Antibiotic resistant strains of Salmonella can be identified and tracked through the food chain as they evolve and spread, according to US researchers.
Tracing the transmission of individual strains from agricultural environments to humans through the food system is difficult because of the evolution of the bacteria’s resistance patterns, said the team from Penn State's College of Agricultural Sciences.
They found that several subtypes of Salmonella Typhimurium showed up repeatedly in the frozen collection of samples taken from cows, pigs and chickens in their Animal Diagnostic Laboratory.
The researchers looked at 84 unique Salmonella Typhimurium isolates collected from 2008 to 2011.
Antibiotic-resistant isolates identified in humans is steadily increasing, suggesting that the spread of antibiotic-resistant strains is a major threat to public health, said DiMarzio et al.
Identify and tracking during evolution
Michael DiMarzio, working under the direction of Edward Dudley, associate professor and Casida Development Professor of Food Science, developed a method for identifying and tracking strains of Salmonella enterica serological variant Typhimurium as they evolve and spread.
Resistance patterns change quickly so it had been impossible to determine where some highly resistant strains are coming from.
DiMarzio developed the approach to identify antibiotic resistant strains of Salmonella Typhimurium focusing on virulence genes and regions of the bacteria's DNA known as clustered regularly interspaced short palindromic repeats (CRISPRs).
CRISPRs are present in many foodborne pathogens and the researchers demonstrated that the 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 said.
"Our research demonstrates that CRISPRs are a novel tool for tracing the transmission of antibiotic-resistant Salmonella Typhimurium from farm to fork."
Salmonella Typhimurium accounts for at least 15% of clinically reported salmonellosis infections in humans nationally.
"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.
"Now more than ever, it is imperative to effectively monitor the transmission of Salmonella Typhimurium throughout the food system to implement effective control measures."
DiMarzio noted they identified subsets of the overall Salmonella bacteria population that seem to be more prone to acquiring antibiotic resistance.
"Our challenge now is to learn what makes those strains different -- why do some strains acquire resistance while others don't, even though both are circulating widely among animal populations?" he said.
Source: Antimicroibal Agents and Chemotherapy
Online ahead of print, doi: 10.1128/AAC.00913-13
“Antibiotic Resistance in Salmonella enterica Serovar Typhimurium Associates with CRISPR Sequence Type”
Authors: Michael DiMarzio, Nikki Shariat, Subhashinie Kariyawasam, Rodolphe Barrangou and Edward G. Dudley