Finally, the combination of both techniques was found to be an easy and useful method of obtaining double knockout mutants of A. baumannii. Results

Replacement of the A. baumannii omp33 gene A PCR product containing a kanamycin resistance cassette flanked by 500 bp of the regions surrounding the omp33 gene (Figure 1a, Table 1) was introduced into the A. baumannii ATCC 17978 strain by electroporation. After selection on kanamycin-containing plates, the A. baumannii Δomp33::Km mutant was obtained. The frequency of generation of mutants by gene replacement was approximately 10-7. The PCR tests with VX-689 clinical trial locus-specific primers revealed that 2 of 15 clones obtained had replaced the wild-type gene by the kanamycin cassette (Figure 1b). In addition, allelic replacement in mutant NVP-AUY922 molecular weight clones was further confirmed by sequencing the PCR products obtained (data Napabucasin price not shown). Figure 1 omp33 replacement. (a) Schematic representation of the linear DNA constructed for the omp33 gene replacement, which was completely deleted. The oligonucleotides used (small arrows) are listed in Table 2. (b) Screening of omp33 A. baumannii mutants generated by gene replacement. The numbers at the top are bacterial colony numbers. WT, Wild-type control with 2115 bp. Colonies 5 and 7 (lanes 5* and 7*) with 2214 bp (2115 bp – 834 bp [from omp33 deletion] + 933 bp [from kanamycin insertion])

were sequenced to confirm gene replacement. Lambda DNA-Hind III and ϕX174 DNA-Hae III Mix (Finnzymes) was used as a size marker (M). The lengths of PCR products and of some molecular size marker fragments are also indicated. Table 1 Genes of A. baumannii strain ATCC 17978 inactivated

in the present study. Product Name Gene locationa Lengthb Locus tagc Accession number Outer membrane protein (Omp33) 3789880 to 3790566 228 A1S_3297 YP_001086288.1 Transcriptional regulator SoxR 1547914 to 1548219 101 A1S_1320 YP_001084350.1 Transcriptional regulator OxyR 1150365 to 1151153 262 A1S_0992 YP_001084026.1 a A. baumannii ATCC 17978 chromosomal coordinates for each gene. b The length is expressed as number of amino acids. c Based on National Center for Biotechnology Information http://​www.​ncbi.​nlm.​nih.​gov Disruption of the A. baumannii omp33 gene The gene disruption method was Suplatast tosilate also used to inactivate the omp33 gene. Gene disruption was carried out by cloning a 387-pb internal fragment of the omp33 gene into the pCR-BluntII-TOPO, to obtain the pTOPO33int plasmid (Figure 2a). After transformation of the recombinant plasmid into the A. baumannii ATCC 17978 strain and selection on kanamycin-containing plates, the A. baumannii omp33::TOPO mutant was obtained. The frequency of generation of mutants by gene disruption was approximately 10-5. PCR tests with locus-specific primers revealed that all the clones analyzed (10 of approximately 100) contained fragments of the expected size (Figure 2b).