10.1371/journal.pgen.1010174.

This article is a rework of a press release issued by Quadram Institute Bioscience. Material has been edited for length and content.

Meet the Author
Molly Campbell
Molly Campbell
Senior Science Writer