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High DNA Uptake Capacity of International Clone II Acinetobacter baumannii Detected by a Novel Planktonic Natural Transformation Assay

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High DNA Uptake Capacity of International Clone II Acinetobacter baumannii Detected by a Novel Planktonic Natural Transformation Assay

Yuan Hu et al. Front Microbiol.

Abstract

Acquisition of novel resistance genes is a key driver of multidrug resistance in the nosocomial pathogen Acinetobacter baumannii. To investigate the DNA uptake ability among clinical A. baumannii strains, a planktonic salt-free transformation assay was developed. A total of 142 clinical A. baumannii isolates with divergent genetic distance were selected, and 86 of them belong to international clonal lineage II (ICL2). Using this new transformation assay, 38% of the clinical A. baumannii isolates were natural competent. Among the multidrug-resistant (MDR) isolates, the transformable isolates all belonging to the ICLs, and showed significant higher transformation frequency compared with sensitive isolates. In addition, some of the ICL2 isolates triggered competence much earlier than the sensitive isolates with similar transformation frequencies. This may give them more opportunities to obtain successful transformation in their natural environment and provides an important clue to explain the severe drug resistance and clinical successfulness of ICL2.

Keywords: Acinetobacter baumannii; DNA uptake; competence induction; international clonal lineage 2; natural transformation.

Figures

FIGURE 1
FIGURE 1
Transformability of Acinetobacter baumannii cells under different conditions. (A) Transformation assays with LB-no salt, LB, NB, and TSB with strains HN85 and W068. (B) Transformability of A. baumannii strain W068 and HN85 in LB-no salt broth with the addition of different concentrations of sodium chloride (0, 0.2 0.4, and 0.6%, wt/vol). (C) Transformability of A. baumannii strain HN85 and W068 in LB-no salt broth and LB-no salt with the addition of KCl (0.64%), NaHCO3 (0.72%), and KH2PO4 (1.25%). (D) Transformability of A. baumannii strain HN85 in LB-no salt broth with the addition of different concentrations of sucrose (0, 20, 40, 80, 160, and 320 mM).
FIGURE 2
FIGURE 2
(A) PFGE typing and transformation frequencies of the 97 MDR clinical A. baumannii strains. Nine clusters were designated and the similarity values were provided. (B) PFGE typing and transformation frequencies of the 45 sensitive clinical A. baumannii strains. Error bars represent ± standard deviation. The results of MLST are provided for direct comparison. Isolates became transformable are marked by colored squares, and the colors indicate the MLST STs of the isolates. ST2(2-2-2-2-2-2-2), ST1(1-1-1-1-5-1-1), N10(1-1-1-1-5-2-1), ST1 like, ST40(1-2-2-2-5-1-14), N6(1-2-7-2-5-1-14), ST40 like, ST452(7-2-2-1-8-4-4), ST153(38-1-14-3-12-1-5), N2(12-1-7-1-7-2-4), ST338(8-5-5-26-13-1-2), N5(3-2-7-1-7-1-4), and ST240(3-3-2-5-7-2-51).
FIGURE 3
FIGURE 3
The competence induction time of A. baumannii isolates with different transformation frequencies. (A) Growth curve of the MDR A. baumannii isolate HN85 and the sensitive isolate S12920 in LB-no salt and LB, respectively. (B) The competence induction time of 20 A. baumannii isolates. The bar chart shows the transformation frequency, the color of the bar indicate the STs of the isolates, as shown in Figure 2. The sensitive strains were marked by blue fonts. The scatter plot shows the time point at which transformant colonies were first obtained.

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