Overexpression of OqxAB and MacAB efflux pumps contributes to eravacycline resistance and heteroresistance in clinical isolates of <i>Klebsiella pneumoniae</i>

This study investigated the characteristics and mechanisms of eravacycline resistance and heteroresistance in clinical <i>Klebsiella pneumoniae</i> isolates. A total of 393 clinical <i>K</i>. <i>pneumoniae</i> isolates were collected and subjected to eravacycline and tigecycline MIC determinations using the agar dilution method. Eravacycline heteroresistance was assessed by a population analysis profile (PAP). The expression levels of efflux pumps and their regulators were determined by quantitative reverse-transcription PCR (qRT-PCR). This study identified 67 eravacycline-nonsusceptible isolates; among the extended-spectrum β-lactamase (ESBL)-positive isolates, eravacycline-nonsusceptible isolates were detected more frequently than tigecycline-nonsusceptible isolates (21.7% vs. 9.4%, <i>p</i> = 0.001). The study sample was observed to include 20 <i>K</i>. <i>pneumoniae</i> isolates with eravacycline heteroresistance. Compared to the reference strain, <i>oqxA</i> or <i>oqxB</i> overexpression was observed in nine eravacycline-nonsusceptible isolates (range, 35.64–309.02-fold) and 13 eravacycline-heteroresistant isolates (8.42–296.34-fold). The overexpression of <i>macA</i> or <i>macB</i> was detected in 12 eravacycline-heteroresistant isolates (3.23–28.35-fold). Overexpression of the efflux pump regulator gene <i>ramA</i> was observed in 11 eravacycline-nonsusceptible isolates (3.33–94.05-fold) and 18 eravacycline-heteroresistant isolates (3.89–571.70-fold). The eravacycline MICs were increased by one–fourfold by overexpression of <i>oqxAB</i> or <i>macAB</i> in three eravacycline-sensitive isolates. In conclusion, the overexpression of OqxAB and MacAB efflux pumps and the transcriptional regulator RamA were suggested to be involved in <i>K</i>. <i>pneumoniae</i> eravacycline resistance and heteroresistance.