CHARACTERIZATION OF SALMONELLA TYPHIMURIUM OF ANIMAL ORGIN OBTAINED FROM THE NATIONAL ANTIMICROBIAL RESISTANCE MONITORING SYSTEMS (NARMS)

Authors: ZHAO, S - FDA-DAFM-OR; Cray, Paula; FRIEDMAN, S - FDA-DAFM-OR; MCDERMOTT, P - FDA-DAFM-OR; WALKER, R - FDA-DAFM-OR; QAIYUMI, S - FDA-DAFM-OR; FOLEY, S - FDA-DAFM-OR; HUBERT, S - FDA-DAFM-OR; AYERS, S - FDA-DAFM-OR; ENGLISH, L - FDA-DAFM-OR

Submitted to: Foodborne Pathogens and Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2005
Publication Date: 6/1/2006
Citation: Zhao, S., Cray, P.J., Friedman, S., Mcdermott, P.F., Walker, R.D., Qaiyumi, S., Foley, S.L., Hubert, S.K., Ayers, S., English, L. 2005. Characterization of salmonella typhimurium of animal orgin obtained from the national antimicrobial resistance monitoring systems (narms). Foodborne Pathogens and Disease. 2(2):169-181.

Interpretive Summary: Salmonella is a zoonotic pathogen which can be transferred from animals to humans, most often through consumption of contaminated food. Infection with Salmonella can cause mild to severe gastroenteritis in humans while infection in food animals is often without clinical signs of disease. Antimicrobials are not required to treat mild to moderate gastroenteritis in humans, but may be required for severe infection, especially if sepsis has occurred. The emergence of multi-drug resistant Salmonella reduces the therapeutic options in cases of severe disease, and has been shown to be associated with an increased burden of illness in humans and animals. In this study, 588 S. Typhimurium (including var. Copenhagen) isolates obtained from either animal diagnostic specimens (n=199) or food animals after slaughter/processing (n=389) were examined for antimicrobial susceptibility and characterized at the molecular level. Seventy-six percent (448/588) of isolates were resistant to at least one antimicrobial. Salmonella isolates displayed resistance most often to streptomycin (63%), tetracycline (61%), ampicillin (61%), and to a lesser extent, chloramphenicol (36%), ceftiofur (15%), gentamicin (9%), and nalidixic acid (4%), with more resistance observed among diagnostic isolates. Salmonella recovered from turkeys (n=38) exhibited the highest rates of resistance. Mobile genetic elements, which are capable of moving resistance genes between bacteria, were present in 51% of all isolates. A total of 311 pulsed-field gel electrophoresis patterns were generated indicating a genetically diverse population. These data demonstrated a varied spectrum of antimicrobial resistance, including several multidrug resistant groups of isolates from both diagnostic and slaughter/processing samples. These data are necessary to enable scientists, commodity groups, physicians, veterinarians, government regulators, and animal industry personnel to make accurate treatment choices when necessary and are important when developing new ways to combat multiple drug resistance among Salmonella.

Technical Abstract: Background: Salmonella Typhimurium remains one of the most common causes of salmonellosis in animals and humans in the United States. The emergence of multi-drug resistant Salmonella reduces the therapeutic options in cases of invasive infections, and has been shown to be associated with an increased burden of illness. Methods: In this study, 588 S. Typhimurium (including var. Copenhagen) isolates obtained from either animal diagnostic specimens (n=199) or food animals after slaughter/processing (n=389) were examined for antimicrobial susceptibility, presence of class-1 integrons, and characterized using pulsed-field gel electrophoresis and phage typing. Results: Seventy-six percent (448/588) of isolates were resistant to at least one antimicrobial. Salmonella isolates displayed resistance most often to streptomycin (63%), tetracycline (61%), ampicillin (61%), and to a lesser extent, chloramphenicol (36%), ceftiofur (15%), gentamicin (9%), and nalidixic acid (4%), with more resistance observed among diagnostic isolates. Salmonella recovered from turkeys (n=38) exhibited the highest rates of resistance, with 92% of isolates resistant to least one antimicrobial, and 58% resistant to > 10 antimicrobials. Class 1 integrons were present in 51% of all isolates. Five integron associated resistance genes (aadA, aadB, pse-1, oxa-2 and dhfr) were identified. A total of 311 PFGE patterns were generated using XbaI, indicating a genetically diverse population. The largest PFGE cluster contained 146 isolates, including DT104 isolates obtained from all seven animal species. Conclusions: Results demonstrated a varied spectrum of antimicrobial resistance, including several multidrug resistant clonal groups, among S. Typhimurium and S. Typhimurium var. Copenhagen isolates recovered from both diagnostic and slaughter/processing samples.