Antimicrobial Susceptibility to Azithromycin among Salmonella enterica Isolates from the United States

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 isolates from humans were also tested. For each isolate, MICs of 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 to 32 g/ml, whereas the MICs among the animal and retail meat isolates ranged from 2 to 16 g/ml and 4 to 16 g/ml, respectively. Among Salmonella serotype Typhi isolates, the azithromycin MICs ranged from 4 to 16 g/ml. The highest MIC observed in the present study was 32 g/ml, and it was detected in three human isolates belonging to serotypes Kentucky, Montevideo, and Paratyphi A. Based on our findings, we propose an epidemiological cutoff value (ECOFF) for wild-type Salmonella of <16 g/ml of azithromycin. The susceptibility data provided could be used in combination with clinical outcome data to determine tentative clinical breakpoints for azithromycin and Salmonella enterica.

Antimicrobial resistance among blood culture isolates of Salmonella enterica in New Delhi

“…Indian studies have reported higher MIC90 for azithromycin in S. Typhi and S. Paratyphi A (24 μg/mL) [10,29] compared to Western studies (MIC90 8 μg/mL, MIC range 4-16 μg/mL) [30,31]. In one Indian study, taking a MIC interpretive breakpoint of >16 μg/mL, 36 (33.6%) of Salmonella isolates were azithromycin-resistant, while clinical non-response to azithromycin therapy was observed in 19 patients (both azithromycin susceptible and resistant isolates with MIC range 6-64 μg/mL, MIC90 24 μg/mL were included) [10].…”

Epidemiology, Clinical Presentation, Laboratory Diagnosis, Antimicrobial Resistance, and Antimicrobial Management of Invasive Salmonella Infections

“…Occasional patients in these studies have demonstrated a slow clearance of bacteremia. Several studies have reported azithromycin MIC distributions for typhoidal and nontyphoidal Salmonella strains (501,511,(562)(563)(564)(565). These studies have led to a suggested epidemiologic MIC cutoff value of Յ16 g/ml for wild-type strains (566,567) 570).…”

Macrolide resistance mechanisms inEnterobacteriaceae: Focus on azithromycin

“…for azithromycin (Sj€ olund-Karlsson et al, 2011). Nonetheless, to date no clinical resistance breakpoint has been established for the members of the Enterobacteriaceae family by either CLSI or EUCAST (CLSI, 2013;EUCAST, 2015).…”