Growth-rate dependency of ribosome abundance and translation elongation rate in Corynebacterium glutamicum differs from that in Escherichia coli.

Bacterial growth rate (µ) depends on the protein synthesis capacity of the cell and thus on the number of active ribosomes and their translation elongation rate. The relationship between these fundamental growth parameters have only been described for few bacterial species, in particular Escherichia coli. Here, we analyse the growth-rate dependency of ribosome abundance and translation elongation rate for Corynebacterium glutamicum, a gram-positive model species differing from E. coli by a lower growth temperature optimum and a lower maximal growth rate. We show that, unlike in E. coli, there is little change in ribosome abundance for µ <0.4 h-1 in C. glutamicum and the fraction of active ribosomes is kept above 70% while the translation elongation rate declines 5-fold. Mathematical modelling indicates that the decrease in the translation elongation rate can be explained by a depletion of translation precursors.

Metabolic engineering of Corynebacterium glutamicum for production of scyllo-inositol, a drug candidate against Alzheimer's disease.

Scyllo-inositol has been identified as a potential drug for the treatment of Alzheimer's disease. Therefore, cost-efficient processes for the production of this compound are desirable. In this study, we analyzed and engineered Corynebacterium glutamicum with the aim to develop competitive scyllo-inositol producer strains. Initial studies revealed that C. glutamicum naturally produces scyllo-inositol when cultured with myo-inositol as carbon source. The conversion involves NAD+-dependent oxidation of myo-inositol to 2-keto-myo-inositol followed by NADPH-dependent reduction to scyllo-inositol. Use of myo-inositol for biomass formation was prevented by deletion of a cluster of 16 genes involved in myo-inositol catabolism (strain MB001(DE3)Δiol1). Deletion of a second cluster of four genes (oxiC-cg3390-oxiD-oxiE) related to inositol metabolism prevented conversion of 2-keto-myo-inositol to undesired products causing brown coloration (strain MB001(DE3)Δiol1Δiol2). The two chassis strains were used for plasmid-based overproduction of myo-inositol dehydrogenase (IolG) and scyllo-inositol dehydrogenase (IolW). In BHI medium containing glucose and myo-inositol, a complete conversion of the consumed myo-inositol into scyllo-inositol was achieved with the Δiol1Δiol2 strain. To enable scyllo-inositol production from cheap carbon sources, myo-inositol 1-phosphate synthase (Ino1) and myo-inositol 1-phosphatase (ImpA), which convert glucose 6-phosphate into myo-inositol, were overproduced in addition to IolG and IolW using plasmid pSI. Strain MB001(DE3)Δiol1Δiol2 (pSI) produced 1.8 g/L scyllo-inositol from 20 g/L glucose and even 4.4 g/L scyllo-inositol from 20 g/L sucrose within 72 h. Our results demonstrate that C. glutamicum is an attractive host for the biotechnological production of scyllo-inositol and potentially further myo-inositol-derived products.

Global mRNA decay and 23S rRNA fragmentation in Gluconobacter oxydans 621H.

BACKGROUND: Gluconobacter oxydans is a strictly aerobic Gram-negative acetic acid bacterium used industrially for oxidative biotransformations due to its exceptional type of catabolism. It incompletely oxidizes a wide variety of carbohydrates regio- and stereoselectively in the periplasm using membrane-bound dehydrogenases with accumulation of the products in the medium. As a consequence, only a small fraction of the carbon and energy source enters the cell, resulting in a low biomass yield. Additionally, central carbon metabolism is characterized by the absence of a functional glycolysis and absence of a functional tricarboxylic acid (TCA) cycle. Due to these features, G. oxydans is a highly interesting model organism. Here we analyzed global mRNA decay in G. oxydans to describe its characteristic features and to identify short-lived mRNAs representing potential bottlenecks in the metabolism for further growth improvement by metabolic engineering. RESULTS: Using DNA microarrays we estimated the mRNA half-lives in G. oxydans. Overall, the mRNA half-lives ranged mainly from 3 min to 25 min with a global mean of 5.7 min. The transcripts encoding GroES and GroEL required for proper protein folding ranked at the top among transcripts exhibiting both long half-lives and high abundance. The F-type H+-ATP synthase transcripts involved in energy metabolism ranked among the transcripts with the shortest mRNA half-lives. RNAseq analysis revealed low expression levels for genes of the incomplete TCA cycle and also the mRNA half-lives of several of those were short and below the global mean. The mRNA decay analysis also revealed an apparent instability of full-length 23S rRNA. Further analysis of the ribosome-associated rRNA revealed a 23S rRNA fragmentation pattern exhibiting new cleavage regions in 23S rRNAs which were previously not known. CONCLUSIONS: The very short mRNA half-lives of the H+-ATP synthase, which is likely responsible for the ATP-proton motive force interconversion in G. oxydans under many or most conditions, is notably in contrast to mRNA decay data from other bacteria. Together with the short mRNA half-lives and low expression of some other central metabolic genes it could limit intended improvements of G. oxydans' biomass yield by metabolic engineering. Also, further studies are needed to unravel the multistep process of the 23S rRNA fragmentation in G. oxydans.

Adaptation of commensal proliferating Escherichia coli to the intestinal tract of young children with cystic fibrosis.

The mature human gut microbiota is established during the first years of life, and altered intestinal microbiomes have been associated with several human health disorders. Escherichia coli usually represents less than 1% of the human intestinal microbiome, whereas in cystic fibrosis (CF), greater than 50% relative abundance is common and correlates with intestinal inflammation and fecal fat malabsorption. Despite the proliferation of E. coli and other Proteobacteria in conditions involving chronic gastrointestinal tract inflammation, little is known about adaptation of specific characteristics associated with microbiota clonal expansion. We show that E. coli isolated from fecal samples of young children with CF has adapted to growth on glycerol, a major component of fecal fat. E. coli isolates from different CF patients demonstrate an increased growth rate in the presence of glycerol compared with E. coli from healthy controls, and unrelated CF E. coli strains have independently acquired this growth trait. Furthermore, CF and control E. coli isolates have differential gene expression when grown in minimal media with glycerol as the sole carbon source. While CF isolates display a growth-promoting transcriptional profile, control isolates engage stress and stationary-phase programs, which likely results in slower growth rates. Our results indicate that there is selection of unique characteristics within the microbiome of individuals with CF, which could contribute to individual disease outcomes.

Genomic Analysis of Salmonella enterica Serovar Typhimurium Characterizes Strain Diversity for Recent U.S. Salmonellosis Cases and Identifies Mutations Linked to Loss of Fitness under Nitrosative and Oxidative Stress.

UNLABELLED: Salmonella enterica serovar Typhimurium is one of the most common S. enterica serovars associated with U.S. foodborne outbreaks. S. Typhimurium bacteria isolated from humans exhibit wide-ranging virulence phenotypes in inbred mice, leading to speculation that some strains are more virulent in nature. However, it is unclear whether increased virulence in humans is related to organism characteristics or initial treatment failure due to antibiotic resistance. Strain diversity and genetic factors contributing to differential human pathogenicity remain poorly understood. We reconstructed phylogeny, resolved genetic population structure, determined gene content and nucleotide variants, and conducted targeted phenotyping assays for S. Typhimurium strains collected between 1946 and 2012 from humans and animals in the United States and abroad. Strains from recent U.S. salmonellosis cases were associated with five S. Typhimurium lineages distributed within three phylogenetic clades, which are not restricted by geography, year of acquisition, or host. Notably, two U.S. strains and four Mexican strains are more closely related to strains associated with human immunodeficiency virus (HIV)-infected individuals in sub-Saharan Africa than to other North American strains. Phenotyping studies linked variants specific to these strains in hmpA and katE to loss of fitness under nitrosative and oxidative stress, respectively. These results suggest that U.S. salmonellosis is caused by diverse S. Typhimurium strains circulating worldwide. One lineage has mutations in genes affecting fitness related to innate immune system strategies for fighting pathogens and may be adapting to immunocompromised humans by a reduction in virulence capability, possibly due to a lack of selection for its maintenance as a result of the worldwide HIV epidemic. IMPORTANCE: Nontyphoidal Salmonella bacteria cause an estimated 1.2 million illnesses annually in the United States, 80 million globally, due to ingestion of contaminated food or water. Salmonella Typhimurium is one of the most common serovars associated with foodborne illness, causing self-limiting gastroenteritis and, in approximately 5% of infected patients, systemic infection. Although some S. Typhimurium strains are speculated to be more virulent than others, it is unknown how strain diversity and genetic factors contribute to differential human pathogenicity. Ours is the first study to examine the diversity of S. Typhimurium associated with recent cases of U.S. salmonellosis and to provide some initial correlation between observed genotypes and phenotypes. Definition of specific S. Typhimurium lineages based on such phenotype/genotype correlations may identify strains with greater capability of associating with specific food sources, allowing outbreaks to be more quickly identified. Additionally, defining simple correlates of pathogenesis may have predictive value for patient outcome.

HAMP Domain Rotation and Tilting Movements Associated with Signal Transduction in the PhoQ Sensor Kinase.

UNLABELLED: HAMP domains are α-helical coiled coils that often transduce signals from extracytoplasmic sensing domains to cytoplasmic domains. Limited structural information has resulted in hypotheses that specific HAMP helix movement changes downstream enzymatic activity. These hypotheses were tested by mutagenesis and cysteine cross-linking analysis of the PhoQ histidine kinase, essential for resistance to antimicrobial peptides in a variety of enteric pathogens. These results support a mechanistic model in which periplasmic signals which induce an activation state generate a rotational movement accompanied by a tilt in α-helix 1 which activates kinase activity. Biochemical data and a high-confidence model of the PhoQ cytoplasmic domain indicate a possible physical interaction of the HAMP domain with the catalytic domain as necessary for kinase repression. These results support a model of PhoQ activation in which changes in the periplasmic domain lead to conformational movements in the HAMP domain helices which disrupt interaction between the HAMP and the catalytic domains, thus promoting increased kinase activity. IMPORTANCE: Most studies on the HAMP domain signaling states have been performed with chemoreceptors or the HAMP domain of Af1503. Full-length structures of the HAMP-containing histidine kinases VicK and CpxA or a hybrid between the HAMP domain of Af1503 and the EnvZ histidine kinase agree with the parallel four-helix bundle structure identified in Af1503 and provide snapshots of structural conformations experienced by HAMP domains. We took advantage of the fact that we can easily regulate the activation state of PhoQ histidine kinase to study its HAMP domain in the context of the full-length protein in living cells and provide biochemical evidence for different conformational states experienced by Salmonella enterica serovar Typhimurium PhoQ HAMP domain upon signaling.

S. Typhimurium strategies to resist killing by cationic antimicrobial peptides.

S. Typhimurium is a broad host range Gram-negative pathogen that must evade killing by host innate immune systems to colonize, replicate, cause disease, and be transmitted to other hosts. A major pathogenic strategy of Salmonellae is entrance, survival, and replication within eukaryotic cell phagocytic vacuoles. These phagocytic vacuoles and gastrointestinal mucosal surfaces contain multiple cationic antimicrobial peptides (CAMPs) which control invading bacteria. S. Typhimurium possesses several key mechanisms to resist killing by CAMPs which involve sensing CAMPs and membrane damage to activate signaling cascades that result in remodeling of the bacterial envelope to reduce its overall negative charge with an increase in hydrophobicity to decrease binding and effectiveness of CAMPs. Moreover Salmonellae have additional mechanisms to resist killing by CAMPs including an outer membrane protease which targets cationic peptides at the surface, and specific efflux pumps which protect the inner membrane from damage. This article is part of a Special Issue entitled: Bacterial Resistance to Antimicrobial Peptides.

Genomic analysis of the emergence of 20th century epidemic dysentery.

BACKGROUND: Shigella dysenteriae type 1 (Sd1) causes recurrent epidemics of dysentery associated with high mortality in many regions of the world. Sd1 infects humans at very low infectious doses (10 CFU), and treatment is complicated by the rapid emergence of antibiotic resistant Sd1 strains. Sd1 is only detected in the context of human infections, and the circumstances under which epidemics emerge and regress remain unknown. RESULTS: Phylogenomic analyses of 56 isolates collected worldwide over the past 60 years indicate that the Sd1 clone responsible for the recent pandemics emerged at the turn of the 20th century, and that the two world wars likely played a pivotal role for its dissemination. Several lineages remain ubiquitous and their phylogeny indicates several recent intercontinental transfers. Our comparative genomics analysis reveals that isolates responsible for separate outbreaks, though closely related to one another, have independently accumulated antibiotic resistance genes, suggesting that there is little or no selection to retain these genes in-between outbreaks. The genomes appear to be subjected to genetic drift that affects a number of functions currently used by diagnostic tools to identify Sd1, which could lead to the potential failure of such tools. CONCLUSIONS: Taken together, the Sd1 population structure and pattern of evolution suggest a recent emergence and a possible human carrier state that could play an important role in the epidemic pattern of infections of this human-specific pathogen. This analysis highlights the important role of whole-genome sequencing in studying pathogens for which epidemiological or laboratory investigations are particularly challenging.

PhoPQ regulates acidic glycerophospholipid content of the Salmonella Typhimurium outer membrane.

Gram-negative bacteria have two lipid membranes separated by a periplasmic space containing peptidoglycan. The surface bilayer, or outer membrane (OM), provides a barrier to toxic molecules, including host cationic antimicrobial peptides (CAMPs). The OM comprises an outer leaflet of lipid A, the bioactive component of lipopolysaccharide (LPS), and an inner leaflet of glycerophospholipids (GPLs). The structure of lipid A is environmentally regulated in a manner that can promote bacterial infection by increasing bacterial resistance to CAMP and reducing LPS recognition by the innate immune system. The gastrointestinal pathogen, Salmonella Typhimurium, responds to acidic pH and CAMP through the PhoPQ two-component regulatory system, which stimulates lipid A remodeling, CAMP resistance, and intracellular survival within acidified phagosomes. Work here demonstrates that, in addition to regulating lipid A structure, the S. Typhimurium PhoPQ virulence regulators also regulate acidic GPL by increasing the levels of cardiolipins and palmitoylated acylphosphatidylglycerols within the OM. Triacylated palmitoyl-PG species were diminished in strains deleted for the PhoPQ-regulated OM lipid A palmitoyltransferase enzyme, PagP. Purified PagP transferred palmitate to PG consistent with PagP acylation of both lipid A and PG within the OM. Therefore, PhoPQ coordinately regulates OM acidic GPL with lipid A structure, suggesting that GPLs cooperate with lipid A to form an OM barrier critical for CAMP resistance and intracellular survival of S. Typhimurium.

A chemical biology approach to interrogate quorum-sensing regulated behaviors at the molecular and cellular level.

Small molecule probes have been used extensively to explore biologic systems and elucidate cellular signaling pathways. In this study, we use an inhibitor of bacterial communication to monitor changes in the proteome of Salmonella enterica serovar Typhimurium with the aim of discovering unrecognized processes regulated by AI-2-based quorum-sensing (QS), a mechanism of bacterial intercellular communication that allows for the coordination of gene expression in a cell density-dependent manner. In S. typhimurium, this system regulates the uptake and catabolism of intercellular signals and has been implicated in pathogenesis, including the invasion of host epithelial cells. We demonstrate that our QS antagonist is capable of selectively inhibiting the expression of known QS-regulated proteins in S. typhimurium, thus attesting that QS inhibitors may be used to confirm proposed and elucidate previously unidentified QS pathways without relying on genetic manipulation.

Clostridium difficile toxin expression is inhibited by the novel regulator TcdC.

Clostridium difficile, an emerging nosocomial pathogen of increasing clinical significance, produces two large protein toxins that are responsible for the cellular damage associated with the disease. The precise mechanisms by which toxin synthesis is regulated in response to environmental change have yet to be discovered. The toxin genes (tcdA and tcdB) are located in a pathogenicity locus (PaLoc), along with tcdR and tcdC. TcdR is an alternative RNA polymerase sigma factor that directly activates toxin gene expression, while the inverse relationship between expression of tcdR, tcdA and tcdB genes on the one hand and tcdC on the other has led to the suggestion that TcdC somehow interferes with toxin gene expression. This idea is further supported by the finding that many recent C. difficile epidemic strains in which toxin production is increased carry a common tcdC deletion mutation. In this report we demonstrate that TcdC negatively regulates toxin synthesis both in vivo and in vitro. TcdC destabilizes the TcdR-containing holoenzyme before open complex formation, apparently by interaction with TcdR or TcdR-containing RNA polymerase holoenzyme or both. In addition, we show that the hypertoxigenicity phenotype of C. difficile epidemic strains is not due to their common 18 bp in-frame deletion in tcdC.

Regulation of toxin and bacteriocin gene expression in Clostridium by interchangeable RNA polymerase sigma factors.

The production of major extracellular toxins by pathogenic strains of Clostridium botulinum, Clostridium tetani and Clostridium difficile, and a bacteriocin by Clostridium perfringens is dependent on a related group of RNA polymerase sigma-factors. These sigma-factors (BotR, TetR, TcdR and UviA) were shown to be sufficiently similar that they could substitute for one another in in vitro DNA binding and run-off transcription experiments. In cells, however, the sigma-factors fell into two subclasses. BotR and TetR were able to direct transcription of their target genes in a fully reciprocal manner. Similarly, UviA and TcdR were fully interchangeable. Neither BotR nor TetR could substitute for UviA or TcdR, however, and neither UviA nor TcdR could direct transcription of the natural targets of BotR or TetR. The extent of functional interchangeability of the sigma-factors was attributed to the strong conservation of their subregion 4.2 sequences and the conserved -35 sequences of their target promoters, while restrictions on interchangeability were attributed to variations in their subregion 2.4 sequences and the target site -10 sequences. The four sigma-factors have been assigned to group 5 of the sigma(70) family and seem to have arisen from a common ancestral protein that may have co-evolved with the genes whose transcription they direct. A fifth Clostridiumsigma-factor, sigma(Y) of Clostridium acetobutylicum, resembles the TcdR family, but was not functionally interchangeable with members of this family.

Regulation of toxin and bacteriocin synthesis in Clostridium species by a new subgroup of RNA polymerase sigma-factors.

Many Clostridium species are pathogenic for humans and animals, and most of the resulting diseases, such as tetanus, botulism, gas gangrene and pseudomembranous colitis, are due to the production of potent extracellular toxins. The biochemical mechanisms of action of Clostridium toxins have been extensively studied in the past ten years. However, detailed information about the regulation of toxin gene expression has only recently emerged. TcdR, BotR, TetR and UviA are now known to be related alternative RNA polymerase sigma factors that drive transcription of toxin A and toxin B genes in C. difficile, the neurotoxin genes in C. botulinum and C. tetani, and a bacteriocin gene in C. perfringens. Although the Clostridium sigma factors have some similarity to members of the ECF sigma factor group, they differ sufficiently in structure and function so that they have been assigned to a new group within the sigma(70)-family.