ARS | Publication request: Antimicrobial susceptibility to azithromycin among Salmonella enterica isolated from the United States

Submitted to: Antimicrobial Agents and Chemotherapy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 9, 2011
Publication Date: September 1, 2011
Citation: Karlsson, M.S., Joyce, K., Blickenstaff, K., Ball, T.A., Haro, J.H., Medalla, F., Cray, P.J., Zhao, S., Crump, J., Whichard, J. 2011. Antimicrobial susceptibility to azithromycin among Salmonella enterica isolated from the United States. Antimicrobial Agents and Chemotherapy. 55(9):3985-3989.

Interpretive Summary: Food borne illness is often associated with food of animal origin. In particular, foods contaminated with Salmonella can cause cramping and diarrhea. Most often food borne illness is self-limiting and does not require treatment with antibiotics. However, when antibiotics are needed, bacteria that have developed resistance to the antibiotic may be difficult to treat. In order to determine if a bacteria are resistant to certain antimicrobials, they are tested against different dilutions of a drug. By comparing results to these drug concentrations from large numbers of isolates from different sources, we can provide scientific based recommendation of what drug level is likely equivalent to a resistant profile, meaning the drug may not work if used for treatment. This is also called a resistance breakpoint. Azithromycin is a newer drug which is being considered for use in treatment of Salmonella infections. We tested a large number of isolates from animals, ill humans and retail meats and determined that a concentration greater than or equal 16 micrograms of Azithromycin would be consistent with a resistant profile. Using this data in conjunction with clinical data after treatment with Azithromycin will enable the medical and veterinary professions to definitely identify the resistance breakpoint.

Technical Abstract: Due to emerging resistance to traditional antimicrobial agents such as ampicillin, trimethoprim-sulfamethoxazole and chloramphenicol, azithromycin is increasingly used for the treatment of invasive Salmonella infections. In the present study, 696 isolates of non-Typhi Salmonella collected from humans, food animals and retail meats in the United States were investigated for antimicrobial susceptibility to azithromycin. Seventy two Salmonella enterica serotype Typhi isolated from humans were also tested. For each isolate, minimum inhibitory concentrations (MICs) to azithromycin and 15 other antimicrobial agents were determined by broth microdilution. Among the non-Typhi Salmonella isolates, azithromycin MICs among human isolates ranged from 1-32 µg/mL whereas the MICs among the animal and retail meat isolates ranged from 2-16 µg/mL and 4-16 µg/mL, respectively. Among Salmonella ser. Typhi isolates the azithromycin MICs ranged from 4-16 µg/mL. The highest MIC observed in the present study was 32 µg/mL and was detected in three isolates from humans belonging to serotypes Kentucky, Montevideo and Paratyphi A. Based on our findings, we propose an epidemiological cut-off value (ECOFF) of wild type = 16 µg/mL for azithromycin and Salmonella. The susceptibility data provided could be used in combination with clinical outcome data to determine tentative clinical breakpoints for azithromycin and Salmonella enterica.