Transposon mutagenesis of the anaerobic commensal, Bacteroides fragilis, using the EZ::TN5 transposome

Commensal microbiota from patients with inflammatory bowel disease produce genotoxic metabolites

Microbiota-derived metabolites that elicit DNA damage can contribute to colorectal cancer (CRC). However, the full spectrum of genotoxic chemicals produced by indigenous gut microbes remains to be defined. We established a pipeline to systematically evaluate the genotoxicity of an extensive collection of gut commensals from inflammatory bowel disease patients. We identified isolates from divergent phylogenies whose metabolites caused DNA damage and discovered a distinctive family of genotoxins-termed the indolimines-produced by the CRC-associated species Morganella morganii. A non-indolimine-producing M. morganii mutant lacked genotoxicity and failed to exacerbate colon tumorigenesis in mice. These studies reveal the existence of a previously unexplored universe of genotoxic small molecules from the microbiome that may affect host biology in homeostasis and disease.

Identification of Genes Involved in Antifungal Activity of Burkholderia seminalis Against Rhizoctonia solani Using Tn5 Transposon Mutation Method

Rhizoctonia solani is the causative agent of rice sheath blight disease. In a previous study, we found that the growth of R. solani was inhibited by Burkholderia seminalis strain R456. Therefore, the present study was conducted to identify the genes involved in the antifungal activity of B. seminalis strain R456 by using a Tn5 transposon mutation method. Firstly, we constructed a random insertion transposon library of 997 mutants, out of which 11 mutants showed the defective antifungal activity against R. solani. Furthermore, the 10 antagonism-related genes were successfully identified based on analysis of the Tn5 transposon insertion site. Indeed, this result indicated that three mutants were inserted on an indigenous plasmid in which the same insertion site was observed in two mutants. In addition, the remaining eight mutants were inserted on different genes encoding glycosyl transferase, histone H1, nonribosomal peptide synthetase, methyltransferase, MnmG, sulfate export transporter, catalase/peroxidase HPI and CysD, respectively. Compared to the wild type, the 11 mutants showed a differential effect in bacteriological characteristics such as cell growth, biofilm formation and response to H₂O₂ stress, revealing the complexity of action mode of these antagonism-related genes. However, a significant reduction of cell motility was observed in the 11 mutants compared to the wild type. Therefore, it can be inferred that the antifungal mechanism of the 10 above-mentioned genes may be, at least partially, due to the weakness of cell motility. Overall, the result of this study will be helpful for us to understand the biocontrol mechanism of this bacterium.

Mutagenesis of the rpoS gene involved in alteration of outer membrane composition

BACKGROUND AND OBJECTIVES

rpoS is a bacterial sigma factor of RNA polymerase which is involved in the expression of the genes which control regulons and play a critical role in survival against stresses. Few suitable vectors are available which could be maintained successfully in Flexibacter chinesis cells and could in particular be used as a suicide vector to make mutation in the rpoS gene. The aim of this study was to investigate if rpoS mutagenesis has impact on bacterial morphology in addition to cell division.

MATERIALS AND METHODS

A 0.603 kb BamHI-PstI fragment subclone of pICRPOS38Ω was cloned into linearized pLYLO3. The final construct, pLRPOS38 suicide vector, was introduced into Flexibacter chinesis. Then the cytoplasm of mutant strain and wild-type were investigated by transmission electron microscopy.

RESULTS

After successful subcloning of suicide vector into F. chinesis, based on TEM study, it was demonstrated that mutation in rpoS gene leads to decomposition of outer membrane of F. chinesis.

CONCLUSION

A suitable vector to make suicide mutation in rpoS was constructed for F. chinesi.

Transposon Mutagenesis of Bacteroides fragilis

Bacteroides fragilis is Gram-negative obligatory anaerobe which usually resides in the gut of humans and animals. As an important member of the human gut microbiota it plays a vital role in digestion and absorption of nutrients as well as shaping of host immune system. B. fragilis is also infamous for causing serious infections. Treatment of B. fragilis infections caused emergence of multidrug-resistant strains. Molecular biology tools such as transposon mutagenesis help to decipher and understand commensal and pathogenic faces of B. fragilis. Using two mariner transposon vectors we describe the detailed methodology for the transposon mutagenesis of B. fragilis. We also describe two methods for the identification of transposon integration site (TIS) in transposon mutants. Transposon mutagenesis methods described in this chapter serve as a great tool for studying B. fragilis.

Identification of potential Campylobacter jejuni genes involved in biofilm formation by EZ-Tn5 Transposome mutagenesis

BACKGROUND

Biofilm formation has been suggested to play a role in the survival of Campylobacter jejuni in the environment and contribute to the high incidence of human campylobacteriosis. Molecular studies of biofilm formation by Campylobacter are sparse.

RESULTS

We attempted to identify genes that may be involved in biofilm formation in seven C. jejuni strains through construction of mutants using the EZ-Tn5 Transposome system. Only 14 mutants with reduced biofilm formation were obtained, all from one strain of C. jejuni. Three different genes of interest, namely CmeB (synthesis of multidrug efflux system transporter proteins), NusG (transcription termination and anti-termination protein) and a putative transmembrane protein (involved in membrane protein function) were identified. The efficiency of the EZ::TN5 transposon mutagenesis approach was strain dependent and was unable to generate any mutants from most of the strains used.

CONCLUSIONS

A diverse range of genes may be involved in biofilm formation by C. jejuni. The application of the EZ::TN5 system for construction of mutants in different Campylobacter strains is limited.

Two multidrug-resistant clinical isolates of Bacteroides fragilis carry a novel metronidazole resistance nim gene (nimJ)

Two multidrug-resistant Bacteroides fragilis clinical isolates contain and express a novel nim gene, nimJ, that is not recognized by the "universal" nim primers and can confer increased resistance to metronidazole when introduced into a susceptible strain on a multicopy plasmid. HMW615, an appendiceal isolate, contains at least two copies of nimJ on its genome, while HMW616, an isolate from a patient with sepsis, contains one genomic copy of nimJ. B. fragilis NimJ is phylogenetically closer to Prevotella baroniae NimI and Clostridium botulinum NimA than to the other known Bacteroides Nim proteins. The predicted protein structure of NimJ, based on fold recognition analysis, is consistent with the crystal structures derived for known Nim proteins, and specific amino acid residues important for substrate binding in the active site are conserved. This study demonstrates that the "universal" nim primers will not detect all nim genes with the ability to confer metronidazole resistance, but nimJ alone cannot account for the very high metronidazole MICs of these resistant clinical isolates.

Transposon mutagenesis of Bacteroides fragilis using a mariner transposon vector

The mariner transposon vector pYV07 was tested for use in the mutagenesis of Bacteroides fragilis 638R. The transposon vector efficiently generated mutants in B. fragilis 638R. The transposon disrupted genes were scattered throughout the genome of B. fragilis 638R. This method serves as a powerful tool to study B. fragilis.