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Penn Vet Stories


One Health - Penn Vet’s Collection of 50,000 Salmonella Strains

By: Louisa Shepard Date: Apr 6, 2015

The 50,000 glass vials locked away in a building on Penn Vet’s New Bolton Center campus hold the keys to new research into Salmonella, the number-one cause of foodborne illness in the United States.

Penn Vet researchers hope their findings will lead to new drugs or vaccines for humans, as well as new ways to protect the animals that produce our food.

Dr. Shelley RankinThe extensive collection of Salmonella strains, one of the largest in the United States, has been assembled over the past 18 years, under the supervision of Dr. Charles Benson Professor Emeritus of Microbiology and Dr. Shelley Rankin, Associate Professor of Microbiology at Penn Vet.

The samples are primarily from livestock, including cows, pigs, and chickens, but also include isolates from eggs and the layer-bird environment as part of the Pennsylvania Egg Quality Assurance Program (PEQAP). The strains were collected at New Bolton Center, in addition to those from the Pennsylvania Department of Agriculture and Penn State University. The three entities comprise the Pennsylvania Animal Diagnostic Laboratory System.

Dr. Dieter SchifferliRankin is moving the collection to Penn Vet’s Philadelphia campus this month, to make the strains available to colleague Dr. Dieter Schifferli, who is conducting two separate, but related, studies designed to better understand host adaptation among different strains of Salmonella. Rankin plans to curate the existing collection, and build upon it by collecting new strains from New Bolton Center and other sources.

“The importance of the collection is huge,” said Schifferli, Professor of Microbiology at Penn Vet. “This collection also has metadata: what animal was the strain isolated from, when, where was that animal located, and was the animal sick or were samples collected purely for surveillance.”

Part of the huge reference collection of Salmonella strains.“We can study these strains over time,” he continued. “It’s a fantastic collection that is very useful for the type of genomics analyses we are doing.”

A major public health concern, more than one million people in the United States get sick from Salmonella each year from eggs, meat, and produce contaminated with the bacteria. Salmonella remains the most frequent bacterial cause of foodborne disease in the U.S. and the agent of typhoid fever in developing countries.

The strains in the Penn Vet collection were isolated between 1997 and 2013. “It’s a snapshot in time of Salmonella strains that were present in Pennsylvania livestock,” said Rankin, who has been working with Salmonella for more than 25 years.

A Salmonella culture grows inside this petri dish.Rankin first worked with the Scottish Salmonella Reference Laboratory in Glasgow, and then came to Penn Vet in 1999 to work with Benson, who had been studying Salmonella since the mid-1980s at New Bolton Center.

“The Salmonella strains were collected primarily for surveillance, to know what types of Salmonella were present in Pennsylvania livestock, and to monitor for antibiotic resistance,” Rankin said.

Schifferli and Rankin will use the Salmonella collection for two recently funded studies:

  • Molecular mechanisms that allow bacteria to leave a host’s intestines and cause systemic infection are poorly understood. Penn Vet’s Center for Host-Microbial Interactions has approved a grant to sequence the genome of hundreds of Salmonella isolates in order to determine the molecular mechanisms that cause this organism to infect humans and livestock. Schifferli’s goal is to understand why some Salmonella strains remain in the intestine, and cause mild gastrointestinal illness, while other strains migrate from the intestine and enter the blood stream, causing serious septic infections that can result in morbidity or even mortality in both livestock and humans. This information could support the development of new drugs or vaccines against septicemic Salmonella.
  • The National Institutes of Health and the US Department of Agriculture are funding studies to look at whether Salmonella isolated from different species, such as human, chicken, bovine, and porcine, show host adaptation traits. Simply stated, what makes a Salmonella strain prefer pigs versus humans versus chickens? Rankin and Schifferli will perform whole genome sequence comparisons to look for attributes that may confer host specificity. “We take each one and sequence and compare, using computers for bioinformatics and biostatistics,” Schifferli said. “We are trying to find specific genes and gene sequences that associate with the specific groups.”

Rankin and Schifferli collaborate with government agencies, like the Food and Drug Administration and the Centers for Disease Control, to share strains. And Rankin is prepared to enter into license agreements with commercial or academic institutions to provide Penn Vet’s Salmonella strains for research or test validation.

A PADLS technician prepares eggs for Salmonella testing.The Clinical Microbiology Laboratory at Penn Vet’s Ryan Hospital that Rankin oversees offers a molecular serotyping service using the xMAP Salmonella Serotyping Assay kit by Luminex; serotyping is an essential tool for epidemiological surveillance

Penn Vet has a long history testing for Salmonella. In fact, protocols developed at New Bolton Center helped create the Pennsylvania Egg Quality Assurance Program, a voluntary certification program for egg producers and processors intended to minimize Salmonella enteritidis contamination of chicken eggs. Pennsylvania’s program became the blueprint for the federal Egg Safety Program, adopted in 2010. A 2010 collaboration between Life Technologies Corporation and the University of Pennsylvania led to the launch of the first real-time, polymerase chain reaction (PCR) detection kit to test for Salmonella enteritidis in poultry eggs, providing an approximate 10-fold reduction in waiting time for accurate results, from 10 days to 27 hours.