A comparative study of class 1 integrons in Acinetobacter baumannii

Integrons in the development of antimicrobial resistance: critical review and perspectives

Antibiotic resistance development and pathogen cross-dissemination are both considered essential risks to human health on a worldwide scale. Antimicrobial resistance genes (AMRs) are acquired, expressed, disseminated, and traded mainly through integrons, the key players capable of transferring genes from bacterial chromosomes to plasmids and their integration by integrase to the target pathogenic host. Moreover, integrons play a central role in disseminating and assembling genes connected with antibiotic resistance in pathogenic and commensal bacterial species. They exhibit a large and concealed diversity in the natural environment, raising concerns about their potential for comprehensive application in bacterial adaptation. They should be viewed as a dangerous pool of resistance determinants from the "One Health approach." Among the three documented classes of integrons reported viz., class-1, 2, and 3, class 1 has been found frequently associated with AMRs in humans and is a critical genetic element to serve as a target for therapeutics to AMRs through gene silencing or combinatorial therapies. The direct method of screening gene cassettes linked to pathogenesis and resistance harbored by integrons is a novel way to assess human health. In the last decade, they have witnessed surveying the integron-associated gene cassettes associated with increased drug tolerance and rising pathogenicity of human pathogenic microbes. Consequently, we aimed to unravel the structure and functions of integrons and their integration mechanism by understanding horizontal gene transfer from one trophic group to another. Many updates for the gene cassettes harbored by integrons related to resistance and pathogenicity are extensively explored. Additionally, an updated account of the assessment of AMRs and prevailing antibiotic resistance by integrons in humans is grossly detailed-lastly, the estimation of AMR dissemination by employing integrons as potential biomarkers are also highlighted. The current review on integrons will pave the way to clinical understanding for devising a roadmap solution to AMR and pathogenicity. Graphical AbstractThe graphical abstract displays how integron-aided AMRs to humans: Transposons capture integron gene cassettes to yield high mobility integrons that target res sites of plasmids. These plasmids, in turn, promote the mobility of acquired integrons into diverse bacterial species. The acquisitions of resistant genes are transferred to humans through horizontal gene transfer.

Antimicrobial Resistance in Romania: Updates on Gram-Negative ESCAPE Pathogens in the Clinical, Veterinary, and Aquatic Sectors

Multidrug-resistant Gram-negative bacteria such as Acinetobacter baumannii, Pseudomonas aeruginosa, and members of the Enterobacterales order are a challenging multi-sectorial and global threat, being listed by the WHO in the priority list of pathogens requiring the urgent discovery and development of therapeutic strategies. We present here an overview of the antibiotic resistance profiles and epidemiology of Gram-negative pathogens listed in the ESCAPE group circulating in Romania. The review starts with a discussion of the mechanisms and clinical significance of Gram-negative bacteria, the most frequent genetic determinants of resistance, and then summarizes and discusses the epidemiological studies reported for A. baumannii, P. aeruginosa, and Enterobacterales-resistant strains circulating in Romania, both in hospital and veterinary settings and mirrored in the aquatic environment. The Romanian landscape of Gram-negative pathogens included in the ESCAPE list reveals that all significant, clinically relevant, globally spread antibiotic resistance genes and carrying platforms are well established in different geographical areas of Romania and have already been disseminated beyond clinical settings.

Xenogenetic evolutionary of integrons promotes the environmental pollution of antibiotic resistance genes - Challenges, progress and prospects

Environmental pollution of antibiotic resistance genes (ARGs) has been a great public concern. Integrons, as mobile genetic elements, with versatile gene acquisition systems facilitate the horizontal gene transfer (HGT) and pollution disseminations of ARGs. However, little is understood about the characteristics of ARGs mediated by integrons, which hampers our monitoring and control of the mobile antimicrobial resistance risks. To address these issues, we reviewed 3,322 publications concerning detection methods and pipeline, ARG diversity and evolutionary progress, environmental and geographical distribution, bacterial hosts, gene cassettes arrangements, and based on which to identify ARGs with high risk levels mediated by integrons. Diverse ARGs of 516 subtypes attributed to 12 types were capable of being carried by integrons, with 62 core ARG subtypes prevalent in pollution source, natural and human-related environments. Hosts of ARG-carrying integrons reached 271 bacterial species, most frequently carried by opportunistic pathogens Escherichia coli, Pseudomonas aeruginosa and Klebsiella pneumoniae. Moreover, the observed emergence of ARGs together with their multiple arrangements indicated the accumulation of ARGs mediated by integrons, and thus pose increasing HGT risks under modern selective agents. With the concerns of public health, we urgently call for a better monitoring and control of these high-risk ARGs. Our identified Risk Rank I ARGs (aacA7, blaOXA10, catB3, catB8, dfrA5) with high mobility, reviewed key trends and noteworthy advancements, and proposed future directions could be reference and guidance for standard formulation.

Multi‑locus sequence and drug resistance analysis of Salmonella infection in children with diarrhea in Guangdong to identify the dominant ST and cause of antibiotic‑resistance

Multi-locus sequence typing (MLST) can be used to analyze the homology among the drug resistance gene cassettes in Salmonella and determine the prevalence. Information extracted using this technique can provide a theoretical basis for hospitals to devise protocols to control Salmonella infections. The aim of the present study was to investigate the possible association between drug resistance and integrons in clinical isolates of Salmonella from human fecal samples. Therefore, in the present study, 52 clinical fecal isolates of non-duplicate (i.e., not genome contamination) Salmonella were harvested from children with diarrhea and used for bacterial identification using biochemical tests, drug susceptibility analysis by antibiotic susceptibility testing and serotype identification using an agglutination assay. In total, seven Salmonella housekeeping genes (chorismate synthase, β sliding clamp of DNA polymerase III, uroporphyrinogen-III synthase, histidinol dehydrogenase, phosphoribosylaminoimidazole carboxylase catalytic subunit, 2-oxoglutarate dehydrogenase E1 component and homoserine dehydrogenase) were amplified and sequenced using MLST, before sequence alignment was performed against the Pub MLST database to determine the sequence-typed (ST) strains and construct genotypic evolutionary diagrams. Subsequently, the 52 Salmonella strains were subdivided into 11 serotypes and 11 sequence types. The dominant subtypes were found to be Salmonella typhimurium ST34 and ST19, which were diversely distributed. However, no new subtypes were found. Although the serotypes, including ST19, ST29, ST34, ST40, ST11, ST27, ST469, ST365, ST1499, ST413 and ST588, were closely associated with the MLST subtype, they did not correspond entirely. The detection rate of class I integrons was 38.46% (20/52), but no class II and III integrons were detected. The variable regions of three of 20 class I integrons were found to be amplified, whereas nine gene cassettes, including dihydrofolate reductase A12, open reading frame F, aminoglycoside-adenylyltransferase (aad)A2, aadA22, aadA23, aadA1, cadmium-translocating P-type ATPase 2, lincosamide and linF, were associated with drug resistance. These data suggest that Class I integrons are important factors underlying drug resistance in Salmonella, which may serve a role in the spread of drug resistance and warrant specific focus. In addition, MLST typing and serotyping should be applied cooperatively in epidemiological research.

Molecular typing and antibiotic resistance patterns among clinical isolates of Acinetobacter baumannii recovered from burn patients in Tehran, Iran

Acinetobacter baumannii (A. baumannii) is now considered a highly resistant pathogen to various types of antibiotics. Therefore, tracking the source of its prevalence and continuous control is crucial. This study aimed to determine antibiotic resistance and perform various molecular typing methods on clinical isolates of A. baumannii isolated from hospitalized burn patients in Shahid Motahari Burn Hospital, Tehran, Iran. Hospital isolates were confirmed by phenotypic and molecular methods. Then the sensitivity to different antibiotics was determined using the minimum inhibitory concentration (MIC) method. In order to perform molecular typing, three-locus dual assay multiplex polymerase chain reaction (PCR), multiple-locus variable-number tandem repeat analysis (MLVA), and multilocus sequence typing (MLST) methods were used. Among the 60 isolates collected, the frequencies of multidrug-resistant (MDR) and extensively drug-resistant (XDR) isolates were 90 and 10%, respectively. The most effective antibiotics were colistin with 100% and tigecycline with 83.33% sensitivity. Isolates were 100% resistant to piperacillin/tazobactam and cephalosporins, and 68.3% were resistant to carbapenem. The results of multiplex PCR showed five groups that international clone I (IC I) and IC II were the most common. The MLVA method identified 34 MLVA types (MTs), 5 clusters, and 25 singletons. Multilocus sequence typing results for tigecycline-resistant isolates showed seven different sequence types (STs). Increasing antibiotic resistance in A. baumannii isolates requires careful management to control and prevent the occurrence of the pre-antibiotic era. The results of this study confirm that the population structure of A. baumannii isolates has a high diversity. More extensive studies are needed in Iran to better understand the epidemiology of A. baumannii.

Colistin and Carbapenem-Resistant Acinetobacter baumannii Aci46 in Thailand: Genome Analysis and Antibiotic Resistance Profiling

Resistance to the last-line antibiotics against invasive Gram-negative bacterial infection is a rising concern in public health. Multidrug resistant (MDR) Acinetobacter baumannii Aci46 can resist colistin and carbapenems with a minimum inhibitory concentration of 512 µg/mL as determined by microdilution method and shows no zone of inhibition by disk diffusion method. These phenotypic characteristics prompted us to further investigate the genotypic characteristics of Aci46. Next generation sequencing was applied in this study to obtain whole genome data. We determined that Aci46 belongs to Pasture ST2 and is phylogenetically clustered with international clone (IC) II as the predominant strain in Thailand. Interestingly, Aci46 is identical to Oxford ST1962 that previously has never been isolated in Thailand. Two plasmids were identified (pAci46a and pAci46b), neither of which harbors any antibiotic resistance genes but pAci46a carries a conjugational system (type 4 secretion system or T4SS). Comparative genomics with other polymyxin and carbapenem-resistant A. baumannii strains (AC30 and R14) identified shared features such as CzcCBA, encoding a cobalt/zinc/cadmium efflux RND transporter, as well as a drug transporter with a possible role in colistin and/or carbapenem resistance in A. baumannii. Single nucleotide polymorphism (SNP) analyses against MDR ACICU strain showed three novel mutations i.e., Glu229Asp, Pro200Leu, and Ala138Thr, in the polymyxin resistance component, PmrB. Overall, this study focused on Aci46 whole genome data analysis, its correlation with antibiotic resistance phenotypes, and the presence of potential virulence associated factors.

Colistin Induced Assortment of Antimicrobial Resistance in a Clinical Isolate of Acinetobacter baumannii SD01

BACKGROUND

Colistin was considered as the most effective antibiotic against Acinetobacter baumannii, a widely-known opportunistic pathogen. In recent years, a number of colistin resistant strains have also been reported.

OBJECTIVE

This work is commenced to investigate the contribution of efflux pumps toward resistance to colistin-like cyclic polypeptide antibiotics, since the efflux pumps serve as the escape routes leading to drug-resistance.

METHODS

RNA was extracted from A. baumannii isolates cultured from samples procured by tracheal aspiration of infected patients. The expressions of gene(s) that played major roles in the regulation of efflux pump families and involvement of integron systems were studied using real time PCR. Antimicrobial susceptibility tests were conducted to investigate antibiotic resistance of the isolates.

RESULTS

It was observed that genes coding for sugE, ydhE, ydgE, mdfA, ynfA and tolC significantly contributed to resistance against colistin antibiotics, however, no significant transcriptional change was observed in the efflux pump, MexAB-OprM. Results suggest that A. baumanii readily pumps out colistin via efflux pumps belonging to MATE and SMR family.

CONCLUSION

Integral role of efflux pumps and integron 1 genetic system was elucidated towards evolution of multi-drug resistant strain(s). Therefore, for accurate therapeutics, an early detection of efflux genes is crucial before prescribing against colistin resistant A. baumanii.

Copy Number of an Integron-Encoded Antibiotic Resistance Locus Regulates a Virulence and Opacity Switch in Acinetobacter baumannii AB5075

Acinetobacter baumannii remains a leading cause of hospital-acquired infections. Widespread multidrug resistance in this species has prompted the WHO to name carbapenem-resistant A. baumannii as its top priority for research and development of new antibiotics. Many strains of A. baumannii undergo a high-frequency virulence switch, which is an attractive target for new therapeutics targeting this pathogen. This study reports a novel mechanism controlling the frequency of switching in strain AB5075. The rate of switching from the virulent opaque (VIR-O) to the avirulent translucent (AV-T) variant is positively influenced by the copy number of an antibiotic resistance locus encoded on a plasmid-borne composite integron. Our data suggest that this locus encodes a small RNA that regulates opacity switching. Low-switching opaque variants, which harbor a single copy of this locus, also exhibit decreased virulence. This study increases our understanding of this critical phenotypic switch, while also identifying potential targets for virulence-based A. baumannii treatments.

We describe a novel genetic mechanism in which tandem amplification of a plasmid-borne integron regulates virulence, opacity variation, and global gene expression by altering levels of a putative small RNA (sRNA) in Acinetobacter baumannii AB5075. Copy number of this amplified locus correlated with the rate of switching between virulent opaque (VIR-O) and avirulent translucent (AV-T) cells. We found that prototypical VIR-O colonies, which exhibit high levels of switching and visible sectoring with AV-T cells by 24 h of growth, harbor two copies of this locus. However, a subset of opaque colonies that did not form AV-T sectors within 24 h were found to harbor only one copy. The colonies with decreased sectoring to AV-T were designated low-switching opaque (LSO) variants and were found to exhibit a 3-log decrease in switching relative to that of the VIR-O. Overexpression studies revealed that the element regulating switching was localized to the 5′ end of the aadB gene within the amplified locus. Northern blotting indicated that an sRNA of approximately 300 nucleotides (nt) is encoded in this region and is likely responsible for regulating switching to AV-T. Copy number of the ∼300-nt sRNA was also found to affect virulence, as the LSO variant exhibited decreased virulence during murine lung infections. Global transcriptional profiling revealed that >100 genes were differentially expressed between VIR-O and LSO variants, suggesting that the ∼300-nt sRNA may act as a global regulator. Several virulence genes exhibited decreased expression in LSO cells, potentially explaining their decreased virulence.

Genotyping and molecular characterization of clinical Acinetobacter baumannii isolates from a single hospital in Southwestern Iran

ACINETOBACTER BAUMANNII

(A. baumannii) is a pathogen responsible for nosocomial infections among the hospitalized patients. The aim of this study was to investigate genotyping and molecular characterization and to examine the biofilm formation ability of A. baumannii isolates. In total, 70 A. baumannii isolates were collected from patients admitted to Imam Khomeini Hospital in Ahvaz, Southwestern Iran. Minimum inhibitory concentrations (MIC) test was performed using Vitek 2 system. The presence of genes encoding metallo-β-lactamases, oxacillinases, and integrase and the biofilm formation ability were then evaluated. Multiple locus variable-number tandem repeat (VNTR) analysis (MLVA) typing and multiplex PCR were performed to determine the genetic relationships. The blaOXA-23-like gene had the highest prevalence. The frequency of genes encoding blaSPM, blaIMP, and blaVIM among MDR A. baumannii isolates were 12 (17.1%), 18 (25.7%), and 22 (31.4%), respectively. Moreover, 46 isolates (75.4%) harbored class I integron and 10 isolates (16.39%) carried class II integron. The number of weak, moderate and strong biofilm-producing isolates were 3 (4.3%), 7 (10%), and 55 (78.5%), respectively. The results showed that 70 A. baumannii isolates were grouped into 12 distinct MLVA types with five clusters and four singleton genotypes. In addition, 25 (35.7%) isolates were assigned to international clone (IC) variants, 37 (52.8%) isolates belonged to group 1 (IC II), and 8  (11.4%) isolates belonged to group 2 (IC I). Our findings revealed that the population structure of the A. baumannii isolates was genetically diverse. More focus on genetic variation and antibiotic resistance of A. baumannii isolates are recommended.

Investigation of carbapenemases and aminoglycoside modifying enzymes of Acinetobacter baumannii isolates recovered from patients admitted to intensive care units in a tertiary-care hospital in Brazil

INTRODUCTION:

Acinetobacter baumannii are opportunistic bacteria, highly capable of acquiring antimicrobial resistance through the production of carbapenemases and aminoglycoside modifying enzymes (AMEs).

METHODS:

Carbapenemase and AME genes were investigated in A. baumannii recovered from inpatients of a Brazilian hospital.

RESULTS:

The key genes found were bla_OXA-51-like, the association ISAba1- bla_OXA-23-like, and the AME genes aph(3´)-VI, aac(6´)-Ib, aac(3)-Ia, and aph(3´)-Ia. Different clusters spread through the institution wards.

CONCLUSIONS:

The dissemination of bla_OXA-23-like and AME-carrying A. baumannii through the hospital highlights the need for improved preventive measures to reduce the spread of infection.

Rapid simultaneous detection of bla oxa-23, Ade-B, int-1, and ISCR-1 in multidrug-resistant Acinetobacter baumannii using single-tube multiplex PCR and high resolution melting assay

Objective: The aim of this study was to develop a multiplex PCR system for the rapid and simultaneous detection of bla_oxa-23, Ade-B, int-1, and ISCR-1 genes in multidrug-resistant Acinetobacter baumannii (MDRAB) using high resolution melting (HRM) assay.

Methods: Four pairs of primers were designed, and PCR amplification products were sequenced and compared with NCBI GeneBank sequences to ensure primer specificity. Multiplex PCR was performed using a dedicated HRM reagent, and melting curves and temperatures were able to distinguish the four genes. This method was subsequently used to detect these genes in 79 MDRAB isolates from the Third Affiliated Hospital of Southern Medical University in southern China.

Results: Using the HRM assay, 73 out of 79 isolates were found to carry both bla_oxa-23 and Ade-B, one isolate carried int-1, two isolates carried both int-1 and ISCR-1, and three isolates carried Ade-B, int-1, and ISCR-1. No isolates carried all four genes.

Conclusion: Compared with traditional resistance gene detection methods–PCR and agarose gel electrophoresis-based resistance gene detection methods–the multiplex PCR and HRM assay method was simple, rapid, highly efficient, and cost-effective. Our results showed that bla_oxa-23 and Ade-B were the main resistance genotypes in MDRAB.

Overproduction of efflux pumps caused reduced susceptibility to carbapenem under consecutive imipenem-selected stress in Acinetobacter baumannii

PURPOSE

Acinetobacter baumannii is an important pathogen in the nosocomial infections worldwide. Combining with carbapenemases, efflux pumps and outer membrane proteins (OMPs) have been thought to affect the development of carbapenem resistance in A. baumannii. This study aimed to investigate the contributions of different efflux pumps and OMPs in developing carbapenem resistance in a clinical isolate of A. baumannii and reveal the possible mechanism of overproduction of main efflux pumps.

PATIENTS AND METHODS

In this study, an imipenem-susceptible clinical isolate was identified as A. baumannii and named SZE. Several common carbapenemases were detected by polymerase chain reaction (PCR). Imipenem-selected mutants were selected from SZE by serial subcultivations on Mueller-Hinton agar, and the minimum inhibitory concentration (MIC) was detected. Gene expressions of four families of efflux pumps, five OMPs, and blaOXA-51 were determined by reverse transcription quantitative PCR, and comparisons were made between SZE strain and the imipenem-selected mutants. The adeRS system in SZE and its mutant was sequenced and aligned.

RESULTS

Under consecutive imipenem-selected stress, the MIC to imipenem increased gradually from 0.125 μg/mL to 8 μg/mL. The effect of resistance inducement was almost neutralized when treated with an efflux pump inhibitor. The expression of efflux pumps, adeB, adeG, and adeJ, was increased by 6.9-, 4.0-, and 2.1-fold in mutants, respectively, compared to SZE. A single mutation (G to A) at position 58 was detected in the regulatory adeRS system and possibly upregulated the adeB expression, and then affected the carbapenem resistance in A. baumannii strains.

CONCLUSION

In conclusion, under consecutive imipenem-selected stress in vitro, A. baumannii strain evolved the ability to reduce susceptibility to a variety of antimicrobials by overproduction of efflux pumps. Especially, the resistance-nodulation-cell division super family and a nucleotide mutant in adeRS regulating system caused the overexpression of adeABC.

Wide distribution of carbapenem resistant Acinetobacter baumannii in burns patients in Iran

Antimicrobial resistance in carbapenem non-susceptible Acinetobacter baumannii (CNSAb) is a major public health concern globally. This study determined the antibiotic resistance and molecular epidemiology of CNSAb isolates from a referral burn center in Tehran, Iran. Sixty-nine CNSAb isolates were tested for susceptibility to antimicrobial agents using the E test methodology. Multiple locus variable number tandem repeat analysis (MLVA), Multilocus sequence typing (MLST) and multiplex PCR were performed. PCR assays tested for ambler classes A, B, and D β-lactamases. Detection of ISAba1, characterization of integrons, and biofilm formation were investigated. Fifty-three (77%) isolates revealed XDR phenotypes. High prevalence of bla_OXA-23-like (88%) and bla_PER-1 (54%) were detected. ISAba1 was detected upstream of bla_ADC, bla_OXA-23-like and bla_OXA51-like genes in, 97, 42, and 26% of isolates, respectively. Thirty-one (45%) isolates were assigned to international clone (IC) variants. MLVA identified 56 distinct types with six clusters and 53 singleton genotypes. Forty previously known MLST sequence types forming 5 clonal complexes were identified. The Class 1 integron (class 1 integrons) gene was identified in 84% of the isolates. The most prevalent (33%) cassette combination was aacA4-catB8-aadA1. The IC variants were predominant in the A. baumannii lineage with the ability to form strong biofilms. The XDR-CNSAb from burned patients in Iran is resistant to various antimicrobials, including tigecycline. This study shows wide genetic diversity in CNSAb. Integrating the new Iranian A. baumannii IC variants into the epidemiologic clonal and susceptibility profile databases can help effective global control measures against the XDR-CNSAb pandemic.

Carbapenemase producing bacteria in the food supply escaping detection

Carbapenem antimicrobials are critically important to human health and they are often the only remaining effective antibiotics for treating serious infections. Resistance to these drugs mediated by acquired carbapenemase enzymes is increasingly encountered in gram-negative bacteria and is considered a public health emergency. Animal origin food products are recognized as a potential source of resistant organisms, although carbapenem resistance has only recently been reported. In western countries there are active resistance surveillance programs targeting food animals and retail meat products. These programs primarily target beef, pork and poultry and focus exclusively on E. coli, Salmonella, Campylobacter spp. and Enterococcus spp. This global surveillance strategy does not capture the diversity of foods available nor does it address the presence of resistance gene-bearing mobile genetic elements in non-pathogenic bacterial taxa. To address this gap, a total of 121 seafood products originating in Asia purchased from retail groceries in Canada were tested. Samples were processed using a taxa-independent method for the selective isolation of carbapenem resistant organisms. Isolates were characterized by phenotypic antimicrobial susceptibility testing, PCR and DNA sequencing. Carbapenemase producing bacteria, all bla_OXA-48, were isolated from 4 (3.3%) of the samples tested. Positive samples originated from China (n=2) and Korea (n=2) and included squid, sea squirt, clams and seafood medley. Carbapenemase producing organisms found include Pseudomonas, Stenotrophomonas and Myroides species. These findings suggest that non-pathogenic bacteria, excluded from resistance surveillance programs, in niche market meats may serve as a reservoir of carbapenemase genes in the food supply.

Association of class 1 and 2 integrons with multidrug-resistant Acinetobacter baumannii international clones and Acinetobacter nosocomialis isolates

The Acinetobacter baumannii clonal complex 113/79 (CC113/79) and class 2 integrons predominate in Latin America; a relationship between these characteristics was explored. The presence of integrases was determined in successive hospital Acinetobacter isolates (163 A. baumannii isolates and 72 Acinetobacter nosocomialis isolates). Most isolates had integrons, but class 1 and 2 integrons were present significantly more often in CC109/1 and CC113/79, respectively. The high prevalence of CC113/79 in Latin America may account for the predominance of class 2 integrons.

Prevalence and mapping of a plasmid encoding a type IV secretion system in Acinetobacter baumannii

We investigated the prevalence of a type IV secretion system (T4SS)-bearing plasmid among clinical isolates of carbapenem-resistant Acinetobacter baumannii (CRAB) using plasmid replicon typing. The complete sequence of a T4SS-bearing plasmid, pAB_CC, isolated from A. baumannii TYTH-1 was determined, and a comparative analysis of the T4SS gene modules was performed. Of the 129 isolates studied, GR6 (repAci6) was the most common (45 of 96 isolates) and was strongly linked with the T4SS. A comparative analysis of the T4SS locus in seven plasmid genomes, including pAB_CC, pACICU2, pABKp1, pABTJ1, p1BJAB0714, p2BJAB0868, and p2ABTCDC0715, indicated that fourteen genes on these plasmids were highly conserved compared to those of the F plasmid. Additionally, the chromosomes in the seven representative isolates may be evolutionarily distinct from their intrinsic T4SS-bearing plasmids, suggesting that the two T4SS lineages emerged long before the appearance of EC II. These two lineages are now widespread in A. baumannii strains.