Sniffing out Salmonella

by Lynn Petrak
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There are more than 2,500 Salmonella serotypes, but the CDC tracks only 30 that account for more than 95 percent of human illness confirmed as due to Salmonella.

As pathogens go, Salmonella is as insidious as it is dangerous. According to the US Centers for Disease Control and Prevention (CDC), most of the millions of cases of food poisoning that occur in the US each year are caused by norovirus or by Salmonella-tainted foods. The CDC estimates that more than 1 million people in this country contract Salmonella in a given year, while more than 400 annual deaths have been linked to the pathogen.

Those statistics have long given pause to those who process fresh, raw foods susceptible to Salmonella bacterium, including eggs and poultry. Ensuring the safety of their products from farm-to-plate by working to detect, prevent and eradicate Salmonella has been an industry priority for decades now, as industry organizations, university researchers, scientists and processors have worked together on several fronts.

Many efforts have focused on the early detection of Salmonella closer to the farm segment of the food chain. One way scientists have done that is by working to identify different Salmonella classifications, or serotypes, that are potentially harmful because of their virulence or their propensity to change.

Fortunate findings

One recent study sought to better identify problematic strains that vary in virulence, which are known by scientists as pathotypes. The research project, officially named “Rapid Molecular Pathotyping of Major Salmonella enterica Serotypes Based on Single-Nucleotide Polymorphisms (SNPs) in the Adenylate Cyclase (cyaA) Gene,” was undertaken at the USDA’s Agriculture Research Service Egg Safety and Quality Research Unit in Athens, Ga. Dr. Michael Rothrack and Dr. Jean Guard were the lead researchers on the project.

The project was funded by Tucker, Ga.-based US Poultry and Egg Association (USPOULTRY) and the USPOULTRY Foundation as part of the organizations’ ongoing research-based food-safety initiatives. “One of the challenges in ensuring food safety is finding the source of the Salmonella that is found on a contaminated product. There are many different types of Salmonella and there is a great need to quickly, accurately identify Salmonella on food products so the source can be identified and interventions implemented. The technique described in this research enhances our ability to do this,” explains Dr. John Glisson, vice president of research programs for USPOULTRY.

Guard underscores the importance of serotyping in the detection of Salmonella strains that impact human illness and, accordingly, the tools that fight it. “According to experts at the CDC and the World Health Organization, serotyping of Salmonella is an international language developed over 50 years that is perhaps the most powerful method available for describing the potential of a bacterial pathogen for causing disease,” she explains.

Although there are more than 2,500 serotypes, the CDC tracks only the top 30 associated with illness, points out Guard: “These 30 account for greater than 95 percent of the human illness confirmed as due to Salmonella.” 
 

By using SNPs, future protocols can be altered to identify any emerging strains of Salmonella that can impact the safety of fresh poultry and eggs.

If the study wasn’t quite akin to looking for a needle in a haystack, it was more like differentiating the types of hay in the haystack. “We wanted to be able to quickly and easily detect serotypes that might be economic threats to the poultry industry but that could be confused with Salmonella serotype Enteritidis, the most common cause of food borne salmonellosis in the world,” Guard explains.

For example, she says, the serotypes that threaten the poultry industry, such as Pullorum and Gallinarum, have evolved to sicken chickens rather than humans. “All three serotypes are very related and we were receiving isolates from around the world,” she notes.

During the course of their research, Guard and Rothrock developed reagents and protocols to detect some of the major Salmonella serotypes and, further, to differentiate different isolates within a serotype. The team used single nucleotide polymorphisms (SNPs) of two different genes, which allowed them to detect 89 to 100 percent of a panel of Salmonella isolates for different settings, including poultry production and processing environments. They determined that the adenylate cyclase gene (cyaA) represents an ideal target for assays.

The research project was extensive. According to Guard, between discovery and application research, it was a five-year process.

Ultimately, the results of the project can lead to the development of future protocols that cover larger numbers of serotypes/pathotypes. Food safety professionals working in the poultry industry can now quickly determine which serotypes are present. Moreover, by using SNPs, the protocols can be altered to identify any emerging strains of Salmonella that can impact the safety of fresh poultry and eggs.

The study is a breakthrough in many ways, including the use of what’s known as Intergenic Sequence Ribotyping (ISR), which would have been difficult to be discovered by whole genome sequencing. The upshot from the science is that ISR is an effective tool in the battle against Salmonella-related food poisoning.

“ISR represents a breakthrough because it makes serotyping of Salmonella affordable and accessible to any microbiologist, hospital, veterinarian, private physician, public health laboratory, small consulting laboratory, research group, or anyone qualified to handle samples for isolation of Salmonella. The data can be acquired and examined privately without involving reference laboratories,” Guard notes.

Early warnings

Those in the poultry business have some takeaway from this study’s findings that impact the safety of their products. “Poultry producers are in the position to do their own testing before regulators come on farm, perhaps through a cooperative lab or consultant,” Guard says. She adds that regulators are already looking for problem serotypes when they visit farms and will continue to do so. “Producers need a simple survey method so they can know as early as possible if there are emerging problems so that interventions can be applied immediately and not just after a government agency has reviewed the farm.”

As a result of this project’s findings, those interventions can come at a critical time and make a significant difference for processors. “Consider the ISR method an early warning system designed to be used by anyone at any time,” says Guard, who emphasizes that while there is a process to get the available database on serotypes/pathotypes, there is no charge.

“Our hope is that one day, a farmer will have a cell phone app that allows him or her to receive a monthly Salmonella serotype report.”

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