Determination of Plasmid Segregational Stability in a Growing Bacterial Population

Engineering Escherichia coli for l-homoserine production

l-homoserine, a nonprotein amino acid, is used to synthesize many active substances in the industry. Here, to develop a robust l-homoserine-producing strain, Escherichia coli W3110 was used as a chassis to be engineered. Based on a previous construct with blocked competing routes for l-homoserine synthesis, five genes were overexpressed by promoter replacement strategy to increase the l-homoserine production, including enhancement of precursors for l-homoserine synthesis (ppc, thrA, and asd), reinforcement of the NADPH supply (pntAB) and efflux transporters (rhtA) to improve the l-homoserine production. However, the plasmid losing was to blame for the wildly fluctuating fermentation performance of engineered strains, ranging between 2.1 and 6.2 g/L. Then, a hok/sok toxin/antitoxin system was introduced into the free plasmid expression cassette to maintain the genetic stability of the episomal plasmid; consequently, the plasmid-losing rate sharply decreased, resulting in the engineered strain SHL17, which exhibited excellent stability in l-homoserine production, with 6.3 g/L in shake flasks and 44.4 g/L in a 5-L fermenter without antibiotic addition. This work verified the effective use of the hok/sok toxin/antitoxin system combined with promoter engineering to improve the genetic stability of E. coli episomal plasmids without antibiotics.

Construction of constitutive expression of Eimeria tenella eukaryotic initiation factor U6L5H2 on the surface of Lactobacillus plantarum and evaluation of its immunoprotective efficiency against chicken coccidiosis

Lactobacillus strains exhibit preferable properties that make them attractive candidates for vaccine delivery systems because of their ability to regulate intestinal mucosal immunity in the body. To date, live Lactobacillus delivery vaccines reported for the defense against Eimeria tenella have been inducer-dependent systems whose applications are significantly limited due to their unattainable induction conditions in vivo. Here, a constitutive expression of Lactobacillus plantarum NC8 surface display system was constructed. Then, this system was used to prepare a live oral vaccine to constitutively express the E. tenella U6L5H2 (EtU6) protein on the NC8 surface and to evaluate its protective efficacy against E. tenella challenge in chickens. The results showed that the heterologous protein (EGFP or EtU6) was successfully expressed on the surface of L. plantarum NC8 without any inducer. The immunoprotection of EtU6 with constitutive expression in L. plantarum NC8 system (NC8/Pc-EtU6) was significantly stronger than that of EtU6 with induced expression of L. plantarum NC8 system (NC8/Pi-EtU6) (ACI: 168.28 vs. 152.74) as evidenced by increased body weight, decreased oocyst output and lesion scores. Furthermore, the constitutive system NC8/Pc-EtU6 produced higher levels of specific cecal SIgA, serum IgG, transcription of cytokines IFN-γ and IL-2, and lymphocyte proliferation than the induced system NC8/Pi-EtU6. These results indicate that, compared to the inducible system, the constitutive surface display system of L. plantarum has the advantages of continuously expressing antigens in vivo and stimulating the host immune system. It could be an ideal platform for vaccine expression. The live vector vaccine for coccidiosis constructed by this constitutive system greatly improves the application potential in chicken production and provides a novel platform for the prevention of coccidiosis in chickens.

The roles of HicBA and a novel toxin-antitoxin-like system, TsxAB, in the stability of IncX4 resistance plasmids in Escherichia coli

OBJECTIVES

To identify toxin-antitoxin (TA)-like plasmid stability loci on IncX4 plasmids.

METHODS

TA-like loci were identified bioinformatically and their contribution to stability of the IncX4 plasmid pJIE143 was tested in optimal growth conditions in vitro. The conservation of the TA-like loci identified was analysed within an updated IncX plasmid database.

RESULTS

A novel TA-like locus, tsxAB, was identified on the IncX4 plasmid pJIE143, carrying the important plasmid-borne antibiotic resistance gene blaCTX-M-15. pJIE143 (the WT plasmid) and its tsxA mutant are stable for 80 bacterial generations in the absence of selective pressure but a tsxB deletion mutant of pJIE143 is relatively quickly lost without positive selection (91.1% ± 1.5% loss after 50 generations). Nine IncX subclasses were identified among 272 fully sequenced IncX plasmids, dominated by those identified as IncX3, IncX1 and IncX4 subclasses in PlasmidFinder. The novel TA-like locus, tsxAB, appears to be a feature of IncX4 plasmids, being present in 64 of 67 so identified, but only present in a single IncX1 plasmid (of 79 identified) and present in no other IncX plasmids.

CONCLUSIONS

tsxAB, a novel TA-like stability locus, is highly conserved in IncX4 plasmids associated with transmission of important antibiotic resistance genes. Previous in silico analysis indicated that IncX4 encodes only HicBA among the known TA systems. Here we show that HicBA does not contribute to plasmid stability in optimal growth conditions for Escherichia coli and instead demonstrate this role for a completely novel TA-like system, TsxAB, that appears both necessary and sufficient for E. coli addiction to IncX4 resistance plasmids.

Development of a Sensitive Escherichia coli Bioreporter Without Antibiotic Markers for Detecting Bioavailable Copper in Water Environments

The whole-cell bioreporters based on the cop-operon sensing elements have been proven specifically useful in the assessment of bioavailable copper ions in water environments. In this study, a series of experiments was conducted to further improve the sensitivity and robustness of bioreporters. First, an Escherichia coli △copA△cueO△cusA mutant with three copper transport genes knocked out was constructed. Then, the copAp::gfpmut2 sensing element was inserted into the chromosome of E. coli △copA△cueO△cusA by gene knock-in method to obtain the bioreporter strain E. coli WMC-007. In optimized assay conditions, the linear detection range of Cu²⁺ was 0.025–5 mg/L (0.39–78.68 μM) after incubating E. coli WMC-007 in Luria–Bertani medium for 5 h. The limit of detection of Cu²⁺ was 0.0157 mg/L (0.25 μM). Moreover, fluorescence spectrometry and flow cytometry experiments showed more environmental robustness and lower background fluorescence signal than those of the sensor element based on plasmids. In addition, we found that the expression of GFPmut2 in E. coli WMC-007 was induced by free copper ions, rather than complex-bound copper, in a dose-dependent manner. Particularly, the addition of 40 mM 3-(N-Morpholino)propanesulfonic acid buffer to E. coli WMC-007 culture enabled accurate quantification of bioavailable copper content in aqueous solution samples within a pH range from 0.87 to 12.84. The copper recovery rate was about 95.88–113.40%. These results demonstrate potential applications of E. coli WMC-007 as a bioreporter to monitor copper contamination in acidic mine drainage, industrial wastewater, and drinking water. Since whole-cell bioreporters are relatively inexpensive and easy to operate, the combination of this method with other physicochemical techniques will in turn provide more specific information on the degree of toxicity in water environments.

Dissemination of the bla NDM-5 Gene via IncX3-Type Plasmid among Enterobacteriaceae in Children

The emergence of CRE strains resistant to multiple antibiotics is considered a substantial threat to human health. Therefore, all the efforts to provide a detailed molecular transmission mechanism of specific drug resistance can contribute positively to prevent the further spread of multidrug-resistant bacteria. Although the new superbug harboring bla_NDM-5 has been reported in many countries, it was mostly identified among E. coli strains, and the gene transfer mechanism has not been fully recognized and studied. In this work, we identified 22 bla_NDM-5-positive strains in different species of Enterobacteriaceae, including 16 Klebsiella pneumoniae strains, four Klebsiella aerogenes strains, and two Escherichia coli strains, which indicated the horizontal gene transfer of bla_NDM-5 among Enterobacteriaceae strains in pediatric patients. Moreover, bla_NDM-5 was located on a 46-kb IncX3 plasmid, which is possibly responsible for this widespread horizontal gene transfer. The different genetic contexts of the bla_NDM-5 gene indicated some minor evolutions of the plasmid, based on the complete sequences of the bla_NDM-5 plasmids. These findings are of great significance to understand the transmission mechanism of drug resistance genes, develop anti-infection treatment, and take effective infection control measures.

The continuous emergence of novel New Delhi metallo-β-lactamase-5 (NDM-5)-producing Enterobacteriaceae isolates is receiving more and more public attention. Twenty-two NDM-5-producing strains were identified from 146 carbapenemase-producing Enterobacteriaceae (CRE) strains isolated from pediatric patients between January and March 2017, indicating that the bla_NDM-5 gene has spread to children. All 22 isolates, including 16 Klebsiella pneumoniae strains, four Klebsiella aerogenes strains, and two Escherichia coli strains, showed significantly high resistance to β-lactam antibiotics (except aztreonam) but remained susceptible to tigecycline and colistin. K. pneumoniae and K. aerogenes strains were respectively defined as homologous clonal isolates by pulsed-field gel electrophoresis (PFGE). Multilocus sequence typing (MLST) results confirmed the genetic relatedness with all K. pneumoniae strains belonging to sequence type (ST) 48. Two E. coli isolates (ST617 and ST1236) were considered genetically unrelated. Twenty-two bla_NDM-5 plasmids were positive for the IncX3 amplicon and showed almost identical profiles after digestion with HindIII and EcoRI. Four representative strains (K. pneumoniae K725, K. aerogenes CR33, E. coli Z214, and E. coli Z244) were selected for further study. Plasmids harboring bla_NDM-5 showed strong stability in both clinical isolates and transconjugants, without apparent plasmid loss after 100 serial generations. S1-PFGE followed by Southern blot analysis demonstrated that the bla_NDM-5 gene was located on an ∼46-kb plasmid. Plasmid sequences of pNDM-K725, pNDM-CR33, and pNDM-Z214 were almost identical but were slightly different from that of pNDM-Z244. Compared with pNDM-Z244, ΔISAba125 and partial copies of IS3000 were missing. The genetic backgrounds of the bla_NDM-5 gene in four strains were slightly different from that of the typical pNDM_MGR194. This study comprehensively characterized the horizontal gene transfer of the bla_NDM-5 gene among different Enterobacteriaceae isolates in pediatric patients, and the IncX3-type plasmid was responsible for the spread.

IMPORTANCE The emergence of CRE strains resistant to multiple antibiotics is considered a substantial threat to human health. Therefore, all the efforts to provide a detailed molecular transmission mechanism of specific drug resistance can contribute positively to prevent the further spread of multidrug-resistant bacteria. Although the new superbug harboring bla_NDM-5 has been reported in many countries, it was mostly identified among E. coli strains, and the gene transfer mechanism has not been fully recognized and studied. In this work, we identified 22 bla_NDM-5-positive strains in different species of Enterobacteriaceae, including 16 Klebsiella pneumoniae strains, four Klebsiella aerogenes strains, and two Escherichia coli strains, which indicated the horizontal gene transfer of bla_NDM-5 among Enterobacteriaceae strains in pediatric patients. Moreover, bla_NDM-5 was located on a 46-kb IncX3 plasmid, which is possibly responsible for this widespread horizontal gene transfer. The different genetic contexts of the bla_NDM-5 gene indicated some minor evolutions of the plasmid, based on the complete sequences of the bla_NDM-5 plasmids. These findings are of great significance to understand the transmission mechanism of drug resistance genes, develop anti-infection treatment, and take effective infection control measures.

Identification of IncA/C Plasmid Replication and Maintenance Genes and Development of a Plasmid Multilocus Sequence Typing Scheme

Plasmids of incompatibility group A/C (IncA/C) are becoming increasingly prevalent within pathogenic Enterobacteriaceae They are associated with the dissemination of multiple clinically relevant resistance genes, including bla_CMY and bla_NDM Current typing methods for IncA/C plasmids offer limited resolution. In this study, we present the complete sequence of a bla_NDM-1-positive IncA/C plasmid, pMS6198A, isolated from a multidrug-resistant uropathogenic Escherichia coli strain. Hypersaturated transposon mutagenesis, coupled with transposon-directed insertion site sequencing (TraDIS), was employed to identify conserved genetic elements required for replication and maintenance of pMS6198A. Our analysis of TraDIS data identified roles for the replicon, including repA, a toxin-antitoxin system; two putative partitioning genes, parAB; and a putative gene, 053 Construction of mini-IncA/C plasmids and examination of their stability within E. coli confirmed that the region encompassing 053 contributes to the stable maintenance of IncA/C plasmids. Subsequently, the four major maintenance genes (repA, parAB, and 053) were used to construct a new plasmid multilocus sequence typing (PMLST) scheme for IncA/C plasmids. Application of this scheme to a database of 82 IncA/C plasmids identified 11 unique sequence types (STs), with two dominant STs. The majority of bla_NDM-positive plasmids examined (15/17; 88%) fall into ST1, suggesting acquisition and subsequent expansion of this bla_NDM-containing plasmid lineage. The IncA/C PMLST scheme represents a standardized tool to identify, track, and analyze the dissemination of important IncA/C plasmid lineages, particularly in the context of epidemiological studies.