Synergistic bactericidal activities of tobramycin with ciprofloxacin and azithromycin against Klebsiella pneumoniae.

Trans-translation is a unique bacterial ribosome rescue system that plays important roles in the tolerance to environmental stresses. It is composed of an ssrA-encoded tmRNA and a protein SmpB. In this study, we examined the role of trans-translation in antibiotic tolerance in Klebsiella pneumoniae and explored whether the inhibition of this mechanism could enhance the bactericidal activities of antibiotics. We found that deletion of the ssrA gene reduced the survival of K. pneumoniae after treatment with kanamycin, tobramycin, azithromycin, and ciprofloxacin, indicating an important role of the trans-translation in bacterial antibiotic tolerance. By using a modified ssrA gene with a 6×His tag we demonstrated that tobramycin suppressed the azithromycin and ciprofloxacin-elicited activation of trans-translation. The results were further confirmed with a trans-translation reporter system that is composed of a normal mCherry gene and a gfp gene without the stop codon. Compared to each individual antibiotic, combination of tobramycin with azithromycin or ciprofloxacin synergistically enhanced the killing activities against planktonic K. pneumoniae cells and improved bacterial clearance in a murine cutaneous abscess infection model. In addition, the combination of tobramycin and ciprofloxacin increased the bactericidal activities against biofilm-associated cells. Overall, our results suggest that the combination of tobramycin with azithromycin or ciprofloxacin is a promising strategy in combating K. pneumoniae infections.