In vitro and in vivo intracellular killing effects of tigecycline against clinical nontyphoid Salmonella isolates using ceftriaxone as a comparator

Salmonella is an important, worldwide food-borne pathogen. Resistance to fluoroquinolones and cephalosporins has been increasingly reported, and new therapeutic agents are desperately needed. In this study, we evaluated the in vitro antimicrobial susceptibility of clinical nontyphoidal Salmonella isolates to tigecycline. Antibacterial activity of tigecycline, ceftriaxone, and ciprofloxacin were investigated by time-kill studies and the murine peritonitis model. The MIC ₅₀/MIC ₉₀ values of tigecycline, ceftriaxone, and ciprofloxacin against 76 Salmonella isolates were 0.25/0.5, 1/8, and 0.125/0.5 μg/ml, respectively. The intracellular inhibitory activity of tigecycline at 0.5 μg/ml (1x MIC) against Salmonella isolates in human peripheral blood mononuclear cells was sustained for 24 h. In a mouse peritonitis model, tigecycline reduced the extracellular and intracellular bacterial counts from 10 ⁷ CFU/ml and 10 ⁵ CFU/ml, respectively, to an undetectable level within 96 h. The results were similar to those obtained with ceftriaxone. The survival rate of mice exposed to tigecycline after being infected by an inoculum of 1 × 10 ⁵ CFU was 80%, and that of mice exposed to ceftriaxone was 100%. When the inoculum was increased to 1.3 × 10 ⁶ CFU, the survival rate of mice treated by tigecycline was 20%, and that of mice exposed to ceftriaxone was 0% (P = 0.2). When a ceftriaxone- and ciprofloxacin-resistant but tigecycline- susceptible isolate was tested, mice treated by tigecycline had a higher survival rate than those treated by ceftriaxone (15/20 [75%] versus 6/20 [30%]; P = 0.011). Our results suggest that tigecycline is at least as effective as ceftriaxone for murine Salmonella infections and warrants further clinical investigations to delineate its potential against human Salmonella infections.