Genetic structure of four plasmids found in Acinetobacter baumannii isolate D36 belonging to lineage 2 of global clone 1

Genomic analysis of diverse environmental Acinetobacter isolates identifies plasmids, antibiotic resistance genes, and capsular polysaccharides shared with clinical strains

Acinetobacter baumannii, an important pathogen known for its widespread antibiotic resistance, has been the focus of extensive research within its genus, primarily involving clinical isolates. Consequently, data on environmental A. baumannii and other Acinetobacter species remain limited. Here, we utilized Illumina and Nanopore sequencing to analyze the genomes of 10 Acinetobacter isolates representing 6 different species sourced from aquatic environments in South Australia. All 10 isolates were phylogenetically distinct compared to clinical and other non-clinical Acinetobacter strains, often tens of thousands of single-nucleotide polymorphisms from their nearest neighbors. Despite the genetic divergence, we identified pdif modules (sections of mobilized DNA) carrying clinically important antimicrobial resistance genes in species other than A. baumannii, including carbapenemase oxa58, tetracycline resistance gene tet(39), and macrolide resistance genes msr(E)-mph(E). These pdif modules were located on plasmids with high sequence identity to those circulating in globally distributed A. baumannii ST1 and ST2 clones. The environmental A. baumannii isolate characterized here (SAAb472; ST350) did not possess any native plasmids; however, it could capture two clinically important plasmids (pRAY and pACICU2) with high transfer frequencies. Furthermore, A. baumannii SAAb472 possessed virulence genes and a capsular polysaccharide type analogous to clinical strains. Our findings highlight the potential for environmental Acinetobacter species to acquire and disseminate clinically important antimicrobial resistance genes, underscoring the need for further research into the ecology and evolution of this important genus.IMPORTANCEAntimicrobial resistance (AMR) is a global threat to human, animal, and environmental health. Studying AMR in environmental bacteria is crucial to understand the emergence and dissemination of resistance genes and pathogens, and to identify potential reservoirs and transmission routes. This study provides novel insights into the genomic diversity and AMR potential of environmental Acinetobacter species. By comparing the genomes of aquatic Acinetobacter isolates with clinical and non-clinical strains, we revealed that they are highly divergent yet carry pdif modules that encode resistance to antibiotics commonly used in clinical settings. We also demonstrated that an environmental A. baumannii isolate can acquire clinically relevant plasmids and carries virulence factors similar to those of hospital-associated strains. These findings suggest that environmental Acinetobacter species may serve as reservoirs and vectors of clinically important genes. Consequently, further research is warranted to comprehensively understand the ecology and evolution of this genus.

Co-transfer of IncFII/IncFIB and IncFII plasmids mediated by IS26 facilitates the transmission of mcr-8.1 and tmexCD1-toprJ1

PURPOSE

This study aimed to characterise the whole-genome structure of two clinical Klebsiella pneumoniae strains co-harbouring mcr-8.1 and tmexCD1-toprJ1, both resistant to colistin and tigecycline.

METHODS

K. pneumoniae strains TGC-02 (ST656) and TGC-05 (ST273) were isolated from urine samples of different patients hospitalised at separate times in 2021. Characterisation involved antimicrobial susceptibility testing (AST), conjugation assays, whole-genome sequencing (WGS), and bioinformatics analysis. Comparative genomic analysis was conducted on mcr-8.1-carrying and tmexCD1-toprJ1-carrying plasmids.

RESULTS

Both K. pneumoniae isolates displayed a multidrug-resistant phenotype, exhibiting resistance or reduced susceptibility to ampicillin, ampicillin/sulbactam, cefazolin, aztreonam, amikacin, gentamicin, tobramycin, ciprofloxacin, levofloxacin, nitrofurantoin, trimethoprim/sulfamethoxazole, apramycin, tigecycline and colistin. WGS analysis revealed that clinical strain TGC-02 carried the TmexCD1-toprJ1 gene on a 200-Kb IncFII/IncFIB-type plasmid, while mcr-8 was situated on a 146-Kb IncFII-type plasmid. In clinical strain TGC-05, TmexCD1-toprJ1 was found on a 300-Kb IncFIB/IncHI1B/IncR-type plasmid, and mcr-8 was identified on a 137-Kb IncFII/IncFIA-type plasmid. Conjugation experiments assessed the transferability of these plasmids. While transconjugants were not obtained for TGC-05 despite multiple screening with tigecycline or colistin, pTGC-02-tmex and pTGC-02-mcr8 from clinical K. pneumoniae TGC-02 demonstrated self-transferability through conjugation. Notably, the rearrangement of pTGC-02-tmex and pTGC-02-mcr8 via IS26-based homologous recombination was observed. Moreover, the conjugative and fusion plasmids of the transconjugant co-harboured the tmexCD1-toprJ1 gene cluster and mcr-8.1, potentially resulting from IS26-based homologous recombination.

CONCLUSION

The emergence of colistin- and tigecycline-resistant K. pneumoniae strains is concerning, and effective surveillance measures should be implemented to prevent further dissemination.

Variation in the plasmid backbone and dif module content of R3-T33 Acinetobacter plasmids

The predominant type of plasmids found in Acinetobacter species encode a Rep_3 initiation protein and many of these carry their accessory genes in dif modules. Here, available sequences of the 14 members of the group of Rep_3 plasmids typed as R3-T33, using a threshold of 95% identity in the repA gene, were compiled and compared. These plasmids were from various Acinetobacter species. The pdif sites were identified allowing the backbone and dif modules to be defined. As for other Rep_3 plasmids carrying dif modules, orfX encoding a protein of unknown function was found downstream of repA followed by a pdif site in the orientation XerC binding site-spacer-XerD binding site. Most backbones (n = 12) also included mobA and mobC genes but the two plasmids with the most diverged repA and orfX genes had different backbone contents. Although the gene content of the plasmid backbone was largely conserved, extensive recombinational exchange was detected and only two small groups carried identical or nearly identical backbones. Individual plasmids were associated with 1 to 13 dif modules. Many different dif modules were identified, including ones containing antibiotic or chromate resistance genes and several toxin/antitoxin gene pairs. In some cases, modules carrying the same genes were significantly diverged. Generally, the orientation of the pdif sites alternated such that C modules (XerC binding sites internal) alternated with D modules (XerD binding sites internal). However, fusions of two dif modules via mutational inactivation or loss of a pdif site were also detected.

The extensively antibiotic resistant ST111 Acinetobacter baumannii isolate RBH2 carries an extensive mobile element complement of plasmids, transposons and insertion sequences

The complete genome of RBH2, a sporadic, carbapenem resistant ST111 Acinetobacter baumannii isolate from Brisbane, Australia was determined and analysed. RBH2 is extensively resistant and the chromosome includes two transposons carrying antibiotic resistance genes, AbaR4 (oxa23 in Tn2006) and Tn7::Tn2006 (dfrA1, sat2, aadA1, oxa23). The chromosome also includes two copies of Tn6175, a transposon carrying putative copper resistance genes, and 1-17 copies of six different insertion sequences. RBH2 has six plasmids ranging in size from 6 kb - 141 kb, four carrying antibiotic resistance genes. Plasmids pRBH2-1 (aadB) and pRBH2-2 (aphA6 in TnaphA6) were found to be essentially identical to known plasmids pRAY*-v1 and pS21-1, respectively. The largest plasmids, pRBH2-5 (oxa23 in AbaR4) and pRBH2-6 (oxa23 in AbaR4::ISAba11 and sul2, tet(B), strA and strB in Tn6172) have known transfer-proficient relatives. pRBH2-5, an RP-T1 (RepAci6) plasmid, also carries a different putative copper resistance transposon related to Tn6177 found in pS21-2. The backbone of pRBH2-5 is related to those of previously described RepAci6 plasmids pAb-G7-2 and pA85-3 but has some distinctive features. Three different RepAci6 backbone types were distinguished, Type 1 (pAb-G7-2), Type 2 (pA85-3) and Type 3 (pRBH2-5 and pS21-2). pRBH2-6 is closely related to pAB3 and their backbones differ by only 5 SNPs. Plasmids pRBH2-3 and pRBH2-4 do not carry antibiotic resistance genes. pRBH2-3 does not include an identifiable rep gene and is a novel plasmid type. pRBH2-4 is of the R3-T3 type and includes segments of the larger pABTJ2 that heads this group. Other ST111 genomes carry different plasmids.

The evolutionary tale of eight novel plasmids in a colistin-resistant environmental Acinetobacter baumannii isolate

Acinetobacter baumannii is an important opportunistic pathogen known for its high levels of resistance to many antibiotics, particularly those considered last resorts such as colistin and carbapenems. Plasmids of this organism are increasingly associated with the spread of clinically important antibiotic resistance genes. Although A. baumannii is a ubiquitous organism, to date, most of the focus has been on studying strains recovered from clinical samples ignoring those isolated in the environment (soil, water, food, etc.). Here, we analysed the genetic structures of eight novel plasmids carried by an environmental colistin-resistant A. baumannii (strain E-072658) recovered in a recycled fibre pulp in a paper mill in Finland. It was shown that E-072658 carries a new variant of the mcr-4 colistin resistance gene (mcr-4.7) in a novel Tn3-family transposon (called Tn6926) carried by a novel plasmid p8E072658. E-072658 is also resistant to sulphonamide compounds; consistent with this, the sul2 sulphonamide resistance gene was found in a pdif module. E-072658 also carries six additional plasmids with no antibiotic resistance genes, but they contained several pdif modules shared with plasmids carried by clinical strains. Detailed analysis of the genetic structure of all eight plasmids carried by E-072658 showed a complex evolutionary history revealing genetic exchange events within the genus Acinetobacter beyond the clinical or environmental origin of the strains. This work provides evidence that environmental strains might act as a source for some of the clinically significant antibiotic resistance genes.

Insertion sequences related to ISAjo2 target pdif and dif sites and belong to a new IS family, the IS1202 family

Several insertion sequences (IS) found in various Acinetobacter species exhibit target specificity. They are found, in the same orientation, 5 bp from the XerC binding site of the pdif sites associated with dif modules in Acinetobacter plasmids, and searches revealed they are also found near chromosomal dif sites of Acinetobacter species. These IS are 1.5 kb long, bounded by 24-26 bp imperfect terminal inverted repeats (TIRs) and encode a large transposase of 441-457 aa. They generate 5 bp target site duplications (TSDs). Structural predictions of the ISAjo2 transposase, TnpAjo2, modelled on TnsB of Tn7 revealed two N-terminal HTH domains followed by an RNaseH fold (DDE domain), a β barrel and a C-terminal domain. Similar to Tn7, the outer IS ends are 5'-TGT and ACA-3', and an additional Tnp binding site, corresponding to the internal portion of the IR, is found near each end. However, the Acinetobacter IS do not encode further proteins related to those required by Tn7 for targeted transposition, and the transposase may interact directly with XerC bound to a dif-like site. We propose that these IS, currently in the IS1202 group in the not characterized yet (NCY) category in ISFinder, are part of a distinct IS1202 family. Other IS listed as in the IS1202 group encode transposases related to TnpAjo2 (25-56 % amino acid identity) and have similar TIRs but fall into three groups based on the TSD length (3-5, >15, 0 bp). Those with 3-5 bp TSDs may also target dif-like sites but targets were not found for the other groups.

Detection and Typing of Plasmids in Acinetobacter baumannii Using rep Genes Encoding Replication Initiation Proteins

Plasmids found in Acinetobacter species contribute to the spread of antibiotic resistance genes. They appear to be largely confined to this genus and cannot be typed with available tools and databases. Here, a method for distinguishing and typing these plasmids was developed using a curated, non-redundant set of 621 complete sequences of plasmids from Acinetobacter baumannii. Plasmids were separated into 3 groups based on the Pfam domains of the encoded replication initiation (Rep) protein and a fourth group that lack an identifiable Rep protein. The rep genes of each Rep-encoding group (n = 13 Rep_1, n = 107 RepPriCT_1, n = 351 Rep_3) were then clustered using a threshold of >95% nucleotide identity to define 80 distinct types. Five Rep_1 subgroups, designated R1_T1 to R1-T5, were identified and a sixth reported recently was added. Each R1 type corresponded to a conserved small plasmid sequence. The RepPriCT_1 plasmids fell into 5 subgroups, designated RP-T1 to RP-T5 and the Rep_3 plasmids comprised 69 distinct types (R3-T1 to R3-T69). Three R1, 2 RP and 32 R3 types are represented by only a single plasmid. Over half of the plasmids belong to the 4 most abundant types: the RP-T1 plasmids (n = 97), which include conjugation genes and are often associated with various acquired antibiotic resistance genes, and R3-T1, R3-T2 and R3-T3 (n = 95, 30 and 45, respectively). To facilitate typing and the identification of plasmids in draft genomes using this framework, we established the Acinetobacter Typing database containing representative nucleotide and protein sequences of the type markers (https://github.com/MehradHamidian/AcinetobacterPlasmidTyping). IMPORTANCE Though they contribute to the dissemination of genes that confer resistance to clinically important carbapenem and aminoglycoside antibiotics used to treat life-threatening Acinetobacter baumannii infections, plasmids found in Acinetobacter species have not been well studied. As these plasmids do not resemble those found in other Gram-negative pathogens, available typing systems are unsuitable. The plasmid typing system developed for A. baumannii plasmids with an identifiable rep gene will facilitate the classification and tracking of sequenced plasmids. It will also enable the detection of plasmid-derived contigs present in draft genomes that are widely ignored currently. Hence, it will assist in the tracking of resistance genes and other genes that affect survival in the environment, as they spread through the population. As identical or similar plasmids have been found in other Acinetobacter species, the typing system will also be broadly applicable in identifying plasmids in other members of the genus.

Dynamic state of plasmid genomic architectures resulting from XerC/D-mediated site-specific recombination in Acinetobacter baumannii Rep_3 superfamily resistance plasmids carrying blaOXA-58 - and TnaphA6-resistance modules

The acquisition of bla _OXA genes encoding different carbapenem-hydrolyzing class-D β-lactamases (CHDL) represents a main determinant of carbapenem resistance in the nosocomial pathogen Acinetobacter baumannii. The bla_OXA-58 gene, in particular, is generally embedded in similar resistance modules (RM) carried by plasmids unique to the Acinetobacter genus lacking self-transferability. The ample variations in the immediate genomic contexts in which bla_OXA-58 -containing RMs are inserted among these plasmids, and the almost invariable presence at their borders of non-identical 28-bp sequences potentially recognized by the host XerC and XerD tyrosine recombinases (pXerC/D-like sites), suggested an involvement of these sites in the lateral mobilization of the gene structures they encircle. However, whether and how these pXerC/D sites participate in this process is only beginning to be understood. Here, we used a series of experimental approaches to analyze the contribution of pXerC/D-mediated site-specific recombination to the generation of structural diversity between resistance plasmids carrying pXerC/D-bounded bla _OXA-58- and TnaphA6-containing RM harbored by two phylogenetically- and epidemiologically-closely related A. baumannii strains of our collection, Ab242 and Ab825, during adaptation to the hospital environment. Our analysis disclosed the existence of different bona fide pairs of recombinationally-active pXerC/D sites in these plasmids, some mediating reversible intramolecular inversions and others reversible plasmid fusions/resolutions. All of the identified recombinationally-active pairs shared identical GGTGTA sequences at the cr spacer separating the XerC- and XerD-binding regions. The fusion of two Ab825 plasmids mediated by a pair of recombinationally-active pXerC/D sites displaying sequence differences at the cr spacer could be inferred on the basis of sequence comparison analysis, but no evidence of reversibility could be obtained in this case. The reversible plasmid genome rearrangements mediated by recombinationally-active pairs of pXerC/D sites reported here probably represents an ancient mechanism of generating structural diversity in the Acinetobacter plasmid pool. This recursive process could facilitate a rapid adaptation of an eventual bacterial host to changing environments, and has certainly contributed to the evolution of Acinetobacter plasmids and the capture and dissemination of bla _OXA-58 genes among Acinetobacter and non-Acinetobacter populations co-residing in the hospital niche.

GR13-type plasmids in Acinetobacter potentiate the accumulation and horizontal transfer of diverse accessory genes

Carbapenem and other antibiotic resistance genes (ARGs) can be found in plasmids in Acinetobacter, but many plasmid types in this genus have not been well-characterized. Here we describe the distribution, diversity and evolutionary capacity of rep group 13 (GR13) plasmids that are found in Acinetobacter species from diverse environments. Our investigation was prompted by the discovery of two GR13 plasmids in A. baumannii isolated in an intensive care unit (ICU). The plasmids harbour distinct accessory genes: pDETAB5 contains bla_NDM-1 and genes that confer resistance to four further antibiotic classes, while pDETAB13 carries putative alcohol tolerance determinants. Both plasmids contain multiple dif modules, which are flanked by pdif sites recognized by XerC/XerD tyrosine recombinases. The ARG-containing dif modules in pDETAB5 are almost identical to those found in pDETAB2, a GR34 plasmid from an unrelated A. baumannii isolated in the same ICU a month prior. Examination of a further 41 complete, publicly available plasmid sequences revealed that the GR13 pangenome consists of just four core but 1186 accessory genes, 123 in the shell and 1063 in the cloud, reflecting substantial capacity for diversification. The GR13 core genome includes genes for replication and partitioning, and for a putative tyrosine recombinase. Accessory segments encode proteins with diverse putative functions, including for metabolism, antibiotic/heavy metal/alcohol tolerance, restriction-modification, an anti-phage system and multiple toxin–antitoxin systems. The movement of dif modules and actions of insertion sequences play an important role in generating diversity in GR13 plasmids. Discrete GR13 plasmid lineages are internationally disseminated and found in multiple Acinetobacter species, which suggests they are important platforms for the accumulation, horizontal transmission and persistence of accessory genes in this genus.

Phylogenomics of two ST1 antibiotic-susceptible non-clinical Acinetobacter baumannii strains reveals multiple lineages and complex evolutionary history in global clone 1

Acinetobacter baumannii is an opportunistic pathogen that is difficult to treat due to its resistance to extreme conditions, including desiccation and antibiotics. Most strains causing outbreaks around the world belong to two main global lineages, namely global clones 1 and 2 (GC1 and GC2). Here, we used a combination of Illumina short read and MinION (Oxford Nanopore) long-read sequence data with a hybrid assembly approach to complete the genome sequence of two antibiotic-sensitive GC1 strains, Ex003 and Ax270, recovered in Lebanon from water and a rectal swab of a cat, respectively. Phylogenetic analysis of Ax270 and Ex003 with 186 publicly available GC1 genomes revealed two major clades, including five main lineages (L1–L5), and four single-isolate lineages outside of the two clades. Ax270 and Ex003, along with AB307-0294 and MRSN7213 (both predicted antibiotic-susceptible isolates) represent these individual lineages. Antibiotic resistance islands and transposons interrupting the comM gene remain important features in L1–L5, with L1 associated with the AbaR-type resistance islands, L2 with AbaR4, L3 strains containing either AbaR4 or its variants as well as Tn6022::ISAba42, and L4 and L5 associated with Tn6022 or its variants. Analysis of the capsule (KL) and outer core (OCL) polysaccharide loci further revealed a complex evolutionary history probably involving many recombination events. As more genomes become available, more GC1 lineages continue to emerge. However, genome sequence data from more diverse geographical regions are needed to draw a more accurate population structure of this globally distributed clone.

Increased Capsule Thickness and Hypermotility Are Traits of Carbapenem-Resistant Acinetobacter baumannii ST3 Strains Causing Fulminant Infection

Background

Acinetobacter baumannii is a successful nosocomial pathogen, causing severe, life-threatening infections in hospitalized patients, including pneumonia and bloodstream infections. The spread of carbapenem-resistant Acinetobacter baumannii (CRAB) strains is a major health threat worldwide. The successful spread of CRAB is mostly due to its highly plastic genome. Although some virulence factors associated with CRAB have been uncovered, many mechanisms contributing to its success are not fully understood.

Methods

Here we describe strains of CRAB that were isolated from fulminant cases in 2 hospitals in Israel. These isolates show a rare hypermucoid (HM) phenotype and were investigated using phenotypic assays, comparative genomics, and an in vivo Galleria mellonella model.

Results

The 3 isolates belonged to the ST3 international clonal type and were closely related to each other, as shown by Fourier-transform infrared spectroscopy and phylogenetic analyses. These isolates possessed thickened capsules and a dense filamentous extracellular polysaccharides matrix as shown by transmission electron microscopy (TEM), and overexpressed the capsule polysaccharide synthesis pathway-related wzc gene.

Conclusions

The HM isolates possessed a unique combination of virulence genes involved in iron metabolism, protein secretion, adherence, and membrane glycosylation. HM strains were more virulent than control strains in 2 G. mellonella infection models. In conclusion, our findings demonstrated several virulence factors, all present in 3 CRAB isolates with rare hypermucoid phenotypes.

An outbreak of multiply antibiotic-resistant ST49:ST128:KL11:OCL8 Acinetobacter baumannii isolates at a Sydney hospital

OBJECTIVES

To understand the acquisition of resistance genes by a non-GC1, non-GC2 Acinetobacter baumannii strain responsible for a 4 year outbreak at a Sydney hospital.

METHODS

Representative isolates were screened for resistance to antibiotics. Three were subjected to WGS using Illumina HiSeq. One genome was completed with MinION long reads. Resistance regions were compared with known sequences using bioinformatics.

RESULTS

Isolates were resistant to third-generation cephalosporins, gentamicin and tobramycin, sulfamethoxazole and erythromycin. Sequenced isolates were ST49 (Institut Pasteur scheme) and ST128 (Oxford scheme) and carried KL11 at the capsule locus and OCL8 at the lipooligosaccharide outer core locus. The complete genome of isolate J9 revealed that the resistance genes were all in plasmids; pRAY* contained aadB, and a large plasmid, pJ9-3, contained sul2 and floR genes and a dif module containing the mph(E)-msr(E) macrolide resistance genes. Transposon Tn6168, consisting of a second copy of the chromosomal ampC gene region flanked by ISAba1s, confers resistance to third-generation cephalosporins. Tn6168 is located inside the mph(E)-msr(E) dif module. pJ9-3 includes a set of four dif modules and the orientation of the pdif sites, XerC-XerD or XerD-XerC, alternates. A large transposon, Tn6175, containing tniCABDE transposition genes and genes annotated as being involved in heavy metal metabolism, uptake or export was found in the comM gene. Other ST49:ST128:KL11:OCL8 genomes found in the GenBank WGS database carried Tn6175 but neither of the plasmids carrying the resistance genes.

CONCLUSIONS

An early carbapenem-susceptible A. baumannii outbreak recorded in Australia was caused by an unusual clone that had acquired plasmids carrying antibiotic resistance genes.

AbGRI4, a novel antibiotic resistance island in multiply antibiotic-resistant Acinetobacter baumannii clinical isolates

OBJECTIVES

To investigate the genomic context of a novel resistance island (RI) in multiply antibiotic-resistant Acinetobacter baumannii clinical isolates and global isolates.

METHODS

Using a combination of long and short reads generated from the Oxford Nanopore and Illumina platforms, contiguous chromosomes and plasmid sequences were determined. BLAST-based analysis was used to identify the RI insertion target.

RESULTS

Genomes of four multiply antibiotic-resistant A. baumannii clinical strains, from a US hospital system, belonging to prevalent MLST ST2 (Pasteur scheme) and ST281 (Oxford scheme) clade F isolates were sequenced to completion. A class 1 integron carrying aadB (tobramycin resistance) and aadA2 (streptomycin/spectinomycin resistance) was identified. The class 1 integron was 6.8 kb, bounded by IS26 at both ends, and embedded in a new target location between an α/β-hydrolase and a reductase. Due to its novel insertion site and unique RI composition, we suggest naming this novel RI AbGRI4. Molecular analysis of global A. baumannii isolates identified multiple AbGRI4 RI variants in non-ST2 clonal lineages, including variations in the resistance gene cassettes, integron backbone and insertion breakpoints at the hydrolase gene.

CONCLUSIONS

A novel RI insertion target harbouring a class 1 integron was identified in a subgroup of ST2/ST281 clinical isolates. Variants of the RI suggested evolution and horizontal transfer of the RI across clonal lineages. Long- and short-read hybrid assembly technology completely resolved the genomic context of IS-bounded RIs, which was not possible using short reads alone.

Targeted Conservative Cointegrate Formation Mediated by IS26 Family Members Requires Sequence Identity at the Reacting End

IS26 forms cointegrates using two distinct routes, a copy-in mechanism involving one insertion sequence (IS) and a target and a targeted conservative mechanism involving two ISs in different DNA molecules. In this study, the ability of IS26 and some close relatives, IS1006, IS1008, and a natural hybrid, IS1006/IS1008, which are found predominantly in Acinetobacter spp., to interact was examined. IS1006/1008 consists of 175 bp from IS1006 at the left end, with the remainder from IS1008. These ISs all have the same 14-bp terminal inverted repeats, and the Tnp26, Tnp1006, and Tnp1008 transposases, with pairwise identities of 83.7% to 93.1%, should be able to recognize each other's ends. In a recA-negative Escherichia coli strain, IS1006, IS1008, and IS1006/1008 each formed cointegrates via the copy-in route and via the targeted conservative route, albeit at frequencies for the targeted reaction at least 10-fold lower than for IS26 However, using mixed pairs, targeted cointegration was detected only when IS1008 was paired with the IS1006/1008 hybrid, which also encodes Tnp1008, and the targeted cointegrates formed all arose from a reaction occurring at the end where the DNA sequences are identical. The reaction also occurred at the end with extended DNA identity using IS26 paired with IS26::catA1, an artificially constructed IS26 derivative that includes the catA1 gene. Thus, both identical transposases and identical DNA sequences at the reacting end were required. These features indicate that the targeted conservative pathway proceeds via a single transposase-catalyzed strand transfer, followed by migration and resolution of the Holliday junction formed.IMPORTANCE The IS26 family includes the ISs that are commonly found associated with antibiotic resistance genes in multiply resistant Gram-negative and Gram-positive bacteria. IS26 is most prevalent in Gram-negative species and can generate the clusters of antibiotic resistance genes interspersed with directly oriented IS26 seen in multiply resistant pathogens. This ability relies on the novel dual mechanistic capabilities of IS26 family members. However, the mechanism underlying the recently discovered targeted conservative mode of cointegrate formation mediated by IS26, IS257/IS431, and IS1216, which is unlike any previously studied IS movement mechanism, is not well understood. An important question is what features of the IS and the transposase are key to allowing IS26 family members to undertake targeted conservative reaction. In this study, this question was addressed using mixed-partner crosses involving IS26 and naturally occurring close relatives of IS26 that are found near resistance genes in Acinetobacter baumannii and are widespread in Acinetobacter species.

Analysis of Complete Genome Sequence of Acinetobacter baumannii Strain ATCC 19606 Reveals Novel Mobile Genetic Elements and Novel Prophage

Acinetobacter baumannii isolate ATCC 19606 was recovered in the US prior to 1948. It has been used as a reference and model organism in many studies involving antibiotic resistance and pathogenesis of A. baumannii, while, until recently, a complete genome of this strain was not available. Here, we present an analysis of the complete 3.91-Mbp genome sequence, generated via a combination of short-read sequencing (Illumina) and long-read sequencing (MinION), and show it contains two small cryptic plasmids and a novel complete prophage of size 41.2 kb. We also characterised several regions of the ATCC 19606 genome, leading to the identification of a novel cadmium/mercury transposon, which was named Tn6551. ATCC 19606 is an antibiotic-sensitive strain, but a comparative analysis of all publicly available ST52 strains predicts a resistance to modern antibiotics by the accumulation of antibiotic-resistance genes via plasmids in recent isolates that belong to this sequence type.

What do we know about plasmids carried by members of the Acinetobacter genus?

Several Acinetobacter spp. act as opportunistic pathogens causing healthcare-associated infections worldwide, and in this respect their ability to resist antimicrobial compounds has certainly boosted up their global propagation. Acinetobacter clinical strains have demonstrated a remarkable ability to evolve and become resistant to almost all available drugs in the antimicrobial arsenal, including the last-resort carbapenem β-lactams. The dissemination of antimicrobial resistant genes (ARG), heavy metals-detoxification systems and other traits such as virulence factors is facilitated by mobile genetic elements (MGE) through horizontal gene transfer. Among them, plasmids have been shown to play a critical role in this genus. Despite the continuous increase of Acinetobacter plasmid sequences present in databases, there are no reports describing the basic traits carried by these MGE. To fill this gap, a broad analysis of the Acinetobacter plasmidome was performed. A search for Acinetobacter complete plasmids indicated that 905 sequences have been deposited in the NCBI-GenBank public database, of which 492 are harbored by Acinetobacter baumannii strains. Plasmid-classification schemes based on Rep proteins homology have so far described 23 different groups for A. baumannii (GR1-23), and 16 Acinetobacter Rep3 Groups (AR3G1-16) for the complete genus. Acinetobacter plasmids size ranges from 1.3 to 400 kb. Interestingly, widespread plasmids which are < 20 kb make up 56% of the total present in members of this genus. This led to the proposal of Acinetobacter plasmid assignation to two groups according to their size (< 20 kb and > 20 kb). Usually, smaller plasmids are not self-transmissible, and thereby employ alternative mechanisms of dissemination. For instance, a subgroup of < 20 kb-plasmids belonging to the pRAY-family, lack a rep gene, but encode a relaxase enabling their mobilization by conjugative plasmids. Other subgroup, including small GR2 Acinetobacter plasmids, does not encode a relaxase gene. However, they could still be mobilized by conjugative plasmids which recognize an oriT region carried by these small plasmids. Also, these < 20 kb-plasmids usually carry accessory genes bordered by XerC/D-recombinases recognition sites which have been hypothesized to mediate plasmid plasticity. Conversely, many cases of larger plasmids are self-transmissible and might encode virulence factors and their regulators, thus controlling strain pathogenicity. The ARGs carried by the > 20 kb-plasmids are usually encoded within other MGEs such as transposons, or as part of integrons. It has been recently noted that some of the > 20 kb-plasmids are derived from excised phages, and thus dubbed as phage-like plasmids. All in all, the plethora of plasmids found in strains of this genus and the multiple strategies promoting their evolution and dissemination have certainly contributed to survival of the Acinetobacter members in different habitats, including the clinical environment.

Structure and Evolution of Acinetobacter baumannii Plasmids

Acinetobacter baumannii is an emergent bacterial pathogen that provokes many types of infections in hospitals around the world. The genome of this organism consists of a chromosome and plasmids. These plasmids vary over a wide size range and many of them have been linked to the acquisition of antibiotic-resistance genes. Our bioinformatic analyses indicate that A. baumannii plasmids belong to a small number of plasmid lineages. The general structure of these lineages seems to be very stable and consists not only of genes involved in plasmid maintenance functions but of gene sets encoding poorly characterized proteins, not obviously linked to survival in the hospital setting, and opening the possibility that they improve the parasitic properties of plasmids. An analysis of genes involved in replication, suggests that members of the same plasmid lineage are part of the same plasmid incompatibility group. The same analysis showed the necessity of classifying the Rep proteins in ten new groups, under the scheme proposed by Bertini et al. (2010). Also, we show that some plasmid lineages have the potential capacity to replicate in many bacterial genera including those embracing human pathogen species, while others seem to replicate only within the limits of the Acinetobacter genus. Moreover, some plasmid lineages are widely distributed along the A. baumannii phylogenetic tree. Despite this, a number of them lack genes involved in conjugation or mobilization functions. Interestingly, only 34.6% of the plasmids analyzed here possess antibiotic resistance genes and most of them belong to fourteen plasmid lineages of the twenty one described here. Gene flux between plasmid lineages appears primarily limited to transposable elements, which sometimes carry antibiotic resistance genes. In most plasmid lineages transposable elements and antibiotic resistance genes are secondary acquisitions. Finally, broad host-range plasmids appear to have played a crucial role.

Antimicrobial Resistance of Serial Isolates of Acinetobacter baumannii Colonizing the Rectum of Adult Intensive Care Unit Patients in a Teaching Hospital in Kuwait

Objectives: Outbreak and endemic isolates of Acinetobacter baumannii are known to be polyclonal. In an ongoing study, we hypothesized that the patient gut was the source of the polyclonality where genetic exchanges take place. To test the hypothesis, we collected 270 serial rectal isolates from 32 adult intensive care unit patients over 16 months and investigated their drug resistance profiles. Methods: Antimicrobial susceptibility was determined according to recommended methods. The bla_IMP, bla_VIM, bla_SIM, bla_OXA-23, bla_OXA-24/40, bla_OXA-51, bla_OXA-48, bla_KPC, bla_GES, bla_NDM and bla_OXA-58 were sought by PCR. A subset of 42 isolates were studied for plasmid-mediated resistance. Results: Most of the 270 isolates were multidrug resistant (MDR; with resistances to meropenem of 85.18% and imipenem of 87.04%), but susceptible to colistin and trimethoprim/sulfamethoxazole. There was no correlation between the pattern of resistance and antibiotics administered to treat infections. There was no consistent pattern of resistance or content of carbapenemase genes in serial rectal isolates suggesting polyclonality of the isolates. Genes mediating production of OXA-23, OXA-24/40, IMP, and GES enzymes were carried on plasmids and they mediated resistance to all carbapenems in conjugation studies. Conclusion: A. baumannii colonizing the rectum were polyclonal, MDR, and carbapenem resistance genes were found on plasmids and some plasmids were transferable.

Accumulation of Antibiotic Resistance Genes in Carbapenem-Resistant Acinetobacter baumannii Isolates Belonging to Lineage 2, Global Clone 1, from Outbreaks in 2012-2013 at a Tehran Burns Hospital

Carbapenem-resistant Acinetobacter baumannii strains are among the most critical antibiotic-resistant bacteria causing hospital-acquired infections and treatment failures. The global spread of two clones has been responsible for the bulk of the resistance, in particular, carbapenem resistance. However, there is a substantial gap in our knowledge of which clones and which specific lineages within each clone are circulating in many parts of the world, including Africa and the Middle East region. This is the first genomic analysis of carbapenem-resistant A. baumannii strains from Iran. All the isolates, from a single hospital, belonged to lineage 2 of global clone 1 (GC1) but fell into two groups distinguished by genes in the locus for capsule biosynthesis. The analysis suggests a potential origin of multiply antibiotic-resistant lineage 2 in the Middle East region and highlights the ongoing evolution of carbapenem-resistant GC1 A. baumannii strains. It will enhance future studies on the local and global GC1 population structure.

The worldwide distribution of carbapenem-resistant Acinetobacter baumannii (CRAB) has become a global concern, particularly in countries where antibiotic prescription is not tightly regulated. However, knowledge of the genomic aspects of CRAB from many parts of the world is still limited. Here, 50 carbapenem-resistant A. baumannii isolates recovered at a single hospital in Tehran, Iran, during several outbreaks in 2012 and 2013 were found to be resistant to multiple antibiotics. They were examined using PCR mapping and multilocus sequence typing (MLST). All Iranian strains belonged to sequence type 328 in the Institut Pasteur MLST scheme (ST328_IP), a single-locus variant of ST81_IP, and all Iranian strains contained two carbapenem resistance genes, oxa23 and oxa24. The oxa23 gene is in the transposon Tn2006 in AbaR4, which interrupts the chromosomal comM gene. Phylogenetic analysis using whole-genome sequence (WGS) data for 9 isolates showed that they belonged to the same clade, designated the ST81/ST328 clade, within lineage 2 of global clone 1 (GC1). However, there were two groups that included either KL13 or KL18 at the K locus (KL) for capsular polysaccharide synthesis and either a tet39 or an aadB resistance gene, respectively. The genetic context of the resistance genes was determined, and the oxa24 (OXA-72 variant) and tet39 (tetracycline resistance) genes were each in a pdif module in different plasmids. The aadB gene cassette (which encodes gentamicin, kanamycin, and tobramycin resistance) was harbored by pRAY*, and the aphA6 gene (which encodes amikacin resistance) and sul2 gene (which encodes sulfamethoxazole resistance) were each harbored by a different plasmid. The sequences obtained here will underpin future studies of GC1 CRAB strains from the Middle East region.

IMPORTANCE Carbapenem-resistant Acinetobacter baumannii strains are among the most critical antibiotic-resistant bacteria causing hospital-acquired infections and treatment failures. The global spread of two clones has been responsible for the bulk of the resistance, in particular, carbapenem resistance. However, there is a substantial gap in our knowledge of which clones and which specific lineages within each clone are circulating in many parts of the world, including Africa and the Middle East region. This is the first genomic analysis of carbapenem-resistant A. baumannii strains from Iran. All the isolates, from a single hospital, belonged to lineage 2 of global clone 1 (GC1) but fell into two groups distinguished by genes in the locus for capsule biosynthesis. The analysis suggests a potential origin of multiply antibiotic-resistant lineage 2 in the Middle East region and highlights the ongoing evolution of carbapenem-resistant GC1 A. baumannii strains. It will enhance future studies on the local and global GC1 population structure.

Crucial Role of the Accessory Genome in the Evolutionary Trajectory of Acinetobacter baumannii Global Clone 1

Acinetobacter baumannii is one of the most important nosocomial pathogens able to rapidly develop extensive drug resistance. Here, we study the role of accessory genome in the success of the globally disseminated clone 1 (GC1) with functional and genomic approaches. Comparative genomics was performed with available GC1 genomes (n = 106) against other A. baumannii high-risk and sporadic clones. Genetic traits related to accessory genome were found common and conserved along time as two novel regions of genome plasticity, and a CRISPR-Cas system acquired before clonal diversification located at the same loci as “sedentary” modules. Although identified within hotspot for recombination, other block of accessory genome was also “sedentary” in lineage 1 of GC1 with signs of microevolution as the AbaR0-type genomic island (GI) identified in A144 and in A155 strains which were maintained one month in independent experiments without antimicrobial pressure. The prophage YMC/09/02/B1251_ABA_BP was found to be “mobile” since, although it was shared by all GC1 genomes, it showed high intrinsic microevolution as well as mobility to different insertion sites. Interestingly, a wide variety of Insertion Sequences (IS), probably acquired by the flow of plasmids related to Rep_3 superfamily was found. These IS showed dissimilar genomic location amongst GC1 genomes presumably associated with promptly niche adaptation. On the other hand, a type VI secretion system and three efflux pumps were subjected to deep processes of genomic loss in A. baumannii but not in GC1. As a whole, these findings suggest that preservation of some genetic modules of accessory genome harbored by strains from different continents in combination with great plasticity of IS and varied flow of plasmids, may be central features of the genomic structure of GC1. Competition of A144 and A155 versus A118 (ST 404/ND) without antimicrobial pressure suggested a higher ability of GC1 to grow over a clone with sporadic behavior which explains, from an ecological perspective, the global achievement of this successful pandemic clone in the hospital habitat. Together, these data suggest an essential role of still unknown properties of “mobile” and “sedentary” accessory genome that is preserved over time under different antibiotic or stress conditions.

Mobilisation of a small Acinetobacter plasmid carrying an oriT transfer origin by conjugative RepAci6 plasmids

Most Acinetobacter plasmids are genus specific but their properties have not been investigated. Small plasmids with Rep_3 family replication initiation proteins and iterons are common in Acinetobacter baumannii and often carry antibiotic resistance genes and toxin-antitoxin systems. A RepAci1 plasmid, carrying the carbapenem resistance gene oxa23 in Tn2006 and a RepAci2 plasmid carrying the amikacin (kanamycin and neomycin) resistance gene aphA6 in TnaphA6 were identified. These two plasmids have related rep regions; the consensus 22 bp iteron repeats differ only at three positions and the RepA proteins are 84% identical. However, they were shown to be compatible, whereas the RepAci1 plasmid displaced another RepAci1 plasmid demonstrating that they were incompatible. Despite encoding no mobilisation proteins, the RepAci1 plasmid was transferred to a new host at low frequency when a conjugatively proficient RepAci6 plasmid was present, whereas the RepAci2 plasmid carrying mobA and mobC mobilisation genes was not. Comparison of the sequences of the mobilised and mobilising plasmids revealed a short region of high similarity that is upstream of the predicted mobilisation genes in the RepAci6 plasmid, and has an organisation similar to that of F-type oriT transfer origins. The segment carrying the oriT-like region is present in many RepAci1 plasmids, including ones carrying the cabarpenem resistance genes oxa24 or oxa58 in dif modules, and in some RepAci2 or other Rep_3 plasmids of further types, including one carrying the tet39 tetracycline resistance determinant. These plasmids are also likely to be mobilised, spreading resistance.

Adaptive dif Modules in Permafrost Strains of Acinetobacter lwoffii and Their Distribution and Abundance Among Present Day Acinetobacter Strains

The dif/Xer system of site-specific recombination allows resolution of chromosomal dimers during bacterial DNA replication. Recently, it was also shown to be involved in horizontal transfer of a few known Xer-dependent mobile elements. Here, we show that plasmids of various Acinetobacter species, including clinically important strains, often contain multiple pdif sites that are mainly located within their accessory regions. Chromosomes of Acinetobacter strains may also contain additional dif sites, and their similarity with plasmid pdif sites is higher than with the main chromosomal site dif1. We further identify putative mobile genetic elements containing pdif sites on both flanks of adaptive genes and analyze their distribution in Acinetobacter species. In total, we describe seven mobile elements containing genes with various adaptive functions from permafrost strains of A. lwoffii group. All of them are also spread in modern plasmids of different Acinetobacter species including A. baumannii. We could not detect pdif sites and corresponding mobile elements in closely related bacterial genera, including Psychrobacter and Moraxella. Thus, the widespread distribution of dif modules is a characteristic feature of Acinetobacter species and may contribute to their high adaptability both in the environment and in the clinic.