Tolerance mechanisms and molecular epidemiology of reduced susceptibility to chlorhexidine digluconate in different species of the <i>Acinetobacter baumannii</i> complex

The objective of this study was to compare chlorhexidine digluconate and other antibiotics susceptibility of four species of the<i>Acinetobacter baumannii</i> complex, and further investigate the chlorhexidine digluconate (CHG) tolerance mechanisms and molecular epidemic characteristics. Of 889 <i>A. baumannii</i> complex isolates, <i>A. baumannii, A. nosocomialis, A. pittii</i>, and <i>A. seifertii</i> accounted for 84.2%, 10.9%, 3.4%, and 1.5%. <i>Acinetobacter baumannii</i> was generally resistant to all tested antibiotics, while other three species were commonly more susceptible; 92.1% (313/340) CHG-tolerant <i>A. baumannii</i>, 19.6% (19/97) CHG-tolerant <i>A. nosocomialis</i>, 3.3% (1/30) CHG-tolerant <i>A. pittii</i>, and 15.4% (2/13) CHG-tolerant <i>A. seifertii</i> were identified. Furthermore, compared to <i>A. baumannii</i> ATCC 19606, upregulated expression was found in <i>qacE</i>_Δ1, <i>fabI</i>, and efflux pump encoding genes in CHG-tolerant <i>A. baumannii</i>, but the expression level of <i>oprD</i> was reduced. Additionally, only the expression level of <i>fabI</i> was increased in the CHG-tolerant <i>A. nosocomialis</i>, and the expression level of <i>adeG</i> was increased in the CHG-tolerant <i>A. pittii</i> and <i>A. seifertii</i>. Furthermore, CHG-tolerant <i>A. baumannii</i> may have a relatively high clonal correlation, the predominant sequence type of which was ST208 (90%, 36/40). It is rather necessary to identify specific species members among the <i>A. baumannii</i> complex for clinical treatment options and antibiotics resistance monitoring.