Cloning and Functional Characterization of Putative Escherichia coli ABC Multidrug Efflux Transporter YddA

IS1-mediated chromosomal amplification of the arn operon leads to polymyxin B resistance in Escherichia coli B strains

Polymyxins [colistin and polymyxin B (PMB)] comprise an important class of natural product lipopeptide antibiotics used to treat multidrug-resistant Gram-negative bacterial infections. These positively charged lipopeptides interact with lipopolysaccharide (LPS) located in the outer membrane and disrupt the permeability barrier, leading to increased uptake and bacterial cell death. Many bacteria counter polymyxins by upregulating genes involved in the biosynthesis and transfer of amine-containing moieties to increase positively charged residues on LPS. Although 4-deoxy-l-aminoarabinose (Ara4N) and phosphoethanolamine (PEtN) are highly conserved LPS modifications in Escherichia coli, different lineages exhibit variable PMB susceptibilities and frequencies of resistance for reasons that are poorly understood. Herein, we describe a mechanism prevalent in E. coli B strains that depends on specific insertion sequence 1 (IS1) elements that flank genes involved in the biosynthesis and transfer of Ara4N to LPS. Spontaneous and transient chromosomal amplifications mediated by IS1 raise the frequency of PMB resistance by 10- to 100-fold in comparison to strains where a single IS1 element located 90 kb away from the end of the arn operon has been deleted. Amplification involving IS1 becomes the dominant resistance mechanism in the absence of PEtN modification. Isolates with amplified arn operons gradually lose their PMB-resistant phenotype with passaging, consistent with classical PMB heteroresistance behavior. Analysis of the whole genome transcriptome profile showed altered expression of genes residing both within and outside of the duplicated chromosomal segment, suggesting complex phenotypes including PMB resistance can result from tandem amplification events.IMPORTANCEPhenotypic variation in susceptibility and the emergence of resistant subpopulations are major challenges to the clinical use of polymyxins. While a large database of genes and alleles that can confer polymyxin resistance has been compiled, this report demonstrates that the chromosomal insertion sequence (IS) content and distribution warrant consideration as well. Amplification of large chromosomal segments containing the arn operon by IS1 increases the Ara4N content of the lipopolysaccharide layer in Escherichia coli B lineages using a mechanism that is orthogonal to transcriptional upregulation through two-component regulatory systems. Altogether, our work highlights the importance of IS elements in modulating gene expression and generating diverse subpopulations that can contribute to phenotypic polymyxin B heteroresistance.

Early transcriptional changes of heavy metal resistance and multiple efflux genes in Xanthomonas campestris pv. campestris under copper and heavy metal ion stress

BACKGROUND

Copper-induced gene expression in Xanthomonas campestris pv. campestris (Xcc) is typically evaluated using targeted approaches involving qPCR. The global response to copper stress in Xcc and resistance to metal induced damage is not well understood. However, homologs of heavy metal efflux genes from the related Stenotrophomonas genus are found in Xanthomonas which suggests that metal related efflux may also be present.

METHODS AND RESULTS

Gene expression in Xcc strain BrA1 exposed to 0.8 mM CuSO4.5H2O for 15 minutes was captured using RNA-seq analysis. Changes in expression was noted for genes related to general stress responses and oxidoreductases, biofilm formation, protein folding chaperones, heat-shock proteins, membrane lipid profile, multiple drug and efflux (MDR) transporters, and DNA repair were documented. At this timepoint only the cohL (copper homeostasis/tolerance) gene was upregulated as well as a chromosomal czcCBA efflux operon. An additional screen up to 4 hrs using qPCR was conducted using a wider range of heavy metals. Target genes included a cop-containing heavy metal resistance island and putative metal efflux genes. Several efflux pumps, including a copper resistance associated homolog from S. maltophilia, were upregulated under toxic copper stress. However, these pumps were also upregulated in response to other toxic heavy metals. Additionally, the temporal expression of the coh and cop operons was also observed, demonstrating co-expression of tolerance responses and later activation of part of the cop operon.

CONCLUSIONS

Overall, initial transcriptional responses focused on combating oxidative stress, mitigating protein damage and potentially increasing resistance to heavy metals and other biocides. A putative copper responsive efflux gene and others which might play a role in broader heavy metal resistance were also identified. Furthermore, the expression patterns of the cop operon in conjunction with other copper responsive genes allowed for a better understanding of the fate of copper ions in Xanthomonas. This work provides useful evidence for further evaluating MDR and other efflux pumps in metal-specific homeostasis and tolerance phenotypes in the Xanthomonas genus. Furthermore, non-canonical copper tolerance and resistance efflux pumps were potentially identified. These findings have implications for interpreting MIC differences among strains with homologous copLAB resistance genes, understanding survival under copper stress, and resistance in disease management.

The effects of NDM-5 on Escherichia coli and the screening of interacting proteins

Carbapenem-resistant Escherichia coli (E. coli) strains are widely distributed and spreading rapidly, creating significant challenges for clinical therapeutics. NDM-5, a novel mutant of New Delhi Metallo-β-Lactamase-1 (NDM-1), exhibits high hydrolase activity toward carbapenems. Since the genetic backgrounds of clinically isolated carbapenem-resistant E. coli are heterogeneous, it is difficult to accurately evaluate the impact of blaNDM-5 on antibiotic resistance. Herein, E. coli BL21 was transformed with a plasmid harboring blaNDM-5, and the resultant strain was named BL21 (pET-28a-blaNDM-5). Consistent with the findings of previous studies, the introduction of exogenous blaNDM-5 resulted in markedly greater resistance of E. coli to multiple β-lactam antibiotics. Compared with BL21 (pET-28a), BL21 (pET-28a-blaNDM-5) exhibited reduced motility but a significant increase in biofilm formation capacity. Furthermore, transcriptome sequencing was conducted to compare the transcriptional differences between BL21 (pET-28a) and BL21 (pET-28a-blaNDM-5). A total of 461 differentially expressed genes were identified, including those related to antibiotic resistance, such as genes associated with the active efflux system (yddA, mcbR and emrY), pili (csgC, csgF and fimD), biofilm formation (csgD, csgB and ecpR) and antioxidant processes (nuoG). Finally, the pGS21a plasmid harboring blaNDM-5 was transformed into E. coli Rosetta2, after which the expression of the NDM-5 protein was induced using isopropyl-β-D-thiogalactoside (IPTG). Using glutathione-S-transferase (GST) pull-down assays, total proteins from E. coli were scanned to screen out 82 proteins that potentially interacted with NDM-5. Our findings provide new insight into the identified proteins to identify potential antibiotic targets and design novel inhibitors of carbapenem-resistant bacteria.

A combination of proteomics, genetics, and physiology provides insights into the acid-tolerance phenotype of Pseudomonas pergaminensis F77

Acid tolerance is crucial for the effective and persistent mineral weathering by acid-producing bacteria. Here, the molecular basis of the acid tolerance of mineral-weathering Pseudomonas pergaminensis F77 was identified using proteomics analysis of the strain under acid stress. Then, the acid tolerance of strain F77 and its mutants with deletion of the acid tolerance-related genes orf03767, mcp, resR, nueR, yegD, and fxsA, which are involved in the two-component systems, DNA repair, nucleotide binding, and membrane parts, were compared. Finally, the acid tolerance-related physiological mechanisms of strain F77 and its mutants F77ΔnueR and F77ΔresR under acidic conditions were characterized. The significantly upregulated proteins in the acid-adapted and acid-challenged strain F77 included the proteins involved in metabolic pathways associated with ATPase, membrane components, organic acid transmembrane transporters, response to stimulus, nucleotide binding, ABC transporters, and two-component systems. The cell numbers decreased by 24-100% at pH ≤ 4.50, while the membrane fluidity increased by 22-61% at pH ≤ 5.50 for the mutants F77ΔnueR and F77ΔresR, compared with that of strain F77. The intracellular H+-ATPase activities decreased by 29-33% for the mutant F77ΔnueR at pH ≤ 4.50% and 33-79% for the mutant F77ΔresR at all tested pHs (pH ≤ 7.00); meanwhile, the ratios of intracellular NAD+/NADH decreased by 71-91% for the mutant F77ΔresR at all tested pHs (pH ≤ 7.00), compared with that of strain F77. Furthermore, the intracellular putrescine concentrations were reduced by 40-70% for the mutant F77ΔresR at all tested pHs (pH ≤ 7.00) compared with that of strain F77. Our findings suggested that multiple proteins and metabolic pathways were associated with bacterial acid tolerance and revealed that nueR and resR were involved in acid tolerance based on their modulation of multiple acid tolerance-related physiological functions in strain F77.

Review of knockout technology approaches in bacterial drug resistance research

Gene knockout is a widely used method in biology for investigating gene function. Several technologies are available for gene knockout, including zinc-finger nuclease technology (ZFN), suicide plasmid vector systems, transcription activator-like effector protein nuclease technology (TALEN), Red homologous recombination technology, CRISPR/Cas, and others. Of these, Red homologous recombination technology, CRISPR/Cas9 technology, and suicide plasmid vector systems have been the most extensively used for knocking out bacterial drug resistance genes. These three technologies have been shown to yield significant results in researching bacterial gene functions in numerous studies. This study provides an overview of current gene knockout methods that are effective for genetic drug resistance testing in bacteria. The study aims to serve as a reference for selecting appropriate techniques.

Structure Elucidation, Total Synthesis, Antibacterial In Vivo Efficacy and Biosynthesis Proposal of Myxobacterial Corramycin

Herein, we describe the myxobacterial natural product Corramycin isolated from Corallococcus coralloides. The linear peptide structure contains an unprecedented (2R,3S)-γ-N-methyl-β-hydroxy-histidine moiety. Corramycin exhibits anti-Gram-negative activity against Escherichia coli (E. coli) and is taken up via two transporter systems, SbmA and YejABEF. Furthermore, the Corramycin biosynthetic gene cluster (BGC) was identified and a biosynthesis model was proposed involving a 12-modular non-ribosomal peptide synthetase/polyketide synthase. Bioinformatic analysis of the BGC combined with the development of a total synthesis route allowed for the elucidation of the molecule's absolute configuration. Importantly, intravenous administration of 20 mg kg-1 of Corramycin in an E. coli mouse infection model resulted in 100 % survival of animals without toxic side effects. Corramycin is thus a promising starting point to develop a potent antibacterial drug against hospital-acquired infections.

Phenotype-Guided Comparative Genomics Identifies the Complete Transport Pathway of the Antimicrobial Lasso Peptide Ubonodin in Burkholderia

New antibiotics are needed as bacterial infections continue to be a leading cause of death, but efforts to develop compounds with promising antibacterial activity are hindered by a poor understanding of─and limited strategies for elucidating─their modes of action. We recently discovered a novel lasso peptide, ubonodin, that is active against opportunistic human lung pathogens from the Burkholderia cepacia complex (Bcc). Ubonodin inhibits RNA polymerase, but only select strains were susceptible, indicating that having a conserved cellular target does not guarantee activity. Given the cytoplasmic target, we hypothesized that cellular uptake of ubonodin determines susceptibility. Although Bcc strains harbor numerous nutrient uptake systems, these organisms lack close homologues of the single known lasso peptide membrane receptor, FhuA. Thus, a straightforward homology-driven approach failed to uncover the identity of the ubonodin transporter(s). Here, we used phenotype-guided comparative genomics to identify genes uniquely associated with ubonodin-susceptible Bcc strains, leading to the identification of PupB as the ubonodin outer membrane (OM) receptor in Burkholderia. The loss of PupB renders B. cepacia resistant to ubonodin, whereas expressing PupB sensitizes a resistant strain. We also examine how a conserved iron-regulated transcriptional pathway controls PupB to further tune ubonodin susceptibility. PupB is only the second lasso peptide OM receptor to be uncovered and the first outside of enterobacteria. Finally, we elucidate the full transport pathway for ubonodin by identifying its inner membrane receptor YddA in Burkholderia. Our work provides a complete picture of the mode of action of ubonodin and establishes a general framework for deciphering the transport pathways of other natural products with cytoplasmic targets.

Emergence and Mechanism of Resistance of Tulathromycin Against Mycoplasma hyopneumoniae in a PK/PD Model and the Fitness Costs of 23S rRNA Mutants

Macrolides are widely used in diseases caused by Mycoplasma spp. The new semi-synthetic macrolide antibiotic tulathromycin is currently in wide use for the treatment of respiratory diseases of livestock. The objective of this study was to evaluate the antibacterial effect of tulathromycin against Mycoplasma hyopneumoniae using an in vitro pharmacokinetic/pharmacodynamic (PK/PD) model to reveal mechanisms of antibiotic resistance and to evaluate the fitness of drug-resistant strains. In this study, high performance liquid chromatography-tandem mass spectrometry was used to determine drug concentrations for the in vitro model after dosing. The peak concentrations were in the range 0.3125–20 μg/mL (1 × MIC-64 × MIC). The ratio of the area under the concentration-time curve (AUC) over 72 h divided by the MIC (AUC_72h/MIC) had the highest correlation with the antibacterial effect of tulathromycin against M. hyopneumoniae. Tulathromycin also showed concentration-dependent antimicrobial effects and promoted the emergence of drug-resistant bacteria after being cultured for 168 h and most were mutations in 23S rRNA at site A2058G (E.coli numbering) and only a single isolate was an A2058T (E.coli numbering) mutant. In the presence of reserpine, we determined the MIC of tulathromycin, tilmicosin, tiamulin and tylosin against these drug-resistant bacteria and the strains with efflux pump mechanisms were found among the strains resistant to tilmicosin. Gene expression analysis indicated that the ABC and MATE transporter efflux pump genes RS01935, RS02670, RS01115, RS01970, RS02395 and RS03540 (MATE family efflux transporter) were up-regulated in the three strains (P < 0.05 or P < 0.01). These investigations provide guidance for clinical administration of tulathromycin and elucidate the mechanism and fitness cost of drug resistance in M. hyopneumoniae.

Core oligosaccharide portion of lipopolysaccharide plays important roles on multiple antibiotic resistance in Escherichia coli

Gram-negative bacteria are intrinsically resistant to antibiotics due to the presence of the cell envelope, but mechanisms are still not fully understood. In this study, a series of mutants that lack one or more major components associated with the cell envelope were constructed from Escherichia coli K-12 W3110. WJW02 can only synthesize Kdo₂-lipid A which lacks the core oligosaccharide portion of lipopolysaccharide. WJW04, WJW07 and WJW08 were constructed from WJW02 by deleting the gene clusters relevant to the biosynthesis of exopolysaccharide, flagella and fimbria, respectively. WJW09, WJW010 and WJW011 cells cannot synthesize exopolysaccharide, flagella and fimbria, respectively. Comparing to the wild type W3110, mutants WJW02, WJW04, WJW07 and WJW08 cells showed decreased resistance to more than 10 different antibacterial drugs, but not the mutants WJW09, WJW010 and WJW011. This indicates that the core oligosaccharide portion of lipopolysaccharide plays important roles on multiple antibiotic resistance in E. coli and the 1^st heptose in core oligosaccharide portion is critical. Furthermore, the removal of the core oligosaccharide of LPS leads to influences on cell wall morphology, cell phenotypes, porins, efflux systems, and the respond behaviors to antibiotic stimulation. The results demonstrated the important role of lipopolysaccharide on the antibiotic resistance of Gram-negative bacteria.