ARS | Publication request: Strain differences among Salmonella serotypes

Submitted to: International Poultry Scientific Forum
Publication Type: Abstract Only
Publication Acceptance Date: October 16, 2012
Publication Date: N/A

Technical Abstract: Foodborne pathogens are constantly adapting to circumvent intervention strategies. The ability to detect and overcome these adaptations are critical to ensure a safe food supply. We determined genotypic and/or phenotypic differences between Salmonella recovered from broiler chicks after comingling with seeder chicks gavaged with a cocktail of two Salmonella serovars (Kentucky and Heidelberg) resistant to either tetracycline (KentuckytetR; KtetR) or streptomycin (HeidelbergstrR; HstrR) with known antimicrobial resistance profiles. Twenty Salmonella Heidelberg (HstrR) and 21 S. Kentucky (KtetR) isolates were selected for further characterization. Two strains each of HstrR and KtetR, parent and inoculum, were included as reference strains. Pulsed-field gel electrophoresis (PFGE) profiles using XbaI revealed a 100% genetic similarity between the HstrR parent and inoculum isolates and 19/20 recovered isolates; one isolate exhibited 96.6% similarity. Genetic similarity between the parent and inoculum KtetR isolates and 17/21 recovered isolates was 100%; four other isolates exhibited at least 97.2% similarity. An additional PFGE analysis using blnI restriction revealed two clusters; one with 18 isolates demonstrating greater than 95% similarity to the HstrR inoculum strain and one with two isolates showing greater than 95% similarity to the HstrR parent strain and greater than 89% similarity between the two clusters. Two clusters were detected from the KtetR isolates; one with 19 isolates showing greater than 92% similarity to the KtetR inoculum strain and one with two isolates showing greater than 88% similarity to the KtetR parent strain with one outlying isolate and greater than 82% similarity overall. Antimicrobial resistance profiles ranged from three pan-susceptible isolates (two HstrR and one KtetR) to 11 multidrug resistant MDR profiles. The most common MDR profile was resistance to amoxicillin/clavulanic acid, ampicillin, cefoxitin, ceftiofur, streptomycin and tetracycline. PCR was used to probe for four resistance genes, two tetracycline (tetA and tetB) and two aminoglycoside genes (strA and aadA1). The resistance genes tetA, tetB, strA, and aadA1 were detected in 11, 10, 15 and 20 out of the 20 recovered HstrR strains, respectively and in 19, 21, 15 and 13 out of the 21 recovered KtetR strains, respectively. Although there was a high degree of similarity in PFGE profiles, the variability in the isolation of resistance genes coupled with variability in resistance profiles warrants further research to determine the source of isolates exhibiting resistance profiles different from those used for the inoculum.