The understanding of why a gene is essential for a bacterium has implications in different research areas, such as bacterial evolution, synthetic biology and biotechnology, making the identification ...of essential genes a very active research field. Bacterial essential genes have been defined, among other criteria, by the inability to obtain viable mutants in such genes. In the case of Escherichia coli this approach led to the construction of the Keio collection of single-gene knockout mutants that contains deletions of all the open reading frames present in the genome with the exception of 303 genes that were found to be essential for the growth of this bacterium. One of the genes that was identified as essential is bcsB, which is involved in synthesis of extracellular cellulose. However, the reason for the essential nature of BcsB for E. coli viability has not been determined. In this work we show that bcsB is essential only in strains that have a functional capacity to synthesize cellulose, presumably due to the activity of BcsB in the translocation of this polymer across the periplasm. Thus, we propose that bcsB is a conditionally essential gene in E. coli.
Pseudomonas aeruginosa is a widespread environmental bacterium and an opportunistic pathogen that represents a health hazard due to its production of virulence factors and its high antibiotic ...resistance. The genome of most of the strains belonging to this bacterial species is highly conserved, and genes coding for virulence-associated traits are part of the species core-genome. Recently, the existence of phylogroups has been documented based on the analysis of whole genome sequences of hundreds of isolates. These clades contain both clinical and environmental strains, which show no particular geographical distribution. The major phylogroups (clades 1 and 2) are characterized by the nearly mutually exclusive production of the virulence effectors secreted by the type three secretion system (T3SS) ExoS and ExoU, respectively. Clade 3 is the most genetically diverse and shares with clade 5, which is closely related to clades 1 and 2, the production of the pore-forming exolysin A, and the lack of T3SS, among other characteristics. Here we analyze the 4955 P. aeruginosa genomes deposited in the Pseudomonas Genome Database and present some hypotheses on the origins of four of the five phylogroups of this bacterial species.
Biosurfactants, tensio-active compounds produced by living cells, are now gaining increasing interest due to their potential applications in many different industrial areas in which to date almost ...exclusively synthetic surfactants have been used. Their unique structures and characteristics are just starting to be appreciated. In addition, biosurfactants are considered to be environmentally 'friendly,' relatively non-toxic and biodegradable. This Microbiology Monographs volume deals with the most recent advances in the field of microbial biosurfactants, such as rhamnolipids, serrawettins, trehalolipids, mannosylerythritol lipids, sophorolipids, surfactin and other lipopeptides. Each chapter reviews the characteristics of an individual biosurfactant including the physicochemical properties, the chemical structures, the role in the physiology of the producing microbes, the biosynthetic pathways, the genetic regulation, and the potential biotechnological applications.
Bacterial genomes are mosaics with fragments showing distinct phylogenetic origins or even being unrelated to any other genetic information (ORFan genes). Thus the analysis of bacterial population ...genetics is in large part a collection of explanations for anomalies in relation to classical population genetic models such as the Wright-Fisher model and the Kingman coalescent that do not adequately describe bacterial population genetics, genomics or evolution. The concept of “species” as an evolutionary coherent biological group that is genetically isolated and shares genetic information through recombination among its members cannot be applied to any bacterial group. Recently, a simple probabilistic model considering the role of strong seed-bank effects in population genetics has been proposed by Blath et al. This model suggests the existence of a genetic pool with high diversity that is not subject to classical selection and extinction. We reason that certain bacterial population genetics anomalies could be explained by the prevalence of strong seed-bank effects among bacteria. To address this possibility we analyzed the genome of the bacterium Azotobacter vinelandii and show that genes that code for functions that are essential for the bacterium biology do not have a relation of ancestry with closely related bacteria, or are ORFan genes. The existence of essential genes that are not inherited from the most recent ancestor cannot be explained by classical population genetics models and is irreconcilable with the current view of genes acquired by horizontal transfer as being accessory or adaptive.
•We propose that strong seed-bank effects shape the evolution of bacteria.•The proposal is based on novel population genetics models.•Models predict that a non-selectable and non-extinguishable genetic pool exists.•Essential bacterial genes are inherited from the ancient genetic pool.•Azotobacter vinelandii genome analysis sustains our theoretical predictions.
Serratia marcescens SmUNAM836 is a multidrug-resistant clinical strain isolated in Mexico City from a patient with chronic obstructive pulmonary disease. Its complete genome sequence was determined ...using PacBio RS II SMRT technology, consisting of a 5.2-Mb chromosome and a 26.3-kb plasmid, encoding multiple resistance determinants and virulence factors.
Pseudomonas aeruginosa is an opportunistic pathogen capable of producing a wide variety of virulence factors, including extracellular rhamnolipids and lipopolysaccharide. Rhamnolipids are ...tenso‐active glycolipids containing one (mono‐rhamnolipid) or two (di‐rhamnolipid) l‐rhamnose molecules. Rhamnosyltransferase 1 (RhlAB) catalyses the synthesis of mono‐rhamnolipid from dTDP‐l‐rhamnose and β‐hydroxydecanoyl‐β‐hydroxydecanoate, whereas di‐rhamnolipid is produced from mono‐rhamnolipid and dTDP‐l‐rhamnose. We report here the molecular characterization of rhlC, a gene encoding the rhamnosyltransferase involved in di‐rhamnolipid (l‐rhamnose‐l‐rhamnose‐β‐hydroxydecanoyl‐β‐hydroxydecanoate) production in P. aeruginosa. RhlC is a protein consisting of 325 amino acids with a molecular mass of 35.9 kDa. It contains consensus motifs that are found in other glycosyltransferases involved in the transfer of l‐rhamnose to nascent polymer chains. To verify the biological function of RhlC, a chromosomal mutant, RTII‐2, was generated by insertional mutagenesis and allelic replacement. This mutant was unable to produce di‐rhamnolipid, whereas mono‐rhamnolipid was unaffected. In contrast, a null rhlA mutant (PAO1‐rhlA) was incapable of producing both mono‐ and di‐rhamnolipid. Complementation of mutant RTII‐2 with plasmid pRTII‐26 containing rhlC restored the level of di‐rhamnolipid production in the recombinant to a level similar to that of the wild‐type strain PAO1. The rhlC gene was located in an operon with an upstream gene (PA1131) of unknown function. A σ54‐type promoter for the PA1131–rhlC operon was identified, and a single transcriptional start site was mapped. Expression of the PA1131–rhlC operon was dependent on the P. aeruginosa rhl quorum‐sensing system, and a well‐conserved lux box was identified in the promoter region. The genetic regulation of rhlC by RpoN and RhlR was in agreement with the observed increasing RhlC rhamnosyltransferase activity during the stationary phase of growth. This is the first report of a rhamnosyltransferase gene responsible for the biosynthesis of di‐rhamnolipid.
Pseudomonas aeruginosa produces the biosurfactants rhamnolipids and 3-(3-hydroxyalkanoyloxy)alkanoic acids (HAAs). In this study, we report the production of one family of rhamnolipids, specifically ...the monorhamnolipids, and of HAAs in a recombinant Escherichia coli strain expressing P. aeruginosa rhlAB operon. We found that the availability in E. coli of dTDP-l-rhamnose, a substrate of RhlB, restricts the production of monorhamnolipids in E. coli. We present evidence showing that HAAs and the fatty acid dimer moiety of rhamnolipids are the product of RhlA enzymatic activity. Furthermore, we found that in the recombinant E. coli, these compounds have the same chain length of the fatty acid dimer moiety as those produced by P. aeruginosa. These data suggest that it is RhlAB specificity, and not the hydroxyfatty acid relative abundance in the bacterium, that determines the profile of the fatty acid moiety of rhamnolipids and HAAs. The rhamnolipids level produced in recombinant E. coli expressing rhlAB is lower than the P. aeruginosa level and much higher than those reported by others in E. coli, showing that this metabolic engineering strategy lead to an increased rhamnolipids production in this heterologous host.
Azotobacter vinelandii produces the exopolysaccharide alginate, which is essential for its differentiation to desiccation-resistant cysts. In different bacterial species, the alternative sigma factor ...sigma(E) regulates the expression of functions related to the extracytoplasmic compartments. In A. vinelandii and Pseudomonas aeruginosa, the sigma(E) factor (AlgU) is essential for alginate production. In both bacteria, the activity of this sigma factor is regulated by the product of the mucA, mucB, mucC, and mucD genes. In this work, we studied the transcriptional regulation of the A. vinelandii algU-mucABCD gene cluster, as well as the role of the mucA and mucC gene products in alginate production. Our results show the existence of AlgU autoregulation and show that both MucA and MucC play a negative role in alginate production.
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