Persistence, recovery and characterization of antimicrobial resistant Salmonella serotypes following in vivo challenge in broiler chickens
Cosby, Douglas Earl
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Salmonella is a major foodborne pathogen linked to poultry and poultry products. However, limited research regarding the in vivo interactions of various non-host specific salmonellae in broiler chicks is available. To evaluate the in vivo competitive effect between serotypes, 600 day of hatch chicks were obtained from a local commercial hatchery and divided between four treatment rooms (n=150/room). Seeder chicks inoculated with 1.8 x 104 cfu of Salmonella Kentucky resistant to tetracycline (SKTetR) and seeder chicks inoculated with 2.3 x 104 cfu of Salmonella Heidelberg resistant to streptomycin (SHStrR) were placed into three replicate treatment rooms; one room was maintained as a negative control. Ceca were removed on days 7 and 28 (n=15) and day 42 (n=30) and cultured for the presence of Salmonella. All control chicks were negative for Salmonella while 74, 77 and 67 % of the ceca sampled from the treatment rooms were positive for SKTetR, SHStrR, or both strains of Salmonella, respectively. After processing with and without chlorine, 23, 15 and 5% were recovered from the chlorinated rinsates, and 46, 20 or 15% from the non-chlorinated rinsates of SHStrR, SKTetR or both strains, respectively. Twenty SHStrR isolates and 21 SKTetR isolates were randomly selected to ensure representatives over sampling days and groups and were characterized by various genotypic and phenotypic tests, parent and inoculum strains were also characterized. Pulsed-field gel electrophoresis (PFGE) profiles using XbaI revealed a minimum of 96.7% and 97.2% genetic similarity within the SHStrR and SKTetR isolates, respectively. Antimicrobial resistance profiles ranged from three pan-susceptible isolates to 11 multidrug resistant profiles (R > two classes of antimicrobials). Streptomycin resistance was detected in 18 of the 20 SHStrR isolates and tetracycline resistance was detected in 20 of the 21 SKTetR isolates. Four resistance genes, tetA, tetB, strA and aadA1, were probed by PCR and detected in 11, 10, 15 and 19 of the SHStrR strains, respectively and in 19, 21, 16 and 14 of the SKTetR strains, respectively. The recovered SHStrR and the SKTetR isolates had a > 96.6% genetic similarity when compared to the Salmonella strains used to inoculate the seeder chicks.