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Bacillus subtilis and Escherichia coli, as widely used microbial species, are of great significance in studying microbial community relationships, adaptive evolution in various niches, engineering cell factories that produce specific products, and designing genome reduction. The pan-genome analysis is an effective method for studying the characteristics and functions of genes among and within species. Many research directions and conclusions usually depend on accurate gene identification and reliable pan-genome results. However, there currently lack enough studies showing how to achieve high-quality pan-genome results between or within certain species. This chapter will take Bacillus subtilis as an example to introduce a stepwise manner for improving the quality of the pan-genome by gradually removing confounding strains step-by-step, and ultimately obtaining a reliable high-quality pan-genome landscape of Bacillus subtilis, which could be used as a quality control protocol in pan-genome analysis pipeline. Finally, we suggest further improving the pan-genome analysis results of Escherichia coli to prove the feasibility and credibility of the quality control protocol for obtaining high-quality pan-genome landscape.We exploited the yeast DAmP mutant collection to identify essential genes that play a role in polyamine resistance. Herein, we described in details the methodology to obtain these genes. This approach is applicable for screening many nontoxic and toxic drugs.Genetic balancer systems, which allow effective capture and maintenance of lethal mutations stably, play an important role in identifying essential genes. Whole-genome sequencing (WGS) followed by bioinformatics analysis, combined with genetic mapping data analysis, allows for an efficient and economical means of identifying genomic mutations in essential genes. Using this approach, we successfully identified 104 essential genes on ChrI, ChrIII, and ChrV in C. elegans. In this report, we described a protocol that sequences the genome of prebalanced Caenorhabditis elegans (C. elegans) strains to carry lethal mutations and identifies candidate causal mutations and candidate essential genes using a robust bioinformatics procedure.Identification of genes essential for structure, function, and survival of a cell type is critical for understanding of the underlying mechanisms. Unfortunately, there is no efficient way to identify such genes. Studies by single-cell RNA sequencing have shown that gene expressions of single cells of the same type are highly heterogeneous. We therefore speculate that the genes expressed in all individual cells of the same type are essential for the cell type, including the housekeeping genes and cell type-specific essential genes. Based on this rationale, we design a high-throughput approach to identify podocyte essential genes. In this approach, mouse podocytes are subjected to ultra-deep single-cell RNA-seq, and the genes expressed in all single podocytes are sorted out and considered as the candidates of podocyte essential genes. The essentiality of these genes for podocytes is assessed by bioinformatics, cross-species conserved expression, association with injury/disease, inclusion of known essential genes, and experimental validation. By comparison with the essential genes of other cell types, podocyte-specific essential genes can be distinguished. This approach applies to any cell types. In this chapter, we describe the approach and detailed methods.Inducible gene expression systems represent powerful tools for studying essential gene function and for validation of drug targets in bacteria. Even if several regulated promoters have been characterized, only a few of them have been successfully used in Mycobacteria. KN-93 Here we describe a successful mycobacterial gene regulation system based on the presence of two chromosomally encoded repressors Pip and TetR, and a tunable promoter (Pptr) that allows a tight regulation of gene expression.Essential genes are those that are indispensable for the survival of organism under specific growth conditions. Investigating essential genes in pathogenic bacteria not only helps to understand vital biological networks but also provides novel targets for drug development. Availability of genetic engineering tools and high-throughput sequencing methods has enabled essential genes identification in many pathogenic gram-positive and gram-negative bacteria. Bacteroides fragilis is one of the major bacteria specific of human gastrointestinal microbiota. When B. fragilis moves out of its niche, it turns into deadly pathogen. Here, we describe detailed method for the essential gene identification in B. fragilis. Generated transposon mutant pool can be used for other applications such as identification of genes responsible for drug resistance in B. fragilis.Functional genomics of bacteria commonly aims at establishing genotype-phenotype links in microorganisms of industrial, technological and biomedical relevance. In this regard, random transposon mutagenesis coupled to high-throughput next-generation sequencing approaches, termed transposon-insertion sequencing (TIS), has emerged as a robust, genome-wide alternative to perform functional genome analysis. Though these approaches have been used in a large number of studies involving pathogenic and clinically relevant bacteria, they have received little attention in the fields of commensal and potentially beneficial bacteria, including probiotic microorganisms. In this chapter, we describe the implementation of the TIS method Transposon-Directed Insertion Sequencing to describe the set of essential genes in a representative strain of a genus encompassing several commensal and potentially probiotic bacteria and discuss considerations when applying similar methodological approaches to other Bifidobacterium species/strains of interest.A powerful method for examining genetic fitness and function on a large scale is to couple saturating transposon mutagenesis with high-throughput sequencing (TnSeq). By mapping where transposon insertions can be tolerated in a genome, it is possible to analyze the fitness of every gene in a genome simultaneously under a given growth condition. While this technique can describe genes as essential or nonessential under those growth conditions, sufficient mutagenesis and sequencing depth can provide more subtle differences in fitness. In this paper, TnSeq was used to analyze gene fitness of two Alphaproteobacteria from different environments the freshwater oligotroph Brevundimonas subvibrioides (Caulobacterales) and the soil plant pathogen Agrobacterium tumefaciens (Rhizobiales) for the purpose of comparing conservation of gene function.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.Identification of essential genes is key to understanding the required processes and gene products of organisms under one or more conditions. Transposon sequencing (Tn-seq) has been used to predict essential genes or ones that conditionally impact fitness in a wide variety of organisms. Here, we describe the generation of genome-scale mutant libraries and the analysis of Tn-seq data to identify essential genes from cultures grown in a single condition as well as those that are conditionally important by analyzing the behavior of these mutant libraries in different growth environments. While we illustrate the approach using data derived from Tn-seq analysis of the α-proteobacteria Rhodobacter sphaeroides and Zymomonas mobilis, the protocols and systems we describe should be generally applicable to a variety of organisms.Transposon-directed insertion site sequencing (TraDIS) combines random transposon mutagenesis and massively parallel sequencing to shed light on bacterial gene function on a genome-wide scale and in a high-throughput manner. The technique has proven to be successful in the determination of the fitness contribution of every gene under specific conditions both in vitro and in vivo. In this contribution, we describe the procedure used for the identification of Escherichia coli K1 genes essential for in vitro growth, survival in pooled human serum and gastrointestinal colonisation in a rodent model of neonatal invasive infection. TraDIS has broad application for systems-level analysis of a wide range of pathogenic, commensular and saprophytic bacteria.Transposon-insertion sequencing (Tn-Seq) allows for identification of bacterial genes and pathways essential for growth under a given condition. A transposon mutant is created by the stable and random integration of a transposable element into a genome of interest, followed by a period of outgrowth and selection for relative fitness on one or more growth media. By pooling hundreds of thousands of mutants, sequencing the transposon-genomic DNA junctions, and mapping sequencing reads to the genome, one can identify an abundance of reads in nonessential insertion regions and the absence of reads in essential regions and thus identify which genes are essential for a given growth condition. By performing this method iteratively across multiple strains and growth conditions, one can define a core essential genome for a species. Here, we describe this methodology in detail and its application for the species Pseudomonas aeruginosa, from generating mutants to the analysis of nonessential versus essential genes using the freely available software “FiTnEss”.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 24 h. 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.