Tracking Antibiotic Resistant Salmonella
August 29, 2013

Tracking Antibiotic Resistant Strains Of Salmonella

Peter Suciu for – Your Universe Online

Researchers at Penn State’s College of Agricultural Sciences are seeking to identify and track strains of antibiotic resistant bacteria as they evolve and spread. The goal is stop the spread of salmonellosis, an infection associated with the bacteria salmonella.

Antibiotic-resistant strains of this bacteria are a major threat to public health, and the number of resistant strains is apparently increasing. Each year in the US, the various strains of salmonella are responsible for an estimated 1 million illnesses, 20,000 hospitalizations and 400 deaths, at an economic cost exceeding $3 billion.

Those infected with salmonella may develop diarrhea, fever and abdominal cramps 12 to 72 hours after infection. The illness usually lasts 4 to 7 days, and most people recover without treatment. However, in some cases – especially those involving the elderly, infants and individuals with impaired immune systems – the diarrhea may be so severe that the patient needs to be hospitalized. In these patients, the salmonella infection may spread from the intestines to the blood stream, and then to other body sites and can cause death unless the person is treated promptly with antibiotics.

According to the Center for Disease Control and Prevention (CDC), since the 1990s there have been 45 salmonella outbreaks linked to live poultry, with a reported 1,563 illnesses, 221 hospitalizations and five deaths.

The CDC also traced salmonella outbreaks to aquatic pet frogs last March and even pet hedgehogs. The greater danger remains the spread of antibiotic resistant bacteria that could spread from farms.

The Penn State researchers have been trying to trace the transmission of individual strains from agricultural environments to humans through the food system. This has been difficult because of the rapid evolution patterns in the salmonella bacteria. Resistance patterns can change rapidly, which has made it difficult to determine where some highly resistant strains may have originated.

Now, however, the researchers are able to find a way to trace the bacteria to the source.

Michael DiMarzio developed a method for identifying and tracking strains of Salmonella Typhimurium as they evolve and spread. DiMarzio is a doctoral candidate in food science, working under the direction of Edward Dudley, associate professor and Casida Development Professor of Food Science.

“Typhimurium infections have exhibited a gradual decline in susceptibility to traditional antibiotics, a trend that is concerning in light of this pathogen's broad host range and its potential to spread antibiotic resistance determinants to other bacteria,” DiMarzio said in a statement. “Now more than ever, it is imperative to effectively monitor the transmission of Salmonella Typhimurium throughout the food system to implement effective control measures.”

The researchers were able to build on previous studies conducted in Dudley’s lab, and DiMarzio developed a new approach that they used to identify antibiotic resistant strains of Typhimurium. Using this approach, he focused on virulence genes and novel regions of the bacteria’s DNA known as clustered regularly interspaced short palindromic repeats, or CRISPRs.

The CRISPRs sequences are present in many food-borne pathogens, and the researchers demonstrated that CRISPR sequences can be used to identify populations of salmonella with common antibiotic-resistance patterns in both animals and humans.

“Specifically, we were able to use CRISPRs to separate isolates by their propensity for resistance to seven common veterinary and human clinical antibiotics,” DiMarzio added. “Our research demonstrates that CRISPRs are a novel tool for tracing the transmission of antibiotic-resistant Salmonella Typhimurium from farm to fork.”

The researchers discovered that several subtypes of Salmonella Typhimurium repeatedly appeared in the frozen collection of salmonella samples that were taken from cows, pigs and chickens at Penn State’s Animal Diagnostic Laboratory.

The team examined 84 unique Salmonella Typhimurium isolates collected between 2008 and 2011. From this collection, the researchers were able to identify subsets of the overall salmonella bacteria population that appeared to be more prone to acquiring antibiotic resistance.

“Our challenge now is to learn what makes those strains different,” noted DiMarzio. “Why do some strains acquire resistance while others don’t, even though both are circulating widely among animal populations? We will need to know that to try to control them.”

The findings of their study were published in the September issue of Antimicrobial Agents and Chemotherapy.