Determinants of Antibacterial Spectrum and Resistance Potential of the Elongation Factor G Inhibitor Argyrin B in Key Gram-Negative Pathogens.

Argyrins are natural products with antibacterial activity against Gram-negative pathogens, such as , , and We previously showed that argyrin B targets elongation factor G (FusA). Here, we show that argyrin B activity against PAO1 (MIC = 8 μg/ml) was not affected by deletion of the MexAB-OprM, MexXY-OprM, MexCD-OprJ, or MexEF-OprN efflux pump. However, argyrin B induced expression of MexXY, causing slight but reproducible antagonism with the MexXY substrate antibiotic ciprofloxacin. Argyrin B activity against increased in a strain with nine efflux pump partner genes deleted. Complementation experiments showed that argyrin was effluxed by AcrAB, AcrEF, and MdtFX. Argyrin B was inactive against Differences between and FusA proteins at key residues for argyrin B interaction implied that natural target sequence variation impacted antibacterial activity. Consistent with this, expression of the sensitive FusA1 protein in conferred argyrin susceptibility, whereas resistant variants did not. Argyrin B was active against (MIC = 4 μg/ml). Spontaneous resistance occurred at high frequency in the bacterium (circa 10), mediated by mutational inactivation of rather than by amino acid substitutions in the target binding region. This strongly suggested that resistance occurred at high frequency through loss of the sensitive FusA1, leaving an alternate argyrin-insensitive elongation factor. Supporting this, an additional -like gene () is present in that was strongly upregulated in response to mutational loss of .