Expression of antibiotic resistance genes in the integrated cassettes of integrons

Genetic background of aminoglycoside-modifying enzymes in various genetic lineages of clinical aminoglycosides-resistant E. coli and K. pneumoniae isolates in Tunisia

AIMS

This study was conducted to evaluate the in vitro activity of clinically relevant aminoglycosides and to determine the prevalence of genes encoding aminoglycoside modifying enzymes (AMEs) and 16S ribosomal RNA (rRNA) methyltransferases among aminoglycoside-resistant E. coli (n = 61) and K. pneumoniae (n = 44) clinical isolates. Associated resistances to beta-lactams and their bla genes as well as the genetic relatedness of isolates were also investigated.

MATERIALS AND METHODS

A total of 105 aminoglycoside-resistant E. coli (n = 61) and K. pneumoniae (n = 44) isolates recovered between March and May 2017 from 100 patients hospitalized in different wards of Charles Nicolle Hospital of Tunis, Tunisia, were studied. Minimal inhibitory concentrations of aminoglycoside compounds were determined by broth microdilution method. Aminoglycosides resistance encoding genes [aph(3´)-Ia, aph(3') IIa, aph(3´)-VIa, ant(2″)-Ia, aac(3)-IIa, aac(3)-IVa, aac(6')-Ib, rmtA, rmtB, rmtC, armA, and npmA] and bla genes were investigated by PCR and sequencing. Genetic relatedness was examined by multilocus sequence typing (MLST) for representative isolates.

RESULTS

High rates of aminoglycoside resistance were found: gentamicin (85.7%), tobramycin (87.6%), kanamycin (78.0%), netilmincin (74.3%), and amikcin (18.0%). Most common AME gene was aac(3)-IIa (42%), followed by aac(6')-Ib (36.2%) and aph(3')-VIa (32.4%). The majority of isolates were resistant to beta-lactams and blaCTX-M-15 was the most common ESBL. The blaNDM-1 and blaOXA-48 were also produced by 1 and 23 isolates, respectively. Novel sequence types have been reported among our isolates and high-risk clonal lineages have been detected, such as E. coli ST43 (ST131 in Achtman MLST scheme) and K. pneumoniae (ST11/ST13).

CONCLUSIONS

The high prevalence of aminoglycoside resistance rates and the diversity of corresponding genes, with diverse β-lactamase enzymes among genetically heterogeneous clinical isolates present a matter of concern.

Distribution and expression of the aac(6')-Im (aacA16) aminoglycoside resistance gene

BACKGROUND

The aac(6')-Im (aacA16) amikacin, netilmicin and tobramycin resistance gene cassette had been circulating globally undetected for many years in a sublineage of Acinetobacter baumannii global clone 2.

OBJECTIVES

To identify sources for the aac(6')-Im fragment found in A. baumannii.

METHODS

MinION long-read sequencing and Unicycler hybrid assemblies were used to determine the genetic context of the aac(6')-Im gene. Quantitative reverse transcriptase PCR was used to measure expression.

RESULTS

Among >60 000 non-Acinetobacter draft genomes in the MRSN collection, the aac(6')-Im gene was detected in Pseudomonas putida and Enterobacter hormaechei isolates recovered from patients in Thailand between 2016 and 2019. Genomes of multiply resistant P. putida MRSN365855 and E. hormaechei MRSN791417 were completed. The class 1 integron containing the aac(6')-Im cassette was in the chromosome in MRSN365855, and in an HI2 plasmid in MRSN791417. However, MRSN791417 was amikacin susceptible and the gene was not expressed due to loss of the Pc promoter of the integron. Further examples of aac(6')-Im in plasmids from or the chromosome of various Gram-negative species were found in the GenBank nucleotide database. The aac(6')-Im context in integrons in pMRSN791417-8 and a Klebsiella plasmid pAMR200031 shared similarities with the aac(6')-Im region of AbGRI2-Im islands in A. baumannii. In other cases, the cassette array including the aac(6')-Im cassette was different.

CONCLUSIONS

The aac(6')-Im gene is widespread, being found so far in several different species and in several different gene cassette arrays. The lack of amikacin resistance in E. hormaechei highlights the importance of correlating resistance gene content and antibiotic resistance phenotype.

Screening Key Sites of Class 2 Integron Integrase that Impact Recombination Efficiency

By capturing and expressing exogenous resistance gene cassettes through site-specific recombination, integrons play important roles in the horizontal transfer of antimicrobial resistant genes among bacteria. The characteristics of integron integrase make it to be a potential gene editing tool enzyme. In this study, a random mutation library using error-prone PCR was constructed, and amino acid residues mutants that impact on attI2 × attC or attC × attC recombination efficiency were screened and analyzed. Thirteen amino acid mutations were identified to be critical impacted on site-specific recombination of IntI2, including the predicted catalyzed site Y301. Nine of 13 mutated amino acid residues that have critically impacted on IntI2 activity were relative concentrated and near the predicted catalyzed site Y301 in the predicted three-dimensional structure indicated the importance of this area in maintain the activity of IntI2. No mutant with obviously increased recombination activity (more than four-fold as high as that of wild IntI2) was found in library screening, except P95S, R100K slightly increased (within two-fold) the excision activity of IntI2, and S243T slightly increased (within two-fold) both excision and integration activity of IntI2. These findings will provide clues for further specific modification of integron integrase to be a tool enzyme as well as establishing a new gene editing system and applied practically.

Identification of promoter activity in gene-less cassettes from Vibrionaceae superintegrons

Integrons are genetic platforms that acquire new genes encoded in integron cassettes (ICs), building arrays of adaptive functions. ICs generally encode promoterless genes, whose expression relies on the platform-associated Pc promoter, with the cassette array functioning as an operon-like structure regulated by the distance to the Pc. This is relevant in large sedentary chromosomal integrons (SCIs) carrying hundreds of ICs, like those in Vibrio species. We selected 29 gene-less cassettes in four Vibrio SCIs, and explored whether their function could be related to the transcription regulation of adjacent ICs. We show that most gene-less cassettes have promoter activity on the sense strand, enhancing the expression of downstream cassettes. Additionally, we identified the transcription start sites of gene-less ICs through 5'-RACE. Accordingly, we found that most of the superintegron in Vibrio cholerae is not silent. These promoter cassettes can trigger the expression of a silent dfrB9 cassette downstream, increasing trimethoprim resistance >512-fold in V. cholerae and Escherichia coli. Furthermore, one cassette with an antisense promoter can reduce trimethoprim resistance when cloned downstream. Our findings highlight the regulatory role of gene-less cassettes in the expression of adjacent cassettes, emphasizing their significance in SCIs and their clinical importance if captured by mobile integrons.

Increasing transposase abundance with ocean depth correlates with a particle-associated lifestyle

Transposases are mobile genetic elements that move within and between genomes, promoting genomic plasticity in microorganisms. In marine microbial communities, the abundance of transposases increases with depth, but the reasons behind this trend remain unclear. Our analysis of metagenomes from the Tara Oceans and Malaspina Expeditions suggests that a particle-associated lifestyle is the main covariate for the high occurrence of transposases in the deep ocean, and this trend holds true for individual genomes as well as in a community-wide sense. We observed a strong and depth-independent correlation between transposase abundance and the presence of biofilm-associated genes, as well as the prevalence of secretory enzymes. This suggests that mobile genetic elements readily propagate among microbial communities within crowded biofilms. Furthermore, we show that particle association positively correlates with larger genome size, which is in turn associated with higher transposase abundance. Cassette sequences associated with transposons are enriched with genes related to defense mechanisms, which are more highly expressed in the deep sea. Thus, while transposons spread at the expense of their microbial hosts, they also introduce novel genes and potentially benefit the hosts in helping to compete for limited resources. Overall, our results suggest a new understanding of deep ocean particles as highways for gene sharing among defensively oriented microbial genomes.IMPORTANCEGenes can move within and between microbial genomes via mobile genetic elements, which include transposases and transposons. In the oceans, there is a puzzling increase in transposase abundance in microbial genomes as depth increases. To gain insight into this trend, we conducted an extensive analysis of marine microbial metagenomes and metatranscriptomes. We found a significant correlation between transposase abundance and a particle-associated lifestyle among marine microbes at both the metagenome and genome-resolved levels. We also observed a link between transposase abundance and genes related to defense mechanisms. These results suggest that as microbes become densely packed into crowded particles, mobile genes are more likely to spread and carry genetic material that provides a competitive advantage in crowded habitats. This may enable deep sea microbes to effectively compete in such environments.

Emergence of carbapenem resistant gram-negative pathogens with high rate of colistin resistance in Egypt: A cross sectional study to assess resistance trends during the COVID-19 pandemic

The current study investigated the temporal phenotypic and genotypic antimicrobial resistance (AMR) trends among multi-drug resistant and carbapenem-resistant Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa recovered from Egyptian clinical settings between 2020 and 2021. Bacterial identification and antimicrobial sensitivity of 111 clinical isolates against a panel of antibiotics were performed. Molecular screening for antibiotic resistance determinants along with integrons and associated gene cassettes was implemented. An alarming rate (98.2%) of these isolates were found to be phenotypically resistant to carbapenem. Although 23.9 % K. pneumoniae isolates were phenotypically resistant to colistin, no mobile colistin resistance (mcr) genes were detected. Among carbapenem-resistant isolates, blaNDM and blaOXA-48-like were the most prevalent genetic determinants and were significantly overrepresented among K. pneumoniae. Furthermore, 84.78% of K. pneumoniae isolates co-produced these two carbapenemase genes. The plasmid-mediated quinolone resistance genes (qnrS and qnrB) were detected among the bacterial species and were significantly more prevalent among K. pneumoniae. Moreover, Class 1 integron was detected in 82% of the bacterial isolates. This study alarmingly reveals elevated resistance to last-resort antibiotics such as carbapenems as well as colistin which impose a considerable burden in the health care settings in Egypt. Our future work will implement high throughput sequencing-based antimicrobial resistance surveillance analysis for characterization of novel AMR determinants. This information could be applied as a step forward to establish a robust antibiotic stewardship program in Egyptian clinical settings, thereby addressing the rising challenges of AMR.

Cassette recombination dynamics within chromosomal integrons are regulated by toxin-antitoxin systems

Integrons are adaptive bacterial devices that rearrange promoter-less gene cassettes into variable ordered arrays under stress conditions, thereby sampling combinatorial phenotypic diversity. Chromosomal integrons often carry hundreds of silent gene cassettes, with integrase-mediated recombination leading to rampant DNA excision and integration, posing a potential threat to genome integrity. How this activity is regulated and controlled, particularly through selective pressures, to maintain such large cassette arrays is unknown. Here, we show a key role of promoter-containing toxin-antitoxin (TA) cassettes as systems that kill the cell when the overall cassette excision rate is too high. These results highlight the importance of TA cassettes regulating the cassette recombination dynamics and provide insight into the evolution and success of integrons in bacterial genomes.

Toxin:antitoxin ratio sensing autoregulation of the Vibrio cholerae parDE2 module

The parDE family of toxin-antitoxin (TA) operons is ubiquitous in bacterial genomes and, in Vibrio cholerae, is an essential component to maintain the presence of chromosome II. Here, we show that transcription of the V. cholerae parDE2 (VcparDE) operon is regulated in a toxin:antitoxin ratio-dependent manner using a molecular mechanism distinct from other type II TA systems. The repressor of the operon is identified as an assembly with a 6:2 stoichiometry with three interacting ParD2 dimers bridged by two ParE2 monomers. This assembly docks to a three-site operator containing 5'- GGTA-3' motifs. Saturation of this TA complex with ParE2 toxin results in disruption of the interface between ParD2 dimers and the formation of a TA complex of 2:2 stoichiometry. The latter is operator binding-incompetent as it is incompatible with the required spacing of the ParD2 dimers on the operator.

A Class 4-like Chromosomal Integron Found in Aeromonas sp. Genomospecies paramedia Isolated from Human Feces

Integrons are genetic elements that store, express and exchange gene cassettes. These elements are characterized by containing a gene that codes for an integrase (intI), a cassette integration site (attI) and a variable region holding the cassettes. Using bioinformatics and molecular biology methods, a functional integron found in Aeromonas sp. 3925, a strain isolated from diarrheal stools, is described. To confirm the integron class, a phylogenetic analysis with amino acid sequences was conducted. The integrase was associated to class 4 integrases; however, it is clearly different from them. Thus, we classified the associated element as a class 4-like integron. We found that the integrase activity is not under the control of the SOS or catabolic repression, since the expression was not increased in the presence of mitomycin or arabinose. The class-4-like integron is located on the chromosome and contains two well-defined gene cassettes: aadA1 that confers resistance to streptomycin and lpt coding for a lipoprotein. It also includes eight Open Reading frames (ORFs) with unknown functions. The strain was characterized through a Multilocus Phylogenetic Analyses (MLPA) of the gyrB, gyrA, rpoD, recA, dnaJ and dnaX genes. The phylogenetic results grouped it into a different clade from the species already reported, making it impossible to assign a species. We resorted to undertaking complete genome sequencing and a phylogenomic analysis. Aeromonas sp. 3925 is related to A. media and A. rivipollensis clusters, but it is clearly different from these species. In silico DNA-DNA hybridization (isDDH) and Average Nucleotide Identity (ANI) analyses suggested that this isolate belongs to the genomospecies paramedia. This paper describes the first class 4-like integron in Aeromonas and contributes to the establishment of genomospecies paramedia.

Novel insights into genetic characteristics of blaGES-encoding plasmids from hospital sewage

Introduction

The prevalence of Guiana extended-spectrum (GES)-type carbapenemase producers is increasing worldwide, and hospital water environments are considered as potential reservoirs. However, the genetic features underlying this resistance are not yet fully understood. This study aimed to characterize blaGES-encoding plasmids from a single-hospital sewage sample in Japan.

Methods

Carbapenemase producers were screened using carbapenemase-selective agar and polymerase chain reaction. Whole-genome sequencing analyzes were performed on the carbapenemase-producing isolates.

Results

Eleven gram-negative bacteria (four Enterobacter spp., three Klebsiella spp., three Aeromonas spp., and one Serratia spp.) with blaGES-24 (n = 6), blaGES-6 (n = 4), and blaGES-5 (n = 1) were isolated from the sewage sample. Five blaGES-24 and a blaGES-5 were localized in IncP-6 plasmids, whereas three blaGES-6 plasmids were localized in IncC plasmids with IncF-like regions. The remaining blaGES-6 and blaGES-24 were, respectively, localized on IncFIB-containing plasmids with IncF-like regions and a plasmid with an IncW-like replication protein. The IncP-6 and IncW-like plasmids had a close genetic relationship with plasmids from Japan, whereas the IncC/IncF-like and IncFIB/IncF-like plasmids were closely related to those from the United States and Europe. All blaGES genes were located on the class 1 integron cassette of the Tn3 transposon-related region, and the IncC/IncF-like plasmid carried two copies of the integron cassette. Eight of the eleven blaGES-encoding plasmids contained toxin-antitoxin system genes.

Discussion

The findings on the plasmids and the novel genetic content from a single wastewater sample extend our understanding regarding the diversity of resistance and the associated spread of blaGES, suggesting their high adaptability to hospital effluents. These findings highlight the need for the continuous monitoring of environmental GES-type carbapenemase producers to control their dissemination.

Genome Study of α-, β-, and γ-Carbonic Anhydrases from the Thermophilic Microbiome of Marine Hydrothermal Vent Ecosystems

Carbonic anhydrases (CAs) are metalloenzymes that can help organisms survive in hydrothermal vents by hydrating carbon dioxide (CO₂). In this study, we focus on alpha (α), beta (β), and gamma (γ) CAs, which are present in the thermophilic microbiome of marine hydrothermal vents. The coding genes of these enzymes can be transferred between hydrothermal-vent organisms via horizontal gene transfer (HGT), which is an important tool in natural biodiversity. We performed big data mining and bioinformatics studies on α-, β-, and γ-CA coding genes from the thermophilic microbiome of marine hydrothermal vents. The results showed a reasonable association between thermostable α-, β-, and γ-CAs in the microbial population of the hydrothermal vents. This relationship could be due to HGT. We found evidence of HGT of α- and β-CAs between Cycloclasticus sp., a symbiont of Bathymodiolus heckerae, and an endosymbiont of Riftia pachyptila via Integrons. Conversely, HGT of β-CA genes from the endosymbiont Tevnia jerichonana to the endosymbiont Riftia pachyptila was detected. In addition, Hydrogenovibrio crunogenus SP-41 contains a β-CA gene on genomic islands (GIs). This gene can be transferred by HGT to Hydrogenovibrio sp. MA2-6, a methanotrophic endosymbiont of Bathymodiolus azoricus, and a methanotrophic endosymbiont of Bathymodiolus puteoserpentis. The endosymbiont of R. pachyptila has a γ-CA gene in the genome. If α- and β-CA coding genes have been derived from other microorganisms, such as endosymbionts of T. jerichonana and Cycloclasticus sp. as the endosymbiont of B. heckerae, through HGT, the theory of the necessity of thermostable CA enzymes for survival in the extreme ecosystem of hydrothermal vents is suggested and helps the conservation of microbiome natural diversity in hydrothermal vents. These harsh ecosystems, with their integral players, such as HGT and endosymbionts, significantly impact the enrichment of life on Earth and the carbon cycle in the ocean.

Molecular characteristics of clinical IMP-producing Klebsiella pneumoniae isolates: novel IMP-90 and integron In2147

BACKGROUND

Since the first report of carbapenem-resistant Klebsiella pneumoniae isolates in China in 2007, the prevalence of CRKP and CRE has increased significantly. However, the molecular characteristics of IMP-producing Klebsiella pneumoniae (IMPKp) are rarely reported.

METHODS

A total of 29 IMPKp isolates were collected from a Chinese tertiary hospital from 2011 to 2017. Clinical IMPKp were identified by VITEK^®MS, and further analyzed by whole-genome DNA sequencing with HiSeq and PacBio RSII sequencer. Sequencing data were analyzed using CSI Phylogeny 1.4, Resfinder, PlasmidFinder and the MLST tool provided by the Centre for Genomic Epidemiology. The analysis results were visualized using iTOL editor v1_1. The open reading frames and pseudogenes were predicted using RAST 2.0 combined with BLASTP/BLASTN searches against the RefSeq database. The databases CARD, ResFinder, ISfinder, and INTEGRALL were performed for annotation of the resistance genes, mobile elements, and other features. The types of bla_IMP in clinical isolates were determined by BIGSdb-Pasteur. Integrons were drawn by Snapgene, and the gene organization diagrams were drawn by Inkscape 0.48.1.

RESULTS

Four novel ST type, including ST5422, ST5423, ST5426 and ST5427 were identified. The IMP-4 and IMP-1 were the dominant IMP type. The majority of bla_IMP-carrying plasmids belonged to IncN and IncHI5. Two novel bla_IMP-carrying integrons (In2146 and In2147) were uncovered. A novel variant bla_IMP-90 presented in novel integron In2147 has been identified.

CONCLUSIONS

IMPKp showed low prevalence in China. Novel molecular characteristics of IMPKp have been identified. Continuous monitoring of IMPKp shall also be carried out in the future.

Analysis of antibiotic resistance gene cassettes in a newly identified Salmonella enterica serovar Gallinarum strain in Korea

Antimicrobial resistant pathogens are a global health threat driven by the indiscriminate use of antimicrobials. Antimicrobial resistance can be acquired by resistance genes encoded by mobile genetic elements. In this study, we identified a strain of Salmonella enterica serovar Gallinarum (SG4021) from an infected chicken in Korea and characterized the presence of resistance genes in its plasmid by whole genome sequencing. The sequence was then compared with that of a plasmid (P2) from strain SG_07Q015, the only other strain of S. Gallinarum isolated in Korea for which a genome sequence is available. The results revealed that both strains harbored nearly identical DNA carrying antibiotic resistance gene cassettes inserted into integron In2 of the transposable element Tn21, namely an aadA1 resistance gene conferring resistance to aminoglycosides and a sul1 resistance gene conferring resistance to sulfonamide. Interestingly, despite the presence of sul1 in SG4021, an antibiotic sensitivity test revealed that it was sensitive to sulfonamides. Further analysis revealed that this disparity was due to the insertion of a ~ 5 kb ISCR16 sequence downstream of the promoter driving sul1 expression in SG4021. Using various mutants, we showed that the insertion of ISCR16 blocked the expression of the sul1 gene from the upstream promoter. Therefore, the functionality of antimicrobial resistance genes determines phenotypic antimicrobial resistance.

Contribution of different class 2 integron elements to fitness costs in multi-drug resistant Escherichia coli and evaluation of their adaptability in “farm-to-table” environments

Integrons play a pivotal role in the dissemination of antimicrobial resistance, because they can capture and express exogenous antimicrobial resistance genes. This study aimed to elucidate the structure and contribution of different elements of class 2 integrons to fitness costs in their host bacteria and evaluate their adaptability to the "farm-to-table" process. We mapped 27 typical class 2 integrons of Escherichia coli isolated from aquatic foods and pork products, each harboring an inactive truncated class 2 integrase gene and the gene cassette (GC) array dfrA1-sat2-aadA1 with strong Pc2A/Pc2B promoters. Notably, the fitness costs associated with class 2 integrons depended on the Pc promoter strength and quantity and content of GCs in the array. Additionally, the costs of integrases were activity-dependent, and a balance was identified between GC capture ability and integron stability, which could explain the inactive truncated integrase identified. Although typical class 2 integrons exhibited low-cost structures in E. coli, the bacteria incurred biological costs, including decreasing growth rates and biofilm formation, in farm-to-table environments, especially under low-nutrient conditions. Nevertheless, sub-inhibitory antibiotic concentrations led to the selection of class 2 integron-carrying bacteria. This study provides important insights into how integrons may travel from preharvest to consumer goods.

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.

Development of constitutive and IPTG-inducible integron promoter-based expression systems for Escherichia coli and Agrobacterium tumefaciens

Broad host range (BHR) expression vector is a vital tool in molecular biology research and application. Currently, most of the plasmid vectors used in Agrobacterium spp. are binary vectors that are designed for plant transformation, and very few are designed for expressing transgenes in Agrobacterium spp. Class 1 integrons are common genetic elements that allow for the efficient capture and expression of antibiotic resistance genes, especially in Gram-negative bacteria. One of its compound promoters, PcS + P2, was used in this study and has been reported to be the strongest class 1 integron constitutive promoter; it is referred to as "integron promoter" (P int) henceforth. Herein, we created two versions of isopropyl-d-thiogalactopyranoside (IPTG)-inducible promoters by substituting and/or inserting lacO sequence(s) into P int. These inducible promoters, which possess different degrees of stringency and inducibility, were used to construct two broad host range expression vectors (pWK102 and pWK103) based on the versatile pGREEN system. This allows them to be stably maintained and replicated in both Escherichia coli and Agrobacterium tumefaciens. Functional validation of these vectors was performed by the expression of the reporter gene, superfolder green fluorescent protein (sfGFP), which was cloned downstream of these promoters. Due to the strong induction and tunable expression of a transgene located downstream to the inducible integron promoter, these vectors may be useful for heterologous gene expression in both E. coli and A. tumefaciens, thus facilitating recombinant protein production and genetic studies in Gram-negative bacteria.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03507-0.

Molecular characterization of class 1 integrons in carbapenem-resistant Enterobacterales isolates

OBJECTIVE

Carbapenem-resistant Enterobacterales (CRE) infections result in higher treatment costs and mortality rates. Integrons play important roles in emergence and spread of antibiotic resistant genes. To get a better understand on the effects of integron on CRE resistance, distribution of common carbapenemase genes and class 1 integron in clinical CRE isolates were investigated.

METHOD

Carbapenemase genes, including bla_KPC, bla_VIM, bla_IMP, bla_NDM, bla_GES, bla_VEB and bla_OXA-23, were screened in 161 CRE isolates and subtypes of these genes were confirmed through sequence analysis. Class 1 integron was screened and common promoter and gene cassette arrays were determined by sequencing. The resistant rates to clinical commonly used antibiotics between integron positive and integron negative CRE isolates were compared.

RESULTS

Of 161 CRE isolates, the most prevalent carbapenemase gene was bla_KPC-2, which was detected in 139 isolates, including 99 Klebsiella pneumoniae. Class 1 integron was detected in 78 isolates. Twenty different gene cassettes, including two carbapenemase genes bla_VEB-1 and bla_IMP-4, and nine different gene cassette arrays, including bla_VEB-1-aadB-arr-2-cmlA5-bla_OXA-10-aadA1, aadB-catB8-bla_OXA-10-aadA1-dfrA1-aacA4 and bla_IMP-4-qacG-aacA4-catB3, were detected. Five types of common promoters were identified. Relative weak promoter PcH1 was the dominant type. Resistant rates of CRE isolates containing class 1 integrons to ceftazidime, amikacin, trimethoprim/sulfamethoxazole and gentamicin were higher than those without class 1 integrons (P < 0.05).

CONCLUSION

Class 1 integrons play important roles in the emergence and spread of CRE resistance. To the best of our knowledge, this is the first report of aadB-catB8-bla_OXA-10-aadA1-dfrA1-aacA4 and bla_IMP-4-qacG-aacA4-catB3 in the same Providencia rettgeri isolate and bla_VEB-1-aadB-arr-2-cmlA5-bla_OXA-10-aadA1 in P. rettgeri.

Off-Target Integron Activity Leads to Rapid Plasmid Compensatory Evolution in Response to Antibiotic Selection Pressure

Integrons are mobile genetic elements that have played an important role in the dissemination of antibiotic resistance. Under stress, the integron can generate combinatorial variation in resistance cassette expression by cassette reshuffling, accelerating the evolution of resistance. However, the flexibility of the integron integrase site recognition motif hints at potential off-target effects of the integrase on the rest of the genome that may have important evolutionary consequences. Here, we test this hypothesis by selecting for increased-piperacillin-resistance populations of Pseudomonas aeruginosa with a mobile integron containing a difficult-to-mobilize β-lactamase cassette to minimize the potential for adaptive cassette reshuffling. We found that integron activity can decrease the overall survival rate but also improve the fitness of the surviving populations. Off-target inversions mediated by the integron accelerated plasmid adaptation by disrupting costly conjugative genes otherwise mutated in control populations lacking a functional integrase. Plasmids containing integron-mediated inversions were associated with lower plasmid costs and higher stability than plasmids carrying mutations albeit at the cost of a reduced conjugative ability. These findings highlight the potential for integrons to create structural variation that can drive bacterial evolution, and they provide an interesting example showing how antibiotic pressure can drive the loss of conjugative genes. IMPORTANCE Tackling the public health challenge created by antibiotic resistance requires understanding the mechanisms driving its evolution. Mobile integrons are widespread genetic platforms heavily involved in the spread of antibiotic resistance. Through the action of the integrase enzyme, integrons allow bacteria to capture, excise, and shuffle antibiotic resistance gene cassettes. This integrase enzyme is characterized by its ability to recognize a wide range of recombination sites, which allows it to easily capture diverse resistance cassettes but which may also lead to off-target reactions with the rest of the genome. Using experimental evolution, we tested the off-target impact of integron activity. We found that integrons increased the fitness of the surviving bacteria through extensive genomic rearrangements of the plasmids carrying the integrons, reducing their ability to spread horizontally. These results show that integrons not only accelerate resistance evolution but also can generate extensive structural variation, driving bacterial evolution beyond antibiotic resistance.

The expression of aminoglycoside resistance genes in integron cassettes is not controlled by riboswitches

Regulation of gene expression is a key factor influencing the success of antimicrobial resistance determinants. A variety of determinants conferring resistance against aminoglycosides (Ag) are commonly found in clinically relevant bacteria, but whether their expression is regulated or not is controversial. The expression of several Ag resistance genes has been reported to be controlled by a riboswitch mechanism encoded in a conserved sequence. Yet this sequence corresponds to the integration site of an integron, a genetic platform that recruits genes of different functions, making the presence of such a riboswitch counterintuitive. We provide, for the first time, experimental evidence against the existence of such Ag-sensing riboswitch. We first tried to reproduce the induction of the well characterized aacA5 gene using its native genetic environment, but were unsuccessful. We then broadened our approach and analyzed the inducibility of all AgR genes encoded in integrons against a variety of antibiotics. We could not observe biologically relevant induction rates for any gene in the presence of several aminoglycosides. Instead, unrelated antibiotics produced mild but consistently higher increases in expression, that were the result of pleiotropic effects. Our findings rule out the riboswitch control of aminoglycoside resistance genes in integrons.

Class C β-Lactamases: Molecular Characteristics

Class C β-lactamases or cephalosporinases can be classified into two functional groups (1, 1e) with considerable molecular variability (≤20% sequence identity). These enzymes are mostly encoded by chromosomal and inducible genes and are widespread among bacteria, including Proteobacteria in particular. Molecular identification is based principally on three catalytic motifs (⁶⁴SXSK, ¹⁵⁰YXN, ³¹⁵KTG), but more than 70 conserved amino-acid residues (≥90%) have been identified, many close to these catalytic motifs. Nevertheless, the identification of a tiny, phylogenetically distant cluster (including enzymes from the genera Legionella, Bradyrhizobium, and Parachlamydia) has raised questions about the possible existence of a C2 subclass of β-lactamases, previously identified as serine hydrolases. In a context of the clinical emergence of extended-spectrum AmpC β-lactamases (ESACs), the genetic modifications observed in vivo and in vitro (point mutations, insertions, or deletions) during the evolution of these enzymes have mostly involved the Ω- and H-10/R2-loops, which vary considerably between genera, and, in some cases, the conserved triplet ¹⁵⁰YXN. Furthermore, the conserved deletion of several amino-acid residues in opportunistic pathogenic species of Acinetobacter, such as A. baumannii, A. calcoaceticus, A. pittii and A. nosocomialis (deletion of residues 304-306), and in Hafnia alvei and H. paralvei (deletion of residues 289-290), provides support for the notion of natural ESACs. The emergence of higher levels of resistance to β-lactams, including carbapenems, and to inhibitors such as avibactam is a reality, as the enzymes responsible are subject to complex regulation encompassing several other genes (ampR, ampD, ampG, etc.). Combinations of resistance mechanisms may therefore be at work, including overproduction or change in permeability, with the loss of porins and/or activation of efflux systems.

Antimicrobial Resistance and Molecular Characterization of Gene Cassettes from Class 1 Integrons in Escherichia coli Strains

To investigate the antimicrobial resistance and molecular characterization of gene cassettes from class 1 integrons in Escherichia coli strains isolated from hospitalized patients. Bacterial identification was conducted using the Vitek-2 Compact system, and antimicrobial susceptibility analysis was performed using the Kirby-Bauer method. Class 1 integrons, integrase genes, the variable regions of integrons and promoters from the isolated E. coli were screened by polymerase chain reaction, and subjected to DNA sequencing. In total, 138 E. coli strains were collected from the hospitalized patients, most from urine specimens (41.30%, 57/138). Antimicrobial resistance to ampicillin (89.86%) was most prevalent, with 79.99% of strains being multidrug-resistant (MDR). The class 1 integron integrase intI1 gene was detected in 67.39% of the isolates (93/138). Three gene cassette arrays and 5 antimicrobial resistance gene cassettes were detected in 69 of the class 1 integron-positive strains. The most common gene cassette array was dfrA17-aadA5. Of the 93 intI1-positive strains, 5 different common promoters were detected. The most prevalent common promoter was PcH1, and most isolates contained the dfrA17-aadA5 gene cassette array. In summary, antimicrobial resistance and MDR were prevalent among E. coli isolates in our city Weifang in Shandong Provence China. Gene cassettes of the class 1 integron variable region mostly conferred resistance to traditional antimicrobials. Weak promoters in the variable regions were predominant in this study. Integrons pose a great threat to the treatment of MDR bacterial infections and further investigations are needed.

Methods for the targeted sequencing and analysis of integrons and their gene cassettes from complex microbial communities

Integrons are microbial genetic elements that can integrate mobile gene cassettes. They are mostly known for spreading antibiotic resistance cassettes among human pathogens. However, beyond clinical settings, gene cassettes encode an extraordinarily diverse range of functions important for bacterial adaptation. The recovery and sequencing of cassettes has promising applications, including: surveillance of clinically important genes, particularly antibiotic resistance determinants; investigating the functional diversity of integron-carrying bacteria; and novel enzyme discovery. Although gene cassettes can be directly recovered using PCR, there are no standardised methods for their amplification and, importantly, for validating sequences as genuine integron gene cassettes. Here, we present reproducible methods for the amplification, sequence processing, and validation of gene cassette amplicons from complex communities. We describe two different PCR assays that either amplify cassettes together with integron integrases, or gene cassettes together within cassette arrays. We compare the performance of Nanopore and Illumina sequencing, and present bioinformatic pipelines that filter sequences to ensure that they represent amplicons from genuine integrons. Using a diverse set of environmental DNAs, we show that our approach can consistently recover thousands of unique cassettes per sample and up to hundreds of different integron integrases. Recovered cassettes confer a wide range of functions, including antibiotic resistance, with as many as 300 resistance cassettes found in a single sample. In particular, we show that class one integrons are collecting and concentrating resistance genes out of the broader diversity of cassette functions. The methods described here can be applied to any environmental or clinical microbiome sample.

Unbridled Integrons: A Matter of Host Factors

Integrons are powerful recombination systems found in bacteria, which act as platforms capable of capturing, stockpiling, excising and reordering mobile elements called cassettes. These dynamic genetic machineries confer a very high potential of adaptation to their host and have quickly found themselves at the forefront of antibiotic resistance, allowing for the quick emergence of multi-resistant phenotypes in a wide range of bacterial species. Part of the success of the integron is explained by its ability to integrate various environmental and biological signals in order to allow the host to respond to these optimally. In this review, we highlight the substantial interconnectivity that exists between integrons and their hosts and its importance to face changing environments. We list the factors influencing the expression of the cassettes, the expression of the integrase, and the various recombination reactions catalyzed by the integrase. The combination of all these host factors allows for a very tight regulation of the system at the cost of a limited ability to spread by horizontal gene transfer and function in remotely related hosts. Hence, we underline the important consequences these factors have on the evolution of integrons. Indeed, we propose that sedentary chromosomal integrons that were less connected or connected via more universal factors are those that have been more successful upon mobilization in mobile genetic structures, in contrast to those that were connected to species-specific host factors. Thus, the level of specificity of the involved host factors network may have been decisive for the transition from chromosomal integrons to the mobile integrons, which are now widespread. As such, integrons represent a perfect example of the conflicting relationship between the ability to control a biological system and its potential for transferability.

Distribution and Molecular Characterization of Functional Class 2 Integrons in Clinical Proteus mirabilis Isolates

Background

Integrons are the main mode of horizontal transmission of drug-resistance genes and are closely related to drug resistance in clinical bacteria. In this study, the distributions of class 1, 2, and 3 integron gene cassettes were investigated in 150 Proteus mirabilis (P. mirabilis) isolates from patients, and molecular characterization of functional class 2 integrons was further analyzed.

Methods

Class 1, 2, and 3 integrons were screened by polymerase chain reaction (PCR) in 150 clinical P. mirabilis isolates. The variable regions of the integrons were determined by restriction analysis and sequencing. Internal stop codons mutations in class 2 integrons and their common promoters were also determined by sequencing. Enterobacterial repetitive intergenic consensus polymerase chain reaction (ERIC-PCR) was used to analyze the phylogenetic relations of class 2 integron-positive isolates.

Results

Class 1 integrons were detected in 69 (46%) of 150 P. mirabilis isolates, and six different gene cassette arrays were detected, with the most prevalent being dfrA32-aadA2. Class 2 integrons were detected in 61 (40.7%) of 150 P. mirabilis isolates, and three different gene cassette arrays were detected, including sat2-aadA1, which was detected for the first time in a class 2 integron. Nearly similar ERIC-PCR fingerprinting patterns were detected in 45 (73.8%) of 61 class 2 integron-positive isolates. The functional class 2 integron was detected in three P. mirabilis isolates having the same gene cassette, dfrA1-sat2-aadA1, in the variable region and four novel open reading frames with unknown functions. Same PintI2 and Pc promoters were detected in these three functional class 2 integron isolates, as was found in other class 2 integron isolates. However, these three strains did not totally show identical homology and drug sensitivity.

Conclusion

Although functional class 2 integrons have low distribution and relatively conserved molecular characteristics, they can still form clinical dissemination and drug resistance expression.

Integron Functionality and Genome Innovation: An Update on the Subtle and Smart Strategy of Integrase and Gene Cassette Expression Regulation

Integrons are considered hot spots for bacterial evolution, since these platforms allow one-step genomic innovation by capturing and expressing genes that provide advantageous novelties, such as antibiotic resistance. The acquisition and shuffling of gene cassettes featured by integrons enable the population to rapidly respond to changing selective pressures. However, in order to avoid deleterious effects and fitness burden, the integron activity must be tightly controlled, which happens in an elegant and elaborate fashion, as discussed in detail in the present review. Here, we aimed to provide an up-to-date overview of the complex regulatory networks that permeate the expression and functionality of integrons at both transcriptional and translational levels. It was possible to compile strong shreds of evidence clearly proving that these versatile platforms include functions other than acquiring and expressing gene cassettes. The well-balanced mechanism of integron expression is intricately related with environmental signals, host cell physiology, fitness, and survival, ultimately leading to adaptation on the demand.

The Natural History of Integrons

Integrons were first identified because of their central role in assembling and disseminating antibiotic resistance genes in commensal and pathogenic bacteria. However, these clinically relevant integrons represent only a small proportion of integron diversity. Integrons are now known to be ancient genetic elements that are hotspots for genomic diversity, helping to generate adaptive phenotypes. This perspective examines the diversity, functions, and activities of integrons within both natural and clinical environments. We show how the fundamental properties of integrons exquisitely pre-adapted them to respond to the selection pressures imposed by the human use of antimicrobial compounds. We then follow the extraordinary increase in abundance of one class of integrons (class 1) that has resulted from its acquisition by multiple mobile genetic elements, and subsequent colonisation of diverse bacterial species, and a wide range of animal hosts. Consequently, this class of integrons has become a significant pollutant in its own right, to the extent that it can now be detected in most ecosystems. As human activities continue to drive environmental instability, integrons will likely continue to play key roles in bacterial adaptation in both natural and clinical settings. Understanding the ecological and evolutionary dynamics of integrons can help us predict and shape these outcomes that have direct relevance to human and ecosystem health.

Metagenomic strategies identify diverse integron-integrase and antibiotic resistance genes in the Antarctic environment

The objective of this study is to identify and analyze integrons and antibiotic resistance genes (ARGs) in samples collected from diverse sites in terrestrial Antarctica. Integrons were studied using two independent methods. One involved the construction and analysis of intI gene amplicon libraries. In addition, we sequenced 17 metagenomes of microbial mats and soil by high-throughput sequencing and analyzed these data using the IntegronFinder program. As expected, the metagenomic analysis allowed for the identification of novel predicted intI integrases and gene cassettes (GCs), which mostly encode unknown functions. However, some intI genes are similar to sequences previously identified by amplicon library analysis in soil samples collected from non-Antarctic sites. ARGs were analyzed in the metagenomes using ABRIcate with CARD database and verified if these genes could be classified as GCs by IntegronFinder. We identified 53 ARGs in 15 metagenomes, but only four were classified as GCs, one in MTG12 metagenome (Continental Antarctica), encoding an aminoglycoside-modifying enzyme (AAC(6´)acetyltransferase) and the other three in CS1 metagenome (Maritime Antarctica). One of these genes encodes a class D β-lactamase (blaOXA-205) and the other two are located in the same contig. One is part of a gene encoding the first 76 amino acids of aminoglycoside adenyltransferase (aadA6), and the other is a qacG2 gene.

Exploring the Global Spread of Klebsiella grimontii Isolates Possessing blaVIM-1 and mcr-9

The spread of plasmid-mediated carbapenemases within Klebsiella oxytoca is well-documented. In contrast, data concerning the closely related species Klebsiella grimontii are scarce. In fact, despite the recent report of the first bla_KPC-2-producing K. grimontii, nothing is known about its clonality and antibiotic resistance patterns. In a retrospective search in our collection, we identified 2 bla_VIM-positive K. oxytoca strains. Whole-genome sequencing with both Illumina and Nanopore indicated that our strains actually belonged to K. grimontii and were of sequence type 172 (ST172) and ST189. Moreover, the two strains were associated with 297-kb IncHI2/HI2A-pST1 and 90.6-kb IncFII(Yp) plasmids carrying bla_VIM-1 together with mcr-9 and bla_VIM-1, respectively. In the IncHI2/HI2A plasmid, bla_VIM-1 was located in a class 1 integron (In110), while mcr-9 was associated with the qseC-qseB-like regulatory elements. Overall, this plasmid was shown to be very similar to those carried by other Enterobacterales isolated from food and animal sources (e.g., Salmonella and Enterobacter spp. detected in Germany and Egypt). The IncFII(Yp) plasmid was unique, and its bla_VIM-1 region was associated with a rare integron (In1373). Mapping of In1373 indicated a possible origin in Austria from an Enterobacter hormaechei carrying a highly similar plasmid. Core-genome phylogenies indicated that the ST172 K. grimontii belonged to a clone of identical Swedish and Swiss strains (≤15 single nucleotide variants [SNVs] to each other), whereas the ST189 strain was sporadic. Surveillance of carbapenemase-producing K. oxytoca strains should be reinforced to detect and prevent the dissemination of new species belonging to the Klebsiella genus.

Cassette recruitment in the chromosomal Integron of Vibrio cholerae

Integrons confer a rapid adaptation capability to bacteria. Integron integrases are able to capture and shuffle novel functions embedded in cassettes. Here, we investigated cassette recruitment in the Vibrio cholerae chromosomal integron during horizontal transfer. We demonstrated that the endogenous integrase expression is sufficiently triggered, after SOS response induction mediated by the entry of cassettes during conjugation and natural transformation, to mediate significant cassette insertions. These insertions preferentially occur at the attIA site, despite the presence of about 180 attC sites in the integron array. Thanks to the presence of a promoter in the attIA site vicinity, all these newly inserted cassettes are expressed and prone to selection. We also showed that the RecA protein is critical for cassette recruitment in the V. cholerae chromosomal integron but not in mobile integrons. Moreover, unlike the mobile integron integrases, that of V. cholerae is not active in other bacteria. Mobile integrons might have evolved from the chromosomal ones by overcoming host factors, explaining their large dissemination in bacteria and their role in antibioresistance expansion.

Bacterial evolution on demand

Bacteria carry antibiotic resistant genes on movable sections of DNA that allow them to select the relevant genes on demand.

Integron activity accelerates the evolution of antibiotic resistance

Mobile integrons are widespread genetic platforms that allow bacteria to modulate the expression of antibiotic resistance cassettes by shuffling their position from a common promoter. Antibiotic stress induces the expression of an integrase that excises and integrates cassettes, and this unique recombination and expression system is thought to allow bacteria to 'evolve on demand' in response to antibiotic pressure. To test this hypothesis, we inserted a custom three-cassette integron into Pseudomonas aeruginosa and used experimental evolution to measure the impact of integrase activity on adaptation to gentamicin. Crucially, integrase activity accelerated evolution by increasing the expression of a gentamicin resistance cassette through duplications and by eliminating redundant cassettes. Importantly, we found no evidence of deleterious off-target effects of integrase activity. In summary, integrons accelerate resistance evolution by rapidly generating combinatorial variation in cassette composition while maintaining genomic integrity.

Comparison of Class 2 Integron Integrase Activities

Integrons play important roles in the dissemination of antimicrobial resistant genes among bacteria. Class 2 integrons usually has an internal stop codon, TAA, in integrase genes (intI2), leading to a truncated integrase, IntI2*. However, a few class 2 integrons with a natural full-length integrase have been reported. In this study, the sequences of natural full-length intI2 were extracted from INTEGRALL database and analyzed. A total of 236 sequences of intI2 were retrieved from INTEGRALL database, only seven of which were natural full-length intI2 genes and could be divided into five types according to their coding amino acid sequence. Quantitative real-time PCR was used to detect gene cassette sat2 integration and excision efficiency catalyzed by different natural full-length IntI2s. The results showed that all five IntI2s could catalyze attI2 × attC_sat2 integration and attC_dfrA1/sat2 × attC_sat2/aadA1 excision in Escherichia coli. Integration and excision frequency catalyzed by IntI2_A176 was highest and was about twofold as high as those catalyzed by IntI2_S175__A176. The secondary structure of the IntI2 was predicted by online software. Polymorphisms of these five IntI2s were limited within residues 172, 174, 175, 176 and 256, and these residues were all far away from the predicted DNA binding regions or catalyzed sites. Influence of amino acid sequence polymorphisms of these natural full-length IntI2s on their catalyzed activities is limited.

Multiple antibiotic resistance and DNA methylation in Enterobacteriaceae isolates from different environments

Antibiotic resistant bacteria with diverse resistance phenotypes and genotypes are ubiquitous in the environments that have become a global health concern. The role of DNA methylation in the dissemination of antibiotic resistance among different environments is currently unclear. We recovered 646 Enterobacteriaceae (Eb) isolates from hospital, livestock manure, municipal wastewater-treatment plants, river sediment and soil for comprehensive analysis of resistance phenotypes, β-lactamase genes, integrons, integron-associated gene cassettes and the levels of DNA methylation. Antibiotic susceptibility testing revealed that approximately 87.31 % isolates were multidrug resistant Eb. The β-lactamase genes were positively detected in 473 isolates with greater diversity in human or animal sourced Eb, while its prevalence was found to be highest in the Eb isolates from the natural environments. Forty-three gene cassettes (28 different types mediated by intI1) were detected in 53 (19.63 %) isolates, with greater diversity in Eb isolates from hospital and livestock manure. The multiple antibiotic resistance index of single strain was positively correlated with the 5-methylcytosine and showed a negative correlation with 6-methylademine. We conclude that the development of antibiotic resistance could possibly be coupled with DNA methylation, which might enhance the antimicrobial resistance and survival capacity of Eb.

Integron gene cassettes harboring novel variants of D-alanine-D-alanine ligase confer high-level resistance to D-cycloserine

Antibiotic resistance poses an increasing threat to global health. To tackle this problem, the identification of principal reservoirs of antibiotic resistance genes (ARGs) plus an understanding of drivers for their evolutionary selection are important. During a PCR-based screen of ARGs associated with integrons in saliva-derived metagenomic DNA of healthy human volunteers, two novel variants of genes encoding a d-alanine-d-alanine ligase (ddl6 and ddl7) located within gene cassettes in the first position of a reverse integron were identified. Treponema denticola was identified as the likely host of the ddl cassettes. Both ddl6 and ddl7 conferred high level resistance to d-cycloserine when expressed in Escherichia coli with ddl7 conferring four-fold higher resistance to D-cycloserine compared to ddl6. A SNP was found to be responsible for this difference in resistance phenotype between the two ddl variants. Molecular dynamics simulations were used to explain the mechanism of this phenotypic change at the atomic scale. A hypothesis for the evolutionary selection of ddl containing integron gene cassettes is proposed, based on molecular docking of plant metabolites within the ATP and d-cycloserine binding pockets of Ddl.

Dissemination of antibiotic resistance genes (ARGs) via integrons in Escherichia coli: A risk to human health

With the induction of various emerging environmental contaminants such as antibiotic resistance genes (ARGs), environment is considered as a key indicator for the spread of antimicrobial resistance (AMR). As such, the ARGs mediated environmental pollution raises a significant public health concern worldwide. Among various genetic mechanisms that are involved in the dissemination of ARGs, integrons play a vital role in the dissemination of ARGs. Integrons are mobile genetic elements that can capture and spread ARGs among environmental settings via transmissible plasmids and transposons. Most of the ARGs are found in Gram-negative bacteria and are primarily studied for their potential role in antibiotic resistance in clinical settings. As one of the most common microorganisms, Escherichia coli (E. coli) is widely studied as an indicator carrying drug-resistant genes, so this article aims to provide an in-depth study on the spread of ARGs via integrons associated with E. coli outside clinical settings and highlight their potential role as environmental contaminants. It also focuses on multiple but related aspects that do facilitate environmental pollution, i.e. ARGs from animal sources, water treatment plants situated at or near animal farms, agriculture fields, wild birds and animals. We believe that this updated study with summarized text, will facilitate the readers to understand the primary mechanisms as well as a variety of factors involved in the transmission and spread of ARGs among animals, humans, and the environment.

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.

Exploring the Animal Waste Resistome: The Spread of Antimicrobial Resistance Genes Through the Use of Livestock Manure

Antibiotic resistance is a public health problem of growing concern. Animal manure application to soil is considered to be a main cause of the propagation and dissemination of antibiotic residues, antibiotic-resistant bacteria (ARB), and antibiotic resistance genes (ARGs) in the soil-water system. In recent decades, studies on the impact of antibiotic-contaminated manure on soil microbiomes have increased exponentially, in particular for taxonomical diversity and ARGs’ diffusion. Antibiotic resistance genes are often located on mobile genetic elements (MGEs). Horizontal transfer of MGEs toward a broad range of bacteria (pathogens and human commensals included) has been identified as the main cause for their persistence and dissemination. Chemical and bio-sanitizing treatments reduce the antibiotic load and ARB. Nevertheless, effects of these treatments on the persistence of resistance genes must be carefully considered. This review analyzed the most recent research on antibiotic and ARG environmental dissemination conveyed by livestock waste. Strategies to control ARG dissemination and antibiotic persistence were reviewed with the aim to identify methods for monitoring DNA transferability and environmental conditions promoting such diffusion.

Role of integrons in the proliferation of multiple drug resistance in selected bacteria occurring in poultry production

1. The increase in microbial resistance, and in particular multiple drug resistance (MDR), is an increasing threat to public health. The uncontrolled use of antibiotics and antibacterial chemotherapeutics in the poultry industry, especially in concentrations too low to cause inhibition, and the occurrence of residues in feed and in the environment play a significant role in the development of resistance among zoonotic food-borne microorganisms.2. Determining the presence and transmission methods of resistance in bacteria is crucial for tracking and preventing antibiotic resistance. Horizontal transfer of genetic elements responsible for drug resistance is considered to be the main mechanism for the spread of antibiotic resistance.3. Of the many well-known genetic elements responsible for horizontal gene transfer, integrons are among the most important factors contributing to multiple drug resistance. The mechanism of bacterial drug resistance acquisition through integrons is one of the essential elements of MDR prevention in animal production.

Antimicrobial Resistance and Molecular Characterization of Gene Cassettes from Class 1 Integrons in Pseudomonas aeruginosa Strains

We investigated the antibiotic-resistance phenotypes and molecularly characterized class 1 integron gene cassettes from 113 Pseudomonas aeruginosa isolates from patients. Primers specific for the class 1 integron integrase (intI1) gene were used to screen for these integrons using polymerase chain reactions (PCRs). The variable regions of the integrons were PCR-amplified and sequenced. Sputum was the most common specimen (69.9%; 79/113) followed by aseptic sites (21.2%; 24/113). Of the 113 isolates with phenotypic resistance to the tested antimicrobials, the highest resistances were to ciprofloxacin (CIP) (26.55%), imipenem (IPM) (23.89%), and meropenem (MEM) (23%). Carbapenem-sensitive P. aeruginosa (CS-PA) isolates displayed 23 patterns, and the predominant multidrug resistance phenotype was CIP-levofloxacin (7.23%, 6/83). Carbapenem-resistant P. aeruginosa (CR-PA) isolates displayed 12 patterns, and the predominant multidrug resistance phenotype was IPM-MEM (23.33%, 7/30). Class 1 integrons were detected in 14 (12.4%, 14/113) isolates, 7.22% (6/83) in CS-PA isolates, and 26.67% (8/30) in CR-PA isolates. Six gene cassette arrays were detected, the most prevalent being aacA4-blaOXA101-aadA5 in five isolates (4.4%, 5/113). Seventeen gene cassettes were detected. The most prevalent antibiotic-resistance gene cassettes were aacA4 (6.2%, 7/113), bla_OXA-1, and bla_OXA-101. Extended-spectrum β-lactamase resistance genes were detected. Some of the genes carried were similar to those in other species, but some had shared characteristics among the P. aeruginosa isolates. Long-standing drug resistance genes appeared to be under elimination in P. aeruginosa, whereas integrons conferring resistance to commonly used clinical drugs such as β-lactamases, fluoroquinolones, and even carbapenems, as well as some other gene elements, were found to be newly integrated.

A Novel, Integron-Regulated, Class C β-Lactamase

AmpC-type β-lactamases severely impair treatment of many bacterial infections, due to their broad spectrum (they hydrolyze virtually all β-lactams, except fourth-generation cephalosporins and carbapenems) and the increasing incidence of plasmid-mediated versions. The original chromosomal AmpCs are often tightly regulated, and their expression is induced in response to exposure to β-lactams. Regulation of mobile ampC expression is in many cases less controlled, giving rise to constitutively resistant strains with increased potential for development or acquisition of additional resistances. We present here the identification of two integron-encoded ampC genes, bla_IDC-1 and bla_IDC-2 (integron-derived cephalosporinase), with less than 85% amino acid sequence identity to any previously annotated AmpC. While their resistance pattern identifies them as class C β-lactamases, their low isoelectric point (pI) values make differentiation from other β-lactamases by isoelectric focusing impossible. To the best of our knowledge, this is the first evidence of an ampC gene cassette within a class 1 integron, providing a mobile context with profound potential for transfer and spread into clinics. It also allows bacteria to adapt expression levels, and thus reduce fitness costs, e.g., by cassette-reshuffling. Analyses of public metagenomes, including sewage metagenomes, show that the discovered ampCs are primarily found in Asian countries.

Engineering a CRISPR Interference System To Repress a Class 1 Integron in Escherichia coli

Microbial multidrug resistance (MDR) poses a huge threat to human health. Bacterial acquisition of MDR relies primarily on class 1 integron-involved horizontal gene transfer (HGT) of antibiotic resistance genes (ARGs). To date, no strategies other than the use of antibiotics can efficiently cope with MDR. Here, we report that an engineered CRISPR interference (CRISPRi) system can markedly reduce MDR by blocking a class 1 integron in Escherichia coli Using CRISPRi to block plasmid R388 class 1 integron, E. coli recombinants showed halted growth upon exposure to relevant antibiotics. A microplate alamarBlue assay showed that both subgenomic RNAs (sgRNAs) R3 and R6 led to 8- and 32-fold decreases in half-maximal inhibitory concentrations (IC50) for trimethoprim and sulfamethoxazole, respectively. Reverse transcription and quantitative PCR (RT-qPCR) revealed that the strain employing sgRNA R6 exhibited 97% and 84% decreases in the transcriptional levels of the dfrB2 cassette and sul1, two typical ARGs, respectively. RT-qPCR analysis also demonstrated that the strain recruiting sgRNA R3 showed a 96% decrease in the transcriptional level of intI1, and a conjugation assay revealed a 1,000-fold decrease in HGT rates of ARGs. Overall, the sgRNA R3 targeting the 31 bp downstream of the Pc promoter on the intI1 nontemplate strand outperformed other sgRNAs in reducing integron activity. Furthermore, this CRISPRi system is reversible, genetically stable, and titratable by varying the concentration of the inducer. To our knowledge, this is the first report on exploiting a CRISPRi system to reduce the class 1 integron in E. coli This study provides valuable insights for future development of CRISPRi-based antimicrobial agents and cellular therapy to suppress MDR.

The Peril and Promise of Integrons: Beyond Antibiotic Resistance

Integrons are bacterial genetic elements that can capture, rearrange, and express mobile gene cassettes. They are best known for their role in disseminating antibiotic-resistance genes among pathogens. Their ability to rapidly spread resistance phenotypes makes it important to consider what other integron-mediated traits might impact human health in the future, such as increased virulence, pathogenicity, or resistance to novel antimicrobial strategies. Exploring the functional diversity of cassettes and understanding their de novo creation will allow better pre-emptive management of bacterial growth, while also facilitating development of technologies that could harness integron activity. If we can control integrons and cassette formation, we could use integrons as a platform for enzyme discovery and to construct novel biochemical pathways, with applications in bioremediation or biosynthesis of industrial and therapeutic molecules. Integron activity thus holds both peril and promise for humans.

Comparative genomics of Aeromonas veronii: Identification of a pathotype impacting aquaculture globally

Aeromonas veronii is a gram-negative species abundant in aquatic environments that causes disease in humans as well as terrestrial and aquatic animals. In the current study, 41 publicly available A. veronii genomes were compared to investigate distribution of putative virulence genes, global dissemination of pathotypes, and potential mechanisms of virulence. The complete genome of A. veronii strain ML09-123 from an outbreak of motile aeromonas septicemia in farm-raised catfish in the southeastern United States was included. Dissemination of A. veronii strain types was discovered in dispersed geographical locations. Isolate ML09-123 is highly similar to Chinese isolate TH0426, suggesting the two strains have a common origin and may represent a pathotype impacting aquaculture in both countries. Virulence of strain ML09-123 in catfish in a dose-dependent manner was confirmed experimentally. Subsystem category disposition showed the majority of genomes exhibit similar distribution of genomic elements. The type I secretion system (T1SS), type II secretion system (T2SS), type 4 pilus (T4P), and flagellum core elements are conserved in all A. veronii genomes, whereas the type III secretion system (T3SS), type V secretion system (T5SS), type VI secretion system (T6SS), and tight adherence (TAD) system demonstrate variable dispersal. Distribution of mobile elements is dependent on host and geographic origin, suggesting this species has undergone considerable genetic exchange. The data presented here lends insight into the genomic variation of A. veronii and identifies a pathotype impacting aquaculture globally.

Promoter activity of ORF-less gene cassettes isolated from the oral metagenome

Integrons are genetic elements consisting of a functional platform for recombination and expression of gene cassettes (GCs). GCs usually carry promoter-less open reading frames (ORFs), encoding proteins with various functions including antibiotic resistance. The transcription of GCs relies mainly on a cassette promoter (P_C), located upstream of an array of GCs. Some integron GCs, called ORF-less GCs, contain no identifiable ORF with a small number shown to be involved in antisense mRNA mediated gene regulation. In this study, the promoter activity of ORF-less GCs, previously recovered from the oral metagenome, was verified by cloning them upstream of a gusA reporter, proving they can function as a promoter, presumably allowing bacteria to adapt to multiple stresses within the complex physico-chemical environment of the human oral cavity. A bi-directional promoter detection system was also developed allowing direct identification of clones with promoter-containing GCs on agar plates. Novel promoter-containing GCs were identified from the human oral metagenomic DNA using this construct, called pBiDiPD. This is the first demonstration and detection of promoter activity of ORF-less GCs from Treponema bacteria and the development of an agar plate-based detection system will enable similar studies in other environments.

Integron-Derived Aminoglycoside-Sensing Riboswitches Control Aminoglycoside Acetyltransferase Resistance Gene Expression

Class 1 integrons accumulate antibiotic resistance genes by site-specific recombination at aatI-1 sites. Captured genes are transcribed from a promoter located within the integron; for class 1 integrons, the first gene to be transcribed and translated normally encodes an aminoglycoside antibiotic resistance protein (either an acetyltransferase [AAC] or adenyltransferase [AAD]). The leader RNA from the Pseudomonas fluorescens class 1 integron contains an aminoglycoside-sensing riboswitch RNA that controls the expression of the downstream aminoglycoside resistance gene. Here, we explore the relationship between integron-dependent DNA recombination and potential aminoglycoside-sensing riboswitch products of recombination derived from a series of aminoglycoside-resistant clinical strains. Sequence analysis of the clinical strains identified a series of sequence variants that were associated with class I integron-derived aminoglycoside-resistant (both aac and aad) recombinants. For the aac recombinants, representative sequences showed up to 6-fold aminoglycoside-dependent regulation of reporter gene expression. Microscale thermophoresis (MST) confirmed RNA binding. Covariance analysis generated a secondary-structure model for the RNA that is an independent verification of previous models that were derived from mutagenesis and chemical probing data and that was similar to that of the P. fluorescens riboswitch RNA. The aminoglycosides were among the first antibiotics to be used clinically, and the data suggest that in an aminoglycoside-rich environment, functional riboswitch recombinants were selected during integron-mediated recombination to regulate aminoglycoside resistance. The incorporation of a functional aminoglycoside-sensing riboswitch by integron recombination confers a selective advantage for the expression of resistance genes of diverse origins.

Antibiotic Resistance in Salmonella Typhimurium Isolates Recovered From the Food Chain Through National Antimicrobial Resistance Monitoring System Between 1996 and 2016

Salmonella is a major foodborne pathogen which causes widespread contamination and infection worldwide. Salmonella Typhimurium is one of the leading serovars responsible for human and animal salmonellosis, globally. The increasing rate of antibiotic resistance in Salmonella Typhimurium poses a significant global concern, and an improved understanding of the distribution of antibiotic resistance patterns in Salmonella Typhimurium is essential for choosing the suitable antibiotic for the treatment of infections. To evaluate the roles of animal and human in antibiotic resistance dissemination, this study aims to categorize 11,447 S. Typhimurium strains obtained across the food-chain, including food animals, retail meats and humans for 21 years in the United States by analyzing minimum inhibitory concentrations (MICs) values for 27 antibiotics. Random Forest Algorithm and Hierarchical Clustering statistics were used to group the strains according to their minimum inhibitory concentration values. Classification and Regression Tree analysis was used to identify the best classifier for human- and animal-populations’ isolates. We found the persistent population or multi-drug resistant strains of S. Typhimurium across the four time periods (1996∼2000, 2001∼2005, 2006∼2010, 2011∼2016). Importantly, we also detected that there was more diversity in the MIC patterns among S. Typhimurium strains isolated between 2011 and 2016, which suggests significant emergence of diversified multi-drug resistant strains. The most frequently observed (43%) antibiotic resistance patterns found in S. Typhimurium were tetra-resistant pattern ASSuT (ampicillin, streptomycin, sulfonamides, and tetracycline) and the penta-resistant pattern ACSSuT (ampicillin, chloramphenicol, streptomycin, sulfonamides, and tetracycline). Animals (mainly swine and bovine) are the major source for these two frequently found antibiotic resistance patterns. The occurrence of antibiotic resistant strains from humans and chicken is alarming. Strains were mostly susceptible to fluoroquinolones. Together, this study helped in understanding the expansion of dynamics of antibiotic resistance of S. Typhimurium and recommended fluoroquinolones as a possible treatment options against S. Typhimurium infection.

Bats as reservoirs of antibiotic resistance determinants: A survey of class 1 integrons in Grey-headed Flying Foxes (Pteropus poliocephalus)

Increasing reports of antimicrobial resistance in wildlife highlight the significance of a One Health approach to managing resistance. We investigated the prevalence and diversity of class 1 integrons, a genetic determinant of resistance, in grey-headed flying foxes, a large fruit bat species belonging to the order Chiroptera. Class 1 integrons were detected in both wild flying foxes (5.3%) and captive flying foxes (41.2%) housed in wildlife rehabilitation facilities. Genes encoding resistance to aminoglycosides, trimethoprim and beta-lactams, and Qac efflux pumps were detected. Analysis of conserved integron elements and gene cassette arrays indicate the direction of integron transfer is from humans to flying foxes. The detection of two novel gene cassette arrays (5'CS-qacH-aacA34-bla_OXA-21-3'CS and 5'CS-qacF-3'CS strongly suggests acquisition of genes from the environmental resistome into class 1 integrons within the flying fox microbiota. The dynamics of class 1 integrons in flying foxes indicates bats have a role in the emergence of novel antibiotic resistance determinants.

High Prevalence of Antimicrobial Resistance in Gram-Negative Bacteria Isolated from Clinical Settings in Egypt: Recalling for Judicious Use of Conventional Antimicrobials in Developing Nations

This study was designed to investigate, at the molecular level, the antimicrobial resistance mechanisms of different antimicrobial resistance genes, including, extended-spectrum β-lactamases, AmpC β-lactamases, class 1 and 2 integrons, and plasmid-mediated quinolone resistance genes of Gram-negative bacteria isolated from clinical settings in Egypt. A total of 126 nonduplicate Gram-negative isolates were recovered from different clinical samples taken from hospitalized patients in Egypt in 2014. Antimicrobial susceptibility testing showed that, 93.6% (118/126) of the isolates had a multidrug-resistant phenotype. Interestingly, we reported a high level of antimicrobial resistance nearly for all tested antibiotics; to our knowledge, this is the first report from Egypt indicating very high level of antibiotic resistance in Egypt. Polymerase chain reaction screening and DNA sequencing revealed that, 75.4% (95/126) of the isolates harbored at least one extended-spectrum β-lactamase-encoding gene, with bla_CTX-M being the most prevalent (65.9%), followed by bla_SHV (46.8%). The AmpC β-lactamase, bla_CMY, was detected in 7.1% (9/126) of bacterial isolates, with bla_CMY-42 being the most prevalent. Class 1 integrons were detected in 50.8% (64/126) of the isolates, and class 2 integrons were detected in 2.4% (3/126) of the isolates. The plasmid-mediated quinolone resistance gene, qnr, was detected in 58.7% (74/126) of the tested isolates, with qnrS being the most prevalent. Several antimicrobial resistance determinants were identified in Egypt for the first time, such as SHV-27, SHV-28, SHV-33, SHV-63, SHV-71, SHV-82, SHV-142, CMY-42, CMY-6, and the new CMY-72 like. This study highlights the importance of the conscious use of conventional antimicrobials to overcome the multidrug resistance problem.

Mobile Genetic Elements Associated with Antimicrobial Resistance

Strains of bacteria resistant to antibiotics, particularly those that are multiresistant, are an increasing major health care problem around the world. It is now abundantly clear that both Gram-negative and Gram-positive bacteria are able to meet the evolutionary challenge of combating antimicrobial chemotherapy, often by acquiring preexisting resistance determinants from the bacterial gene pool. This is achieved through the concerted activities of mobile genetic elements able to move within or between DNA molecules, which include insertion sequences, transposons, and gene cassettes/integrons, and those that are able to transfer between bacterial cells, such as plasmids and integrative conjugative elements. Together these elements play a central role in facilitating horizontal genetic exchange and therefore promote the acquisition and spread of resistance genes. This review aims to outline the characteristics of the major types of mobile genetic elements involved in acquisition and spread of antibiotic resistance in both Gram-negative and Gram-positive bacteria, focusing on the so-called ESKAPEE group of organisms (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter spp., and Escherichia coli), which have become the most problematic hospital pathogens.