Efficient transposon mutagenesis mediated by an IPTG-controlled conditional suicide plasmid

An evolutionarily conserved metabolite inhibits biofilm formation in Escherichia coli K-12

Methylerythritol cyclodiphosphate (MEcPP) is an intermediate in the biosynthesis of isoprenoids in plant plastids and in bacteria, and acts as a stress signal in plants. Here, we show that MEcPP regulates biofilm formation in Escherichia coli K-12 MG1655. Increased MEcPP levels, triggered by genetic manipulation or oxidative stress, inhibit biofilm development and production of fimbriae. Deletion of fimE, encoding a protein known to downregulate production of adhesive fimbriae, restores biofilm formation in cells with elevated MEcPP levels. Limited proteolysis-coupled mass spectrometry (LiP-MS) reveals that MEcPP interacts with the global regulatory protein H-NS, which is known to repress transcription of fimE. MEcPP prevents the binding of H-NS to the fimE promoter. Therefore, our results indicate that MEcPP can regulate biofilm formation by modulating H-NS activity and thus reducing fimbriae production. Further research is needed to test whether MEcPP plays similar regulatory roles in other bacteria.

Inducible transposon mutagenesis for genome-scale forward genetics

Transposon insertion sequencing (Tn-seq) is a powerful method for genome-scale functional genetics in bacteria. However, its effectiveness is often limited by a lack of mutant diversity, caused by either inefficient transposon delivery or stochastic loss of mutants due to population bottlenecks. Here, we introduce "InducTn-seq", which leverages inducible mutagenesis for temporal control of transposition. InducTn-seq generates millions of transposon mutants from a single colony, enabling the sensitive detection of subtle fitness defects and transforming binary classifications of gene essentiality into a quantitative fitness measurement across both essential and non-essential genes. Using a mouse model of infectious colitis, we show that InducTn-seq bypasses a highly restrictive host bottleneck to generate a diverse transposon mutant population from the few cells that initiate infection, revealing the role of oxygen-related metabolic plasticity in pathogenesis. Overall, InducTn-seq overcomes the limitations of traditional Tn-seq, unlocking new possibilities for genome-scale forward genetic screens in bacteria.

The Pseudomonas aeruginosa Resistome: Permanent and Transient Antibiotic Resistance, an Overview

One of the most concerning characteristics of Pseudomonas aeruginosa is its low susceptibility to several antibiotics of common use in clinics, as well as its facility to acquire increased resistance levels. Consequently, the study of the antibiotic resistance mechanisms of this bacterium is of relevance for human health. For such a study, different types of resistance should be distinguished. The intrinsic resistome is composed of a set of genes, present in the core genome of P. aeruginosa, which contributes to its characteristic, species-specific, phenotype of susceptibility to antibiotics. Acquired resistance refers to those genetic events, such as the acquisition of mutations or antibiotic resistance genes that reduce antibiotic susceptibility. Finally, antibiotic resistance can be transiently acquired in the presence of specific compounds or under some growing conditions. The current article provides information on methods currently used to analyze intrinsic, mutation-driven, and transient antibiotic resistance in P. aeruginosa.

Improved site-specific mutagenesis in Rhodococcus opacus using a novel conditional suicide plasmid

Rhodococcus opacus PD630 is a biotechnologically important bacterium with metabolic capability for bioremediation, metal recovery, and storage of triacylglycerols. Genome editing by homologous recombination in R. opacus is hampered by a very low combined frequency of DNA transfer and recombination. To improve recombination in the species, a conjugative, conditional suicide plasmid based on the replicon derived from the Corynebacterium glutamicum plasmid pGA1 was constructed and evaluated in R. opacus. The replication of this plasmid is controlled by a dual inducible and repressible promoter system originally developed for Mycobacterium spp. Next, we demonstrated that a derivative of this plasmid containing sacB as a counterselection marker and homologous regions of R. opacus could be used for homologous recombination, and that the problem of obtaining recombinants had been solved. Like for other Corynebacteriales, the cell wall of Rhodococcus spp. contains mycolic acids which form a hydrophobic and impermeable outer layer. Mycolic acids are essential for Mycobacterium smegmatis, but not for Corynebacterium glutamicum, and the new vector was used to study if mycolic acid is essential for R. opacus. We found that accD3 that is necessary for mycolic acid synthesis could only be deleted from the chromosome in strains containing a plasmid-encoded copy of accD3. This indicates that mycolic acid is important for R. opacus viability. The conditional suicide vector should be useful for homologous recombination or for delivering gene products like recombinases or Cas proteins and gRNA to Rhodococcus and related genera, while the approach should be applicable for any plasmid needing a plasmid-encoded protein for replication. KEY POINTS: • Improved vector for homologous recombination in R. opacus. • Mycolic acid is important for survival of R. opacus like it is for Mycobacterium. • Similar conditional suicide plasmids may be constructed for other bacteria.

The lipoprotein NlpD in Cronobacter sakazakii responds to acid stress and regulates macrophage resistance and virulence by maintaining membrane integrity

Cronobacter sakazakii, an emerging opportunistic pathogen, is implicated in severe foodborne outbreak infections in premature and full-term infants. Generally, acid tolerance is vital for the pathogenesis of foodborne pathogens; however, its role in C. sakazakii virulence remains largely unknown. To screen out acid-tolerance determinants from transposon mutants, anovel counterselection method using gentamicin and acid was developed. Using the counterselection method and growth assay, we screened several acid-sensitive mutants and found that nlpD encodes an acid-resistance factor in C. sakazakii. 

Compared to the wild-type strain, the nlpD mutant exhibited attenuated virulence in a rat model. Using macrophage THP-1 cells and a pH probe, we verified that nlpD enables bacteria to resist macrophages by resisting acidification. Finally, we confirmed that nlpD maintains C. sakazakii membrane integrity in acid using propidium iodide permeabilization assays via flow cytometry. Our results confirm that nlpD is a novel virulence factor that permits C. sakazakii to survive under acid stress conditions. Considering that NlpD is a conserved lipoprotein located in the bacterial outer membrane, NlpD could be used as a target for drug development.

A mini-Tn5-derived transposon with reportable and selectable markers enables rapid generation and screening of insertional mutants in Gram-negative bacteria

We re-engineered a classic tool for mutagenesis and gene expression studies in Gram-negative bacteria. Our modified Tn5-based transposon contains multiple features that allow rapid selection for mutants, direct quantification of gene expression and straightforward cloning of the inactivated gene. The promoter-less gfp-km cassette provides selection and reporter assay depending on the activity of the promoter upstream of the transposon insertion site. The cat gene facilitates positive antibiotic selection for mutants, while the narrow R6Kγ replication origin forces transposition in recipient strains lacking the pir gene and enables cloning of the transposon flanked with the disrupted gene from the chromosome. The suicide vector pCKD100, a plasmid that could be delivered into recipient cells through biparental mating or electroporation, harbours the modified transposon. We used the transposon to mutagenize Pectobacterium versatile KD100, Pseudumonas coronafaciens PC27R and Escherichia coli 35150N. The fluorescence intensities of mutants expressing high GFP could be quantified and detected qualitatively. Transformation efficiency from conjugation ranged from 1600 to 1900 CFU per ml. We sequenced the upstream flanking regions, identified the putative truncated genes and demonstrated the restoration of the GFP phenotype through marker exchange. The mini-Tn5 transposon was also utilized to construct mutant a library of P. versatile for forward genetic screens.

Genetic determinants of Salmonella enterica critical for attachment and biofilm formation

Salmonella is a bacterial pathogen frequently involved in human gastrointestinal infections including those associated with low-moisture foods such as dehydrated food powders/spices, vegetable seeds, and tree nuts. The survival/persistence of Salmonella on low moisture foods and in dry environments is enhanced by its ability in developing biofilms. This study was undertaken to identify the genetic determinants critical for Salmonella attachment and biofilm formation. E. coli SM10 lambda pir, with a kanamycin resistant marker on mini-Tn10 (mini-Tn10:lacZ:kan^r), an ampicillin resistant marker on the mini-Tn10-bearing suicidal plasmid pLBT and a streptomycin sensitive marker on the SM10 chromosome, was used as a donor (amp^r, kan^r, strep^s), and three Salmonella strains (amp^s, kan^s, strep^r) were used as recipients in a transposon mutagenesis study. The donor and each recipient were co-incubated overnight on tryptic soy agar at 37 °C, and mutant colonies (amp^s, kan^r, strep^r) were subsequently selected. A single-banded degenerate PCR product, amplified from each mutant genome using oligonucleotide primers derived from the end of min-Tn10 and restriction enzyme EcoR I- or Pst I-recognizing sequence, were analyzed using the Sanger sequencing technology. Acquired DNA sequences were compared to those deposited in the Genbank using BLAST search. Cells of Salmonella mutants accumulated either significantly more or less (P < 0.05) biofilms than their parent cells on polystyrene surface. Sequence analysis of degenerate PCR products revealed that the mini-Tn10 from pLBT had inserted into the cdg, trx, fadI or rxt on Salmonella chromosomes. Results of the research will likely help strategize future antimicrobial intervention for control of pathogen attachment and biofilm formation.