Transposon Mutant Library Research Articles

UbiF is involved in acid stress tolerance and symbiotic competitiveness in Rhizobium favelukesii LPU83.

The acidity of soils significantly reduces the productivity of legumes mainly because of the detrimental effects of hydrogen ions on the legume plants, leading to the establishment of an inefficient symbiosis and poor biological nitrogen fixation. We recently reported the analysis of the fully sequenced genome of Rhizobium favelukesii LPU83, an alfalfa-nodulating rhizobium with a remarkable ability to grow, nodulate and compete in acidic conditions. To gain more insight into the genetic mechanisms leading to acid tolerance in R. favelukesii LPU83, we constructed a transposon mutant library and screened for mutants displaying a more acid-sensitive phenotype than the parental strain. We identified mutant Tn833 carrying a single-transposon insertion within LPU83_2531, an uncharacterized short ORF located immediately upstream from ubiF homolog. This gene encodes a protein with an enzymatic activity involved in the biosynthesis of ubiquinone. As the transposon was inserted near the 3' end of LPU83_2531 and these genes are cotranscribed as a part of the same operon, we hypothesized that the phenotype in Tn833 is most likely due to a polar effect on ubiF transcription.We found that a mutant in ubiF was impaired to grow at low pH and other abiotic stresses including 5mM ascorbate and 0.500mM Zn2+. Although the ubiF mutant retained the ability to nodulate alfalfa and Phaseolus vulgaris, it was unable to compete with the R. favelukesii LPU83 wild-type strain for nodulation in Medicago sativa and P. vulgaris, suggesting that ubiF is important for competitiveness. Here, we report for the first time an ubiF homolog being essential for nodulation competitiveness and tolerance to specific stresses in rhizobia.

Identification of novel biofilm genes in avian pathogenic Escherichia coli by Tn5 transposon mutant library.

Avian pathogenic Escherichia coli (APEC) is the main pathogens that inflict the poultry industry. Biofilm as the pathogenic factors of APEC, which can enhance the anti-host immune system of APEC and improve its survival in the environment. In order to screen for new genes related to APEC biofilm. The APEC strain APEC81 was used to construct a mutant library by Tn5 insertion mutagenesis. Moreover the 28 mutant strains with severely weakened biofilm were successfully screened from 1500 mutant strains by crystal violet staining, in which 17 genes were obtained by high-efficiency thermal asymmetric interlaced PCR. The reported genes include 3 flagella genes (fliS, fliD, andfliR), 4 curli fimbriae genes (csgD, csgA, csgF, andcsgG) and 3 type 1 fimbriae genes (fimA, fimD, andfimC). The novel genes include 3 coenzyme genes (gltA, bglX, and mltF) and 4 putative protein genes (yehE, 07045, 11735, 11255). To investigate whether these 17 genes co-regulate the biofilm, the 17 identified genes were deleted from APEC strain APEC81. The results showed that except for the 11735 and 11255 genes, the deletion of 15 genes significantly reduced the biofilm formation ability of APEC81 (P < 0.05). The result of rdar (red, dry and rough) colony morphology showed that curli fimbriae genes (csgD, csgA, csgF, andcsgG) and other functional genes (fimC, glxK, yehE, 07045, and11255) affected the colony morphology. In particular, the hypothetical protein YehE had the greatest influence on the biofilm. It was predicted to have the same structure as the type 1 fimbria protein. When yehE was deleted, the fimE transcription was up-regulated, and the fimA and fimB transcription were down-regulated, resulting in a decrease in type 1 fimbriae. Hence, the yehE mutant significantly reduced the biofilm and the adhesion and invasion ability to cells (P < 0.05). This study identified 5 novel genes (gltA, bglX, mltF, yehE, and07045) related to biofilm formation and confirmed that yehE affects biofilm formation by type 1 fimbriae, which will benefit further study of the mechanism of biofilm regulation in APEC.

Genome-Scale Mutational Analysis of Cathode-Oxidizing Thioclava electrotropha ElOx9T.

Extracellular electron transfer (EET) – the process by which microorganisms transfer electrons across their membrane(s) to/from solid-phase materials – has implications for a wide range of biogeochemically important processes in marine environments. Though EET is thought to play an important role in the oxidation of inorganic minerals by lithotrophic organisms, the mechanisms involved in the oxidation of solid particles are poorly understood. To explore the genetic basis of oxidative EET, we utilized genomic analyses and transposon insertion mutagenesis screens (Tn-seq) in the metabolically flexible, lithotrophic Alphaproteobacterium Thioclava electrotropha ElOx9T. The finished genome of this strain is 4.3 MB, and consists of 4,139 predicted ORFs, 54 contain heme binding motifs, and 33 of those 54 are predicted to localize to the cell envelope or have unknown localizations. To begin to understand the genetic basis of oxidative EET in ElOx9T, we constructed a transposon mutant library in semi-rich media which was comprised of >91,000 individual mutants encompassing >69,000 unique TA dinucleotide insertion sites. The library was subjected to heterotrophic growth on minimal media with acetate and autotrophic oxidative EET conditions on indium tin oxide coated glass electrodes poised at –278 mV vs. SHE or un-poised in an open circuit condition. We identified 528 genes classified as essential under these growth conditions. With respect to electrochemical conditions, 25 genes were essential under oxidative EET conditions, and 29 genes were essential in both the open circuit control and oxidative EET conditions. Though many of the genes identified under electrochemical conditions are predicted to be localized in the cytoplasm and lack heme binding motifs and/or homology to known EET proteins, we identified several hypothetical proteins and poorly characterized oxidoreductases that implicate a novel mechanism(s) for EET that warrants further study. Our results provide a starting point to explore the genetic basis of novel oxidative EET in this marine sediment microbe.

PruR and PA0065 Genes Are Responsible for Decreasing Antibiotic Tolerance by Autoinducer Analog-1 (AIA-1) in Pseudomonas aeruginosa.

Pseudomonas aeruginosa infection is considered a high-risk nosocomial infection and is very difficult to eradicate because of its tolerance to antibiotic treatment. A new compound, autoinducer analog-1 (AIA-1), has been demonstrated to reduce antibiotic tolerance, but its mechanisms remain unknown. This study aimed to investigate the mechanisms of AIA-1 in the antibiotic tolerance of P. aeruginosa. A transposon mutant library was constructed using miniTn5pro, and screening was performed to isolate high tolerant mutants upon exposure to biapenem and AIA-1. We constructed a deletion mutant and complementation strain of the genes detected in transposon insertion site determination, pruR and PA0066-65-64, and performed killing assays with antibiotics and AIA-1. Gene expression upon exposure to biapenem and AIA-1 and their relationship to stress response genes were analyzed. High antibiotic tolerance was observed in Tn5-pruR and Tn5-PA0065 transposon mutants and their deletion mutants, ΔpruR and ΔPA0066-65-64. Complemented strains of pruR and PA0066-65-64 with their respective deletion mutants exhibited suppressed antibiotic tolerance. It was determined that deletion of PA0066-65-64 increased rpoS expression, and PA0066-65-64 affects antibiotic tolerance via the rpoS pathway. Additionally, antibiotics and AIA-1 were found to inhibit pruR and PA0066-65-64. This study proposed that pruR and PA0066-65-64 are members of the antibiotic tolerance suppressors.

Neisseria genes required for persistence identified via in vivo screening of a transposon mutant library.

The mechanisms used by human adapted commensal Neisseria to shape and maintain a niche in their host are poorly defined. These organisms are common members of the mucosal microbiota and share many putative host interaction factors with Neisseria meningitidis and Neisseria gonorrhoeae. Evaluating the role of these shared factors during host carriage may provide insight into bacterial mechanisms driving both commensalism and asymptomatic infection across the genus. We identified host interaction factors required for niche development and maintenance through in vivo screening of a transposon mutant library of Neisseria musculi, a commensal of wild-caught mice which persistently and asymptomatically colonizes the oral cavity and gut of CAST/EiJ and A/J mice. Approximately 500 candidate genes involved in long-term host interaction were identified. These included homologs of putative N. meningitidis and N. gonorrhoeae virulence factors which have been shown to modulate host interactions in vitro. Importantly, many candidate genes have no assigned function, illustrating how much remains to be learned about Neisseria persistence. Many genes of unknown function are conserved in human adapted Neisseria species; they are likely to provide a gateway for understanding the mechanisms allowing pathogenic and commensal Neisseria to establish and maintain a niche in their natural hosts. Validation of a subset of candidate genes confirmed a role for a polysaccharide capsule in N. musculi persistence but not colonization. Our findings highlight the potential utility of the Neisseria musculi-mouse model as a tool for studying the pathogenic Neisseria; our work represents a first step towards the identification of novel host interaction factors conserved across the genus.

Genetic Determinants of Surface Accessibility in Staphylococcus aureus.

Bacterial cell walls represent one of the most prominent targets of antibacterial agents. These agents include natural products (e.g., vancomycin) and proteins stemming from the innate immune system (e.g., peptidoglycan-recognition proteins and lysostaphin). Among bacterial pathogens that infect humans, Staphylococcus aureus (S. aureus) continues to impose a tremendous healthcare burden across the globe. S. aureus has evolved countermeasures that can directly restrict the accessibility of innate immune proteins, effectively protecting itself from threats that target key cell well components. We recently described a novel assay that directly reports on the accessibility of molecules to the peptidoglycan layer within the bacterial cell wall of S. aureus. The assay relies on site-specific chemical remodeling of the peptidoglycan with a biorthogonal handle. Here, we disclose the application of our assay to a screen of a nonredundant transposon mutant library for susceptibility of the peptidoglycan layer with the goal of identifying genes that contribute to the control of cell surface accessibility. We discovered several genes that resulted in higher accessibility levels to the peptidoglycan layer and showed that these genes modulate sensitivity to lysostaphin. These results indicate that this assay platform can be leveraged to gain further insight into the biology of bacterial cell surfaces.

Screening for Virulence-Related Genes via a Transposon Mutant Library of Streptococcus suis Serotype 2 Using a Galleria mellonella Larvae Infection Model.

Streptococcus suis (S. suis) is a zoonotic bacterial pathogen causing lethal infections in pigs and humans. Identification of virulence-related genes (VRGs) is of great importance in understanding the pathobiology of a bacterial pathogen. To identify novel VRGs, a transposon (Tn) mutant library of S. suis strain SC19 was constructed in this study. The insertion sites of approximately 1700 mutants were identified by Tn-seq, which involved 417 different genes. A total of 32 attenuated strains were identified from the library by using a Galleria mellonella larvae infection model, and 30 novel VRGs were discovered, including transcription regulators, transporters, hypothetical proteins, etc. An isogenic deletion mutant of hxtR gene (ΔhxtR) and its complementary strain (CΔhxtR) were constructed, and their virulence was compared with the wild-type strain in G. mellonella larvae and mice, which showed that disruption of hxtR significantly attenuated the virulence. Moreover, the ΔhxtR strain displayed a reduced survival ability in whole blood, increased sensitivity to phagocytosis, increased chain length, and growth defect. Taken together, this study performed a high throughput screening for VRGs of S. suis using a G. mellonella larvae model and further characterized a novel critical virulence factor.

Screening transcriptional connections in Staphylococcus aureus using high-throughput transduction of bioluminescent reporter plasmids.

Characterization of transcriptional networks is one of the main strategies used to understand how bacteria interact with their environment. To reveal novel regulatory elements in the human pathogen Staphylococcus aureus, we adapted a traditional transduction protocol to be used in a high-throughput format in combination with the publicly available S. aureus Nebraska Transposon Mutant Library. Specifically, plasmid transductions are performed in 96-well format, so that a single plasmid can be simultaneously transferred into numerous recipient strains. When used in conjunction with bioluminescent reporter constructs, this strategy enables parallel and continuous monitoring of downstream transcriptional effects of hundreds of defined mutations. Here, we use this workflow in a proof-of-concept study to identify novel regulators of the staphylococcal metalloprotease aureolysin. Importantly, this strategy can be utilized with any other bacterium where plasmid transduction is possible, making it a versatile and efficient tool to probe transcriptional regulatory connections.

The menaquinone pathway is important for susceptibility of Staphylococcus aureus to the antibiotic adjuvant, cannabidiol.

Emergence of antibiotic resistant bacteria is evolving at an alarming pace; therefore, we must start turning to alternative approaches. One of these, could be the use of antibiotic adjuvants that enhances the effect of antibiotics towards resistant bacteria. A novel antibiotic adjuvant is cannabidiol (CBD), which we have previously shown can enhance the effect of bacitracin (BAC). BAC targets cell wall synthesis by inhibiting dephosphorylation of the lipid carrier undecaprenyl pyrophosphate prior to recycling across the membrane. However, the mechanism underlying this CBD mediated potentiation of BAC has remained unknown. To explore this, we examined resistance to CBD in Staphylococcus aureus through daily exposures to CBD. By subsequent whole genome sequencing, we observed multiple genes to be mutated, including the farE/farR system encoding a fatty acid efflux pump (FarE) and its regulator (FarR). Importantly, recreation of mutations in these genes showed decreased susceptibility towards the combination of CBD and BAC. Furthermore, we searched the Nebraska Transposon Mutant Library for CBD susceptible strains and identified menH encoding a protein participating in menaquinone biosynthesis. Strains containing deletions in this and other menaquinone related genes showed increased susceptibility towards CBD, while addition of exogenous menaquinone reversed the effect and reduced susceptible towards CBD. These results suggest that CBD potentiates BAC by redirecting the isoprenoid precursor isopentenyl pyrophosphate towards production of menaquinone rather than the lipid carrier undecaprenyl pyrophosphate, which dephosphorylation is inhibited by BAC. This in turn might decrease the level of undecaprenyl pyrophosphate thus enhancing the effect of BAC. Our study illustrates how antibiotic adjuvants may apply to enhance efficacy of antimicrobial compounds.

Study on the inhibitory effect of selective estrogen receptor modulators on Streptococcus mutans.

To screen small-molecule antibacterial drugs and investigate the antibacterial effect and mechanism of selective estrogen receptor modulators (SERMs) against Streptococcus mutans (S.mutans). The minimum inhibitory concentration of 426 Food and Drug Administration (FDA)-approved small-molecule drugs against S. mutans was determined using the microdilution method, and the target of SERMs acting on S. mutans was explored by employing a random transposon mutant library. Among the 426 FDA-approved SERMs, toremiphene, tamoxifen, clomiphene, and raloxifene exhibited excellent antibacterial effects against S.mutans. Results of mutant library screening showed that the two mutant strains were resistant to clomiphene. The gene sequence of the resistant strains showed that the transposon insertion sites were located in the genes of smu_546 and smu_874. SERMs, such as toremifene, tamoxifen, clomiphene, and raloxifene, exerted obvious antibacterial effects on S. mutans, and their targets may be proteins expressed by smu_546 and smu_874 gene.

Bacteriophage S6 requires bacterial cellulose for Erwinia amylovora infection.

Bacteriophages are highly selective in targeting bacteria. This selectivity relies on the specific adsorption of phages to the host cell surface. In this study, a Tn5 transposon mutant library of Erwinia amylovora, the causative agent of fire blight, was screened to identify bacterial receptors required for infection by the podovirus S6. Phage S6 was unable to infect mutants with defects in the bacterial cellulose synthase operon (bcs). The Bcs complex produces and secretes bacterial cellulose, an extracellular polysaccharide associated with bacterial biofilms. Deletion of the bcs operon or associated genes (bcsA, bcsC and bcsZ) verified the crucial role of bacterial cellulose for S6 infection. Application of the cellulose binding dye Congo Red blocked infection by S6. We demonstrate that infective S6 virions degraded cellulose and that Gp95, a phage-encoded cellulase, is involved to catalyse the reaction. In planta S6 did not significantly inhibit fire blight symptom development. Moreover, deletion of bcs genes in E. amylovora did not affect bacterial virulence in blossom infections, indicating that sole application of cellulose targeting phages is less appropriate to biologically control E. amylovora. The interplay between cellulose synthesis, host cell infection and maintenance of the host cell population is discussed.

The periplasmic chaperone protein Psg_2795 contributes to the virulence of Pseudomonas savastanoi pv. glycinea: the causal agent of bacterial blight of soybean.

Pseudomonas savastanoi pv. glycinea (Psg also named P. syringae pv. glycinea and P. amygdali pv. glycinea) is the causative agent of bacterial blight in soybean. The identification of virulence factors is essential for understanding the pathogenesis of Psg. In this study, a mini-Tn5 transposon mutant library of Psg strain PsgNC12 was screened on soybean, and one low-virulent mini-Tn5 mutant, designated as 4573, was identified. Sequence analysis of the 4573-mutant revealed that the mini-Tn5 transposon was inserted in the Psg_2795 gene. Psg_2795 encodes a FimC-domain protein that is highly conserved in Pseudomonas. Further analysis revealed that the mutation and knockout of Psg_2795 results in a reduced virulence phenotype on soybean, decreased motility, weakened bacterial attachment to a glass surface and delayed the population growth within soybean leaves. The phenotype of the 4573-mutant could be complemented nearly to wild-type levels using an intact Psg_2795 gene. Collectively, our results demonstrate that Psg_2795 plays an important role in the virulence, motility, attachment and the population growth of PsgNC12 in soybean. This finding provides a new insight into the function of periplasmic chaperone proteins in a type I pilus and provides reference information for identifying Psg_2795 homologues in P. savastanoi and other bacteria.

Long-read sequencing for identification of insertion sites in large transposon mutant libraries

Transposon insertion site sequencing (TIS) is a powerful method for associating genotype to phenotype. However, all TIS methods described to date use short nucleotide sequence reads which cannot uniquely determine the locations of transposon insertions within repeating genomic sequences where the repeat units are longer than the sequence read length. To overcome this limitation, we have developed a TIS method using Oxford Nanopore sequencing technology that generates and uses long nucleotide sequence reads; we have called this method LoRTIS (Long-Read Transposon Insertion-site Sequencing). LoRTIS enabled the unique localisation of transposon insertion sites within long repetitive genetic elements of E. coli, such as the transposase genes of insertion sequences and copies of the ~ 5 kb ribosomal RNA operon. We demonstrate that LoRTIS is reproducible, gives comparable results to short-read TIS methods for essential genes, and better resolution around repeat elements. The Oxford Nanopore sequencing device that we used is cost-effective, small and easily portable. Thus, LoRTIS is an efficient means of uniquely identifying transposon insertion sites within long repetitive genetic elements and can be easily transported to, and used in, laboratories that lack access to expensive DNA sequencing facilities.

Identification of Methicillin-Resistant Staphylococcus aureus (MRSA) Genetic Factors Involved in Human Endothelial Cells Damage, an Important Phenotype Correlated with Persistent Endovascular Infection.

Methicillin-resistant Staphylococcus aureus (MRSA) is a leading cause of life-threatening endovascular infections. Endothelial cell (EC) damage is a key factor in the pathogenesis of these syndromes. However, genetic factors related to the EC damage have not been well studied. This study aims to identify genetic determinants that impact human EC damage by screening the genome-wide Nebraska Transposon Mutant Library (NTML). A well-established MTT assay was used to test the in vitro damage of human EC cell line (HMEC-1) caused by each mutant strain in the NTML. We first confirmed some global regulators and genes positively impact the EC damage, which is consistent with published results. These data support the utility of the high-throughput approach. Importantly, we demonstrated 317 mutants significantly decreased the EC damage, while only 6 mutants enhanced the EC damage vs. parental JE2 strain. The majority of these genes have not been previously defined to affect human EC damage. Interestingly, many of these newly identified genes are involved in metabolism, genetic and environmental information processing, and cellular processes. These results advance our knowledge of staphylococcal genetic factors related to human EC damage which may provide novel targets for the development of effective agents against MRSA endovascular infection.

Guidelines for metabolomics-guided transposon mutagenesis for microbial natural product discovery.

There is a great discrepancy between the natural product output of cultured microorganisms and their bioinformatically predicted biosynthetic potential, such that most of the molecular diversity contained within microbial reservoirs has yet to be discovered. One of the primary reasons is insufficient expression of natural product biosynthetic gene clusters (BGCs) under standard laboratory conditions. Several methods have been developed to increase production from such "cryptic" BGCs. Among these, we recently implemented mass spectrometry-guided transposon mutagenesis, a forward genetic screen in which mutants that exhibit stimulated biosynthesis of cryptic metabolites, as read out by mass spectrometry, are selected from a transposon mutant library. Herein, we use Burkholderia gladioli as an example and provide guidelines for generating transposon mutant libraries, measuring metabolomic inventories through mass spectrometry, performing comparative metabolomics to prioritize cryptic natural products from the mutant library, and isolating and characterizing novel natural products elicited through mutagenesis. Application of this approach will be useful in both accessing novel natural products from cryptic BGCs and identifying genes involved in their global regulation.

Chemical biology-whole genome engineering datasets predict new antibacterial combinations.

Trimethoprim and sulfamethoxazole are used commonly together as cotrimoxazole for the treatment of urinary tract and other infections. The evolution of resistance to these and other antibacterials threatens therapeutic options for clinicians. We generated and analysed a chemical-biology-whole-genome data set to predict new targets for antibacterial combinations with trimethoprim and sulfamethoxazole. For this we used a large transposon mutant library in Escherichia coli BW25113 where an outward-transcribing inducible promoter was engineered into one end of the transposon. This approach allows regulated expression of adjacent genes in addition to gene inactivation at transposon insertion sites, a methodology that has been called TraDIS-Xpress. These chemical genomic data sets identified mechanisms for both reduced and increased susceptibility to trimethoprim and sulfamethoxazole. The data identified that over-expression of FolA reduced trimethoprim susceptibility, a known mechanism for reduced susceptibility. In addition, transposon insertions into the genes tdk, deoR, ybbC, hha, ldcA, wbbK and waaS increased susceptibility to trimethoprim and likewise for rsmH, fadR, ddlB, nlpI and prc with sulfamethoxazole, while insertions in ispD, uspC, minC, minD, yebK, truD and umpG increased susceptibility to both these antibiotics. Two of these genes’ products, Tdk and IspD, are inhibited by AZT and fosmidomycin respectively, antibiotics that are known to synergise with trimethoprim. Thus, the data identified two known targets and several new target candidates for the development of co-drugs that synergise with trimethoprim, sulfamethoxazole or cotrimoxazole. We demonstrate that the TraDIS-Xpress technology can be used to generate information-rich chemical-genomic data sets that can be used for antibacterial development.

Delving into defence: identifying the Pseudomonas protegens Pf-5 gene suite involved in defence against secreted products of fungal, oomycete and bacterial rhizosphere competitors.

Competitive behaviours of plant growth promoting rhizobacteria (PGPR) are integral to their ability to colonize and persist on plant roots and outcompete phytopathogenic fungi, oomycetes and bacteria. PGPR engage in a range of antagonistic behaviours that have been studied in detail, such as the production and secretion of compounds inhibitory to other microbes. In contrast, their defensive activities that enable them to tolerate exposure to inhibitory compounds produced by their neighbours are less well understood. In this study, the genes involved in the Pseudomonas protegens Pf-5 response to metabolites from eight diverse rhizosphere competitor organisms, Fusarium oxysporum, Rhizoctonia solani, Gaeumannomyces graminis var. tritici, Pythium spinosum, Bacillus subtilis QST713, Pseudomonas sp. Q2-87, Streptomyces griseus and Streptomyces bikiniensis subspecies bikiniensi, were examined. Proximity induced excreted metabolite responses were confirmed for Pf-5 with all partner organisms through HPLC before culturing a dense Pf-5 transposon mutant library adjacent to each of these microbes. This was followed by transposon-directed insertion site sequencing (TraDIS), which identified genes that influence Pf-5 fitness during these competitive interactions. A set of 148 genes was identified that were associated with increased fitness during competition, including cell surface modification, electron transport, nucleotide metabolism, as well as regulatory genes. In addition, 51 genes were identified for which loss of function resulted in fitness gains during competition. These included genes involved in flagella biosynthesis and cell division. Considerable overlap was observed in the set of genes observed to provide a fitness benefit during competition with all eight test organisms, indicating commonalities in the competitive response to phylogenetically diverse micro-organisms and providing new insight into competitive processes likely to take place in the rhizosphere.

Identification and Analysis of Essential Genes in Streptococcus mutans with Transposon Sequencing.

Transposon sequencing (Tn-seq) has greatly accelerated the rate at which gene function can be profiled in microbial organisms. This technique has been applied to the study of the dental caries pathogen Streptococcus mutans where it has been used to generate large transposon mutant libraries. Coupled with high-throughput sequencing and bioinformatics tools, culture of these transposon mutant libraries has facilitated the identification of essential and conditional essential genes. In this chapter, we describe a procedure for performing Tn-seq studies in S. mutans that covers pooled transposon mutant construction, in vitro culture, and DNA library sequencing and data analysis.

A Method of Transposon Insertion Sequencing in Comprehensively Identifying Vibrio vulnificus Genes Required for Growth in Human Serum.

One of the most powerful approaches to detect the loci that enable a pathogen to cause disease is the creation of a high-density transposon mutant library by transposon insertion sequencing (TIS) and the screening of the library using an adequate in vivo and/or ex vivo model of the disease. Here we describe the procedure for detection of the putative loci required for a septicemic pathogen to cause sepsis in humans by using TIS plus an ex vivo model of septicaemia: to grow the pathogen in fresh and inactivated human serum. We selected V. vulnificus because it is a highly invasive pathogen capable of spreading from an infection site to the bloodstream, causing sepsis and death in less than 24h. To survive and proliferate in blood (or host serum), the pathogen requires mechanisms to overcome the innate immune defenses and metabolic limitations of this host niche. Initially, genes under-represented for insertions can be used to estimate the V. vulnificus essential gene set. Analysis of the relative abundance of insertion mutants in the library after exposure to serum would detect which genes are essential for the pathogen to overcome the diverse limitations imposed by serum.

Identification of cbiO Gene Critical for Biofilm Formation by MRSA CFSa36 Strain Isolated from Pediatric Patient with Cystic Fibrosis.

The colonization of Staphylococcus aureus, especially methicillin-resistant S. aureus (MRSA), has a detrimental effect on the respiratory care of pediatric patients with cystic fibrosis (CF). In addition to being resistant to multiple antibiotics, S. aureus also has the ability to form biofilms, which makes the infection more difficult to treat and eradicate. In this study, we examined the ability of S. aureus strains isolated from pediatric patients with CF to form biofilms. We screened a transposon mutant library of MRSA and identified a putative cobalt transporter ATP binding domain (cbiO) that is required for biofilm formation. We discovered that deleting cbiO creating a cbiO null mutant in CFSa36 (an MRSA strain isolated from a patient with cystic fibrosis) significantly hinders the ability of CFSa36 to form biofilm. The complementation of cbiO restored the ability of the cbiO deletion mutant to generate biofilm. Interestingly, we revealed that incorporating extra copper ions to the chemically defined medium (CDM) complemented the function of cbiO for biofilm formation in a dose-dependent manner, while the addition of extra iron ions in CDM enhanced the effect of cbiO null mutation on biofilm formation. In addition, neither the addition of certain extra amounts of copper ions nor iron ions in CDM had an impact on bacterial growth. Taken together, our findings suggest that cbiO mediates biofilm formation by affecting the transportation of copper ions in the MRSA CFSa36 strain. This study provides new insights into the molecular basis of biofilm formation by S. aureus.