Isolation of a genetically accessible thermophilic xylan degrading bacterium from compost

Whole-genome sequence of Geobacillus thermodenitrificans K1041, a genetically tractable strain representative of the genus Geobacillus

We present the whole-genome sequence of Geobacillus thermodenitrificans K1041, a bacterium that was originally identified as a genetically tractable thermophile. The genome consists of a circular chromosome that contains 3,848 genes. The sequence has a total size of 3,755,826 bp with a GC content of 49.18%.

New Platform for Screening Genetic Libraries at Elevated Temperatures: Biological and Genomic Information and Genetic Tools of Geobacillus thermodenitrificans K1041

Geobacillus thermodenitrificans K1041 is an unusual thermophile that is highly transformable via electroporation, making it a promising host for screening genetic libraries at elevated temperatures. In this study, we determined its biological properties, draft genome sequence, and effective vectors and also optimized the electroporation procedures in an effort to enhance its utilization. The organism exhibited swarming motility but not detectable endospore formation, and growth was rapid at 60°C under neutral and relatively low-salt conditions. Although the cells showed negligible acceptance of shuttle plasmids from general strains of Escherichia coli, methylation-controlled plasmids from dam mutant strains were efficiently accepted, suggesting circumvention of a restriction-modification system in G. thermodenitrificans K1041. We optimized the electroporation procedure to achieve efficiencies of 10³ to 10⁵ CFU/μg for five types of plasmids, which exhibited the different copy numbers and segregational stabilities in G. thermodenitrificans K1041. Some sets of plasmids were compatible. Moreover, we observed substantial plasmid-directed production of heterologous proteins in the intracellular or extracellular environments. Our successful construction of a library of promoter mutants using K1041 cells as hosts and subsequent screening at elevated temperatures to identify improved promoters revealed that G. thermodenitrificans K1041 was practical as a library host. The draft genomic sequence of the organism contained 3,384 coding genes, including resA and mcrB genes, which are involved in restriction-modification systems. Further examination revealed that in-frame deletions of resA increased transformation efficiencies, but mcrB deletion had no effect. The ΔresA mutant exhibited transformation efficiencies of >10⁵ CFU/μg for some plasmids. IMPORTANCE Geobacillus thermodenitrificans K1041 has yet to be fully characterized. Although it is transformable via electroporation, it rarely accepts Escherichia coli-derived plasmids. This study clarified the biological and genomic properties of G. thermodenitrificans K1041. Additionally, we developed an electroporation procedure resulting in efficient acceptance of E. coli-derived plasmids. This procedure produced transformants using small amounts of plasmids immediately after the ligation reaction. Thus, G. thermodenitrificans K1041 was identified as a host for screening promoter mutants at elevated temperatures. Furthermore, because this strain efficiently produced heterologous proteins, it could serve as a host for screening thermostable proteins encoded in random mutant libraries or metagenomes. We also generated a ΔresA mutant that exhibited transformation efficiencies of >10⁵ CFU/μg, which were highest in cases of electroporation-based transformation of Geobacillus spp. with E. coli-derived plasmids. Our findings provide a new platform for screening diverse genetic libraries at elevated temperatures.

CRISPR/Cas genome editing systems in thermophiles: Current status, associated challenges, and future perspectives

Thermophiles, offering an attractive and unique platform for a broad range of applications in biofuels and environment protections, have received a significant attention and growing interest from academy and industry. However, the exploration and exploitation of thermophilic organisms have been hampered by the lack of a powerful genome manipulation tool to improve production efficiency. At current, the clustered Regularly Interspaced Short Palindromic Repeat (CRISPR)/CRISPR associated (Cas) system has been successfully exploited as a competent, simplistic, and powerful tool for genome engineering both in eukaryotes and prokaryotes. Indeed, with the significant efforts made in recent years, some thermostable Cas9 proteins have been well identified and characterized and further, some thermostable Cas9-based editing tools have been successfully established in some representative obligate thermophiles. In this regard, we reviewed the current status and its progress in CRISPR/Cas-based genome editing system towards a variety of thermophilic organisms. Despite the potentials of these progresses, multiple factors/barriers still have to be overcome and optimized for improving its editing efficiency in thermophiles. Some insights into the roles of thermostable CRISPR/Cas technologies for the metabolic engineering of thermophiles as a thermophilic microbial cell factory were also fully analyzed and discussed.

A systematic review of the genera Geobacillus and Parageobacillus: their evolution, current taxonomic status and major applications

The genus Geobacillus, belonging to the phylum Firmicutes, is one of the most important genera and comprises thermophilic bacteria. The genus Geobacillus was erected with the taxonomic reclassification of various Bacillus species. Taxonomic studies of Geobacillus remain in progress. However, there is no comprehensive review of the characteristic features, taxonomic status and study of various applications of this interesting genus. The main aim of this review is to give a comprehensive account of the genus Geobacillus. At present the genus acomprises 25 taxa, 14 validly published (with correct name), nine validly published (with synonyms) and two not validly published species. We describe only validly published species of the genera Geobacillus and Parageobacillus. Vegetative cells of Geobacillus species are Gram-strain-positive or -variable, rod-shaped, motile, endospore-forming, aerobic or facultatively anaerobic, obligately thermophilic and chemo-organotrophic. Growth occurs in the pH range 6.08.5 and a temperature of 37-75 °C. The major cellular fatty acids are iso-C15:o, iso-C16:0 and iso-C17:o. The main menaquinone type is MK-7. The G-+C content of the DNA ranges between 48.2 and 58 mol%. The genus Geobacillus is widely distributed in nature, being mostly found in many extreme locations such as hot springs, hydrothermal vents, marine trenches, hay composts, etc. Geobacillus species have been widely exploited in various industrial and biotechnological applications, and thus are promising candidates for further studies in the future.

A plasmid vector that directs hyperproduction of recombinant proteins in the thermophiles Geobacillus species

Geobacillus spp. are moderate thermophiles that have great potential for use in diverse applications. For effective utilization of the species, genetic tools have been extensively studied; however, an overexpression vector remains to be developed. Here we constructed a plasmid vector that can shuttle between Escherichia coli and Geobacillus spp., and which contained a maltose-inducible promoter from Geobacillus kaustophilus HTA426. Although the vector (termed pGKE119) was originally designed for basal gene expression, it surprisingly directed robust protein production in G. kaustophilus. Protein production essentially occurred in an auto-inducible manner without maltose; however, some proteins were produced more efficiently in the presence of maltose. Although the productivity was affected by culture conditions, three proteins were successfully produced with abundance ratios of 12-27% (on a total protein basis) and yields of 77-170 mg (per L culture). pGKE119 directed substantial protein production even in Geobacillus subterraneus, Geobacillus thermoglucosidasius, and Geobacillus thermoleovorans. This suggests that pGKE119 can use a range of Geobacillus spp. as hosts and widely expand their genetic toolbox. Because Geobacillus spp. are highly proliferative bacteria that are distinct from organisms used as protein production hosts, pGKE119 may also provide a novel platform for hyperproduction of recombinant proteins.

Engineering Geobacillus thermodenitrificans to introduce cellulolytic activity; expression of native and heterologous cellulase genes

Background

Consolidated bioprocessing (CBP) is a cost-effective approach for the conversion of lignocellulosic biomass to biofuels and biochemicals. The enzymatic conversion of cellulose to glucose requires the synergistic action of three types of enzymes: exoglucanases, endoglucanases and β-glucosidases. The thermophilic, hemicellulolytic Geobacillus thermodenitrificans T12 was shown to harbor desired features for CBP, although it lacks the desired endo and exoglucanases required for the conversion of cellulose. Here, we report the expression of both endoglucanase and exoglucanase encoding genes by G. thermodenitrificans T12, in an initial attempt to express cellulolytic enzymes that complement the enzymatic machinery of this strain.

Results

A metagenome screen was performed on 73 G. thermodenitrificans strains using HMM profiles of all known CAZy families that contain endo and/or exoglucanases. Two putative endoglucanases, GE39 and GE40, belonging to glucoside hydrolase family 5 (GH5) were isolated and expressed in both E. coli and G. thermodenitrificans T12. Structure modeling of GE39 revealed a folding similar to a GH5 exo-1,3-β-glucanase from S. cerevisiae. However, we determined GE39 to be a β-xylosidase having pronounced activity towards p-nitrophenyl-β-d-xylopyranoside. Structure modelling of GE40 revealed its protein architecture to be similar to a GH5 endoglucanase from B. halodurans, and its endoglucanase activity was confirmed by enzymatic activity against 2-hydroxyethylcellulose, carboxymethylcellulose and barley β-glucan. Additionally, we introduced expression constructs into T12 containing Geobacillus sp. 70PC53 endoglucanase gene celA and both endoglucanase genes (M1 and M2) from Geobacillus sp. WSUCF1. Finally, we introduced expression constructs into T12 containing the C. thermocellum exoglucanases celK and celS genes and the endoglucanase celC gene.

Conclusions

We identified a novel G. thermodenitrificans β-xylosidase (GE39) and a novel endoglucanase (GE40) using a metagenome screen based on multiple HMM profiles. We successfully expressed both genes in E. coli and functionally expressed the GE40 endoglucanase in G. thermodenitrificans T12. Additionally, the heterologous production of active CelK, a C. thermocellum derived exoglucanase, and CelA, a Geobacillus derived endoglucanase, was demonstrated with strain T12. The native hemicellulolytic activity and the heterologous cellulolytic activity described in this research provide a good basis for the further development of G. thermodenitrificans T12 as a host for consolidated bioprocessing.

Electronic supplementary material

The online version of this article (10.1186/s12896-018-0453-y) contains supplementary material, which is available to authorized users.

Talaromyces borbonicus, sp. nov., a novel fungus from biodegraded Arundo donax with potential abilities in lignocellulose conversion

A novel fungal species able to synthesize enzymes with potential synergistic actions in lignocellulose conversion was isolated from the biomass of Arundo donax during biodegradation under natural conditions in the Gussone Park of the Royal Palace of Portici (Naples, Italy). In this work, this species was subjected to morphological and phylogenetic analyses. Sequencing of its genome was performed, resulting in 28 scaffolds that were assembled into 27.05 Mb containing 9744 predicted genes, among which 396 belong to carbohydrate-active enzyme (CAZyme)-encoding genes. Here we describe and illustrate this previously unknown species, which was named Talaromyces borbonicus, by a polyphasic approach combining phenotypic, physiological, and sequence data.

Genetic Tools and Techniques for Recombinant Expression in Thermophilic Bacillaceae

Although Escherichia coli and Bacillus subtilis are the most prominent bacterial hosts for recombinant protein production by far, additional species are being explored as alternatives for production of difficult-to-express proteins. In particular, for thermostable proteins, there is a need for hosts able to properly synthesize, fold, and excrete these in high yields, and thermophilic Bacillaceae represent one potentially interesting group of microorganisms for such purposes. A number of thermophilic Bacillaceae including B.methanolicus, B.coagulans, B.smithii, B.licheniformis, Geobacillus thermoglucosidasius, G. kaustophilus, and G. stearothermophilus are investigated concerning physiology, genomics, genetic tools, and technologies, altogether paving the way for their utilization as hosts for recombinant production of thermostable and other difficult-to-express proteins. Moreover, recent successful deployments of CRISPR/Cas9 in several of these species have accelerated the progress in their metabolic engineering, which should increase their attractiveness for future industrial-scale production of proteins. This review describes the biology of thermophilic Bacillaceae and in particular focuses on genetic tools and methods enabling use of these organisms as hosts for recombinant protein production.

In vivo selection of sfGFP variants with improved and reliable functionality in industrially important thermophilic bacteria

BACKGROUND

Fluorescent reporter proteins (FP) have become an indispensable tool for the optimization of microbial cell factories and in synthetic biology per se. The applicability of the currently available FPs is, however, constrained by species-dependent performance and misfolding at elevated temperatures. To obtain functional reporters for thermophilic, biotechnologically important bacteria such as Parageobacillus thermoglucosidasius, an in vivo screening approach based on a mutational library of superfolder GFP was applied.

RESULTS

Flow cytometry-based benchmarking of a set of GFPs, sfGFPs and species-specific codon-optimized variants revealed that none of the proteins was satisfyingly detectable in P. thermoglucosidasius at its optimal growth temperature of 60 °C. An undirected mutagenesis approach coupled to fluorescence-activated cell sorting allowed the isolation of sfGFP variants that were extremely well expressed in the chassis background at 60 °C. Notably, a few nucleotide substitutions, including silent mutations, significantly improved the functionality and brightness. The best mutant sfGFP(N39D/A179A) showed an 885-fold enhanced mean fluorescence intensity (MFI) at 60 °C and is the most reliable reporter protein with respect to cell-to-cell variation and signal intensity reported so far. The in vitro spectral and thermostability properties were unaltered as compared to the parental sfGFP protein, strongly indicating that the combination of the amino acid exchange and an altered translation or folding speed, or protection from degradation, contribute to the strongly improved in vivo performance. Furthermore, sfGFP(N39D/A179A) and the newly developed cyan and yellow derivatives were successfully used for labeling several industrially relevant thermophilic bacilli, thus proving their broad applicability.

CONCLUSIONS

This study illustrates the power of in vivo isolation of thermostable proteins to obtain reporters for highly efficient fluorescence labeling. Successful expression in a variety of thermophilic bacteria proved that the novel FPs are highly suitable for imaging and flow cytometry-based studies. This enables a reliable cell tracking and single-cell-based real-time monitoring of biological processes that are of industrial and biotechnological interest.

Characterizing a thermostable Cas9 for bacterial genome editing and silencing

CRISPR-Cas9-based genome engineering tools have revolutionized fundamental research and biotechnological exploitation of both eukaryotes and prokaryotes. However, the mesophilic nature of the established Cas9 systems does not allow for applications that require enhanced stability, including engineering at elevated temperatures. Here we identify and characterize ThermoCas9 from the thermophilic bacterium Geobacillus thermodenitrificans T12. We show that in vitro ThermoCas9 is active between 20 and 70 °C, has stringent PAM-preference at lower temperatures, tolerates fewer spacer-protospacer mismatches than SpCas9 and its activity at elevated temperatures depends on the sgRNA-structure. We develop ThermoCas9-based engineering tools for gene deletion and transcriptional silencing at 55 °C in Bacillus smithii and for gene deletion at 37 °C in Pseudomonas putida. Altogether, our findings provide fundamental insights into a thermophilic CRISPR-Cas family member and establish a Cas9-based bacterial genome editing and silencing tool with a broad temperature range.

Complete Genome Sequence of Geobacillus thermodenitrificans T12, A Potential Host for Biotechnological Applications

In attempt to obtain a thermophilic host for the conversion of lignocellulose derived substrates into lactic acid, Geobacillus thermodenitrificans T12 was isolated from a compost heap. It was selected from over 500 isolates as a genetically tractable hemicellulolytic lactic acid producer, requiring little nutrients. The strain is able to ferment glucose and xylose simultaneously and can produce lactic acid from xylan, making it a potential host for biotechnological applications. The genome of strain T12 consists of a 3.64 Mb chromosome and two plasmids of 59 and 56 kb. It has a total of 3.676 genes with an average genomic GC content of 48.7%. The T12 genome encodes a denitrification pathway, allowing for anaerobic respiration. The identity and localization of the responsible genes are similar to those of the denitrification pathways found in strain NG80-2. The hemicellulose utilization (HUS) locus was identified based on sequence homology against G. stearothermophilus T-6. It appeared that T12 has all the genes that are present in strain T-6 except for the arabinan degradation cluster. Instead, the HUS locus of strain T12 contains genes for both an inositol and a pectate degradation pathway. Strain T12 has complete pathways for the synthesis of purine and pyrimidine, all 20 amino acids and several vitamins except D-biotin. The host-defense systems present comprise a Type II and a Type III restriction-modification system, as well as a CRISPR-Cas Type II system. It is concluded that G. thermodenitrificans T12 is a potentially interesting candidate for industrial applications.

Valorization of untreated rice bran towards bioflocculant using a lignocellulose-degrading strain and its use in microalgal biomass harvest

BACKGROUND

Microalgae are currently considered as a promising feedstock for the production of biofuels and high-value products. However, the efficient harvest of microalgal biomasses from their culture broth is a major challenge. The harvesting of algal biomass by flocculation combined with gravity sedimentation is more convenient and cost-effective than traditional methods such as centrifugation and filtration. Compared to inorganic and chemically synthetic flocculants, bioflocculants are a suitable choice for microalgal harvest due to their biodegradable and nontoxic properties. Nonetheless, the high production costs associated with expensive substrates hinder the commercial applications of bioflocculants. Previous studies have shown that the hydrolysates of lignocellulosic biomasses from dilute acid hydrolysis can be utilized as an inexpensive carbon source for the production of bioflocculants. However, the toxic by-products generated in the dilute acid hydrolysis step limit the efficiency of subsequent fermentation. The strains that produce bioflocculants by using untreated lignocellulosic materials can circumvent the pretreatment process, as well as promote the application of bioflocculants in microalgal harvest.

RESULTS

Under alkaline fermentation conditions, the alkaliphilic strain Bacillus agaradhaerens C9 secreted 1.69 IU/mL of alkali-tolerant xylanase and 0.06 IU/mL of cellulase, indicating that this particular strain can efficiently convert untreated rice bran into bioflocculant (RBBF-C9), thereby circumventing rice bran pretreatment for downstream fermentation. The optimal fermentation conditions that result in the highest bioflocculant yield (12.94 g/L) were as follows: 20 g/L of untreated rice bran, 3 g/L of yeast extract, and 20 g/L of Na2CO3 at 37 °C for 24 h. RBBF-C9 contained 74.12% polysaccharides and 4.51% proteins, and was estimated to be 137 kDa. Furthermore, the bioflocculant RBBF-C9 exhibited good flocculating efficiency (91.05%) of oil alga Chlorella minutissima UTEX2341 when 60 mg/L of RBBF-C9 was added into the algal culture broth.

CONCLUSIONS

This study demonstrated that untreated rice bran is a suitable inexpensive substrate for the production of bioflocculants, and thus provides a novel approach in utilizing rice bran. The extracted bioflocculants may be potentially used in biomass harvesting of the oil algae C. minutissima UTEX2341 from the culture broth.

Lactobacilli and pediococci as versatile cell factories - Evaluation of strain properties and genetic tools

This review discusses opportunities and bottlenecks for cell factory development of Lactic Acid Bacteria (LAB), with an emphasis on lactobacilli and pediococci, their metabolism and genetic tools. In order to enable economically feasible bio-based production of chemicals and fuels in a biorefinery, the choice of product, substrate and production organism is important. Currently, the most frequently used production hosts include Escherichia coli and Saccharomyces cerevisiae, but promising examples are available of alternative hosts such as LAB. Particularly lactobacilli and pediococci can offer benefits such as thermotolerance, an extended substrate range and increased tolerance to stresses such as low pH or high alcohol concentrations. This review will evaluate the properties and metabolism of these organisms, and provide an overview of their current biotechnological applications and metabolic engineering. We substantiate the review by including experimental results from screening various lactobacilli and pediococci for transformability, growth temperature range and ability to grow under biotechnologically relevant stress conditions. Since availability of efficient genetic engineering tools is a crucial prerequisite for industrial strain development, genetic tool development is extensively discussed. A range of genetic tools exist for Lactococcus lactis, but for other species of LAB like lactobacilli and pediococci such tools are less well developed. Whereas lactobacilli and pediococci have a long history of use in food and beverage fermentation, their use as platform organisms for production purposes is rather new. By harnessing their properties such as thermotolerance and stress resistance, and by using emerging high-throughput genetic tools, these organisms are very promising as versatile cell factories for biorefinery applications.