Essential Gene Clusters Involved in Copper Tolerance Identified in Acinetobacter baumannii Clinical and Environmental Isolates

Abstract

Copper is widely used as antimicrobial in agriculture and medicine. Copper tolerance mechanisms of pathogenic bacteria have been proven to be required for both copper tolerance and survival during bacterial infections. Here, we determined both copper-tolerant phenotype and genotype in <i>A. baumannii</i> originated from clinical and environmental samples. Using copper susceptibility testing, copper-tolerant <i>A. baumannii</i> could be found in both clinical and environmental isolates. Genotypic study revealed that representative copper-related genes of the cluster A (<i>cueR</i>), B (<i>pcoAB</i>), and D (<i>oprC</i>) were detected in all isolates, while <i>copRS</i> of cluster C was detected in only copper-tolerant <i>A. baumannii</i> isolates. Moreover, we found that copper-tolerant phenotype was associated with amikacin resistance, while the presence of <i>copRS</i> was statistically associated with <i>bla</i>_NDM-1. We chose the <i>A. baumannii</i> strain AB003 as a representative of copper-tolerant isolate to characterize the effect of copper treatment on external morphology as well as on genes responsible for copper tolerance. The morphological features and survival of <i>A. baumannii</i> AB003 were affected by its exposure to copper, while whole-genome sequencing and analysis showed that it carried fourteen copper-related genes located on four clusters, and cluster C of AB003 was found to be embedded on genomic island G08. Transcriptional analysis of fourteen copper-related genes identified in AB003 revealed that copper treatment induced the expressions of genes of clusters A, B, and D at the micromolar level, while genes of cluster C were over-expressed at the millimolar levels of copper. This study showed that both clinical and environmental <i>A. baumannii</i> isolates have the ability to tolerate copper and carried numerous copper tolerance determinants including intrinsic copper tolerance (clusters A, B, and D) and acquired copper tolerance (cluster C) that could respond to copper toxicity. Our evidence suggests that we need to reconsider the use of copper in hospitals and other medical environments to prevent the selection and spread of copper-tolerant organisms.