Mutations in the listerial proB gene leading to proline overproduction: effects on salt tolerance and murine infection

Modular reconstruction and optimization of the trans-4-hydroxy-L-proline synthesis pathway in Escherichia coli

Background

In recent years, there has been a growing demand for microbial production of trans-4-hydroxy-L-proline (t4Hyp), which is a value-added amino acid and has been widely used in the fields of medicine, food, and cosmetics. In this study, a multivariate modular metabolic engineering approach was used to remove the bottleneck in the synthesis pathway of t4Hyp.

Results

Escherichia coli t4Hyp synthesis was performed using two modules: a α-ketoglutarate (α-KG) synthesis module (K module) and L-proline synthesis with hydroxylation module (H module). First, α-KG attrition was reduced, and then, L-proline consumption was inhibited. Subsequently, to improve the contribution to proline synthesis with hydroxylation, optimization of gene overexpression, promotor, copy number, and the fusion system was performed. Finally, optimization of the H and K modules was performed in combination to balance metabolic flow. Using the final module H1K4 in a shaking flask culture, 8.80 g/L t4Hyp was produced, which was threefold higher than that produced by the W0 strain.

Conclusions

These strategies demonstrate that a microbial cell factory can be systematically optimized by modular engineering for efficient production of t4Hyp.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01884-4.

Improved the lipopeptide production of Bacillus amyloliquefaciens HM618 under co-culture with the recombinant Corynebacterium glutamicum producing high-level proline

The application of antibacterial lipopeptides is limited by high cost and low yield. Herein, the exogenous L-proline significantly improved lipopeptide production by Bacillus amyloliquefaciens HM618. A recombinant Corynebacterium glutamicum producing high levels of proline using genetically modifying proB and putA was used to establish consortium, to improve lipopeptide production of strain HM618. Compared to a pure culture, the levels of iturin A, fengycin, and surfactin in consortium reached 67.75, 39.32, and 37.25 mg L^-1, respectively, an increase of 3.19-, 2.05-, and 1.63-fold over that produced by co-cultures of B. amyloliquefaciens and recombinant C. glutamicum with normal medium. Commercial amylase and recombinant Pichia pastoris with a heterologous amylase gene were used to hydrolyze kitchen waste. A three-strain consortium with recombinant P. pastoris and C. glutamicum increased the lipopeptide production of strain HM618 in medium containing KW. This work provides new strategies to improve lipopeptide production by B. amyloliquefaciens.

Production of l-glutamate family amino acids in Corynebacterium glutamicum: Physiological mechanism, genetic modulation, and prospects

l-glutamate family amino acids (GFAAs), consisting of l-glutamate, l-arginine, l-citrulline, l-ornithine, l-proline, l-hydroxyproline, γ-aminobutyric acid, and 5-aminolevulinic acid, are widely applied in the food, pharmaceutical, cosmetic, and animal feed industries, accounting for billions of dollars of market activity. These GFAAs have many functions, including being protein constituents, maintaining the urea cycle, and providing precursors for the biosynthesis of pharmaceuticals. Currently, the production of GFAAs mainly depends on microbial fermentation using Corynebacterium glutamicum (including its related subspecies Corynebacterium crenatum), which is substantially engineered through multistep metabolic engineering strategies. This review systematically summarizes recent advances in the metabolic pathways, regulatory mechanisms, and metabolic engineering strategies for GFAA accumulation in C. glutamicum and C. crenatum, which provides insights into the recent progress in l-glutamate-derived chemical production.

De Novo Engineering of Corynebacterium glutamicum for l-Proline Production

l-Proline is an important amino acid that has various industrial applications. Industrial l-proline-producing strains are obtained by the mutagenesis of Corynebacterium glutamicum. In this study, the optimized C. glutamicum genome-editing tools were further applied in the de novo construction of a hyper-l-proline-producing strain. Overexpression of a feedback inhibition-resistant γ-glutamic kinase mutant ProBG149K, deletion of a proline dehydrogenase to block l-proline degradation, overexpression of glutamate dehydrogenase to increase glutamate synthesis flux, the mutation of 6-phosphate gluconate dehydrogenase and glucose-6-phosphate-dehydrogenase in the pentose phosphate pathway to enhance NADPH supply, the deletion of pyruvate aminotransferase to decrease the byproduct l-alanine synthesis, and weakening of α-ketoglutarate dehydrogenase to regulate the TCA cycle were combined to obtain ZQJY-9. ZQJY-9 produced 19.68 ± 0.22 g/L of l-proline in flask fermentation and was also demonstrated at the 3 L bioreactor level by fed-batch fermentation producing 120.18 g/L of l-proline at 76 h with the highest productivity of 1.581 g/L/h.

Whole Genome Sequencing and Comparative Genomic Analyses of Lysinibacillus pakistanensis LZH-9, a Halotolerant Strain with Excellent COD Removal Capability

Halotolerant microorganisms are promising in bio-treatment of hypersaline industrial wastewater. Four halotolerant bacteria strains were isolated from wastewater treatment plant, of which a strain LZH-9 could grow in the presence of up to 14% (w/v) NaCl, and it removed 81.9% chemical oxygen demand (COD) at 96 h after optimization. Whole genome sequencing of Lysinibacillus pakistanensis LZH-9 and comparative genomic analysis revealed metabolic versatility of different species of Lysinibacillus, and abundant genes involved in xenobiotics biodegradation, resistance to toxic compound, and salinity were found in all tested species of Lysinibacillus, in which Horizontal Gene Transfer (HGT) contributed to the acquisition of many important properties of Lysinibacillus spp. such as toxic compound resistance and osmotic stress resistance as revealed by phylogenetic analyses. Besides, genome wide positive selection analyses revealed seven genes that contained adaptive mutations in Lysinibacillus spp., most of which were multifunctional. Further expression assessment with Codon Adaption Index (CAI) also reflected the high metabolic rate of L. pakistanensis to digest potential carbon or nitrogen sources in organic contaminants, which was closely linked with efficient COD removal ability of strain LZH-9. The high COD removal efficiency and halotolerance as well as genomic evidences suggested that L. pakistanensis LZH-9 was promising in treating hypersaline industrial wastewater.

Reconstruction of tricarboxylic acid cycle in Corynebacterium glutamicum with a genome-scale metabolic network model for trans-4-hydroxyproline production

trans-4-Hydroxy- l-proline (Hyp) is an abundant component of mammalian collagen and functions as a chiral synthon for the syntheses of anti-inflammatory drugs in the pharmaceutical industry. Proline 4-hydroxylase (P4H) can catalyze the conversion of l-proline to Hyp; however, it is still challenging for the fermentative production of Hyp from glucose using P4H due to the low yield and productivity. Here, we report the metabolic engineering of Corynebacterium glutamicum for the fermentative production of Hyp by reconstructing tricarboxylic acid (TCA) cycle together with heterologously expressing the p4h gene from Dactylosporangium sp. strain RH1. In silico model-based simulation showed that α-ketoglutarate was redirected from the TCA cycle toward Hyp synthetic pathway driven by P4H when the carbon flux from succinyl-CoA to succinate descended to zero. The interruption of the TCA cycle by the deletion of sucCD-encoding the succinyl-CoA synthetase (SUCOAS) led to a 60% increase in Hyp production and had no obvious impact on the growth rate. Fine-tuning of plasmid-borne ProB* and P4H abundances led to a significant increase in the yield of Hyp on glucose. The final engineered Hyp-7 strain produced up to 21.72 g/L Hyp with a yield of 0.27 mol/mol (Hyp/glucose) and a volumetric productivity of 0.36 g·L ^-1 ·hr ^-1 in the shake flask fermentation. To our knowledge, this is the highest yield and productivity achieved by microbial fermentation in a glucose-minimal medium for Hyp production. This strategy provides new insights into engineering C. glutamicum by flux coupling for the fermentative production of Hyp and related products.

A new genome-scale metabolic model of Corynebacterium glutamicum and its application

BACKGROUND

Corynebacterium glutamicum is an important platform organism for industrial biotechnology to produce amino acids, organic acids, bioplastic monomers, and biofuels. The metabolic flexibility, broad substrate spectrum, and fermentative robustness of C. glutamicum make this organism an ideal cell factory to manufacture desired products. With increases in gene function, transport system, and metabolic profile information under certain conditions, developing a comprehensive genome-scale metabolic model (GEM) of C. glutamicum ATCC13032 is desired to improve prediction accuracy, elucidate cellular metabolism, and guide metabolic engineering.

RESULTS

Here, we constructed a new GEM for ATCC13032, iCW773, consisting of 773 genes, 950 metabolites, and 1207 reactions. Compared to the previous model, iCW773 supplemented 496 gene-protein-reaction associations, refined five lumped reactions, balanced the mass and charge, and constrained the directionality of reactions. The simulated growth rates of C. glutamicum cultivated on seven different carbon sources using iCW773 were consistent with experimental values. Pearson's correlation coefficient between the iCW773-simulated and experimental fluxes was 0.99, suggesting that iCW773 provided an accurate intracellular flux distribution of the wild-type strain growing on glucose. Furthermore, genetic interventions for overproducing l-lysine, 1,2-propanediol and isobutanol simulated using OptForce_MUST were in accordance with reported experimental results, indicating the practicability of iCW773 for the design of metabolic networks to overproduce desired products. In vivo genetic modifications of iCW773-predicted targets resulted in the de novo generation of an l-proline-overproducing strain. In fed-batch culture, the engineered C. glutamicum strain produced 66.43 g/L l-proline in 60 h with a yield of 0.26 g/g (l-proline/glucose) and a productivity of 1.11 g/L/h. To our knowledge, this is the highest titer and productivity reported for l-proline production using glucose as the carbon resource in a minimal medium.

CONCLUSIONS

Our developed iCW773 serves as a high-quality platform for model-guided strain design to produce industrial bioproducts of interest. This new GEM will be a successful multidisciplinary tool and will make valuable contributions to metabolic engineering in academia and industry.

Under the microscope: From pathogens to probiotics and back

The review centers on the human gastrointestinal tract; focusing first on the bacterial stress responses needed to overcome the physiochemical defenses of the host, specifically how these stress survival strategies can be used as targets for alternative infection control strategies. The concluding section focuses on recent developments in molecular diagnostics; centring on the shifting paradigm from culture to molecular based diagnostics.

Functional Screening of the Cronobacter sakazakii BAA-894 Genome reveals a role for ProP (ESA_02131) in carnitine uptake

Cronobacter sakazakii is a neonatal pathogen responsible for up to 80% of fatalities in infected infants. Low birth weight infants and neonates infected with C. sakazakii suffer necrotizing enterocolitis, bacteraemia and meningitis. The mode of transmission most often associated with infection is powdered infant formula (PIF) which, with an aw of ∼0.2, is too low to allow most microorganisms to persist. Survival of C. sakazakii in environments subject to extreme hyperosmotic stress has previously been attributed to the uptake of compatible solutes including proline and betaine. Herein, we report the construction and screening of a C. sakazakii genome bank and the identification of ProP (ESA_02131) as a carnitine uptake system.

Evolution of proline biosynthesis: enzymology, bioinformatics, genetics, and transcriptional regulation

Proline is not only an essential component of proteins but it also has important roles in adaptation to osmotic and dehydration stresses, redox control, and apoptosis. Here, we review pathways of proline biosynthesis in the three domains of life. Pathway reconstruction from genome data for hundreds of eubacterial and dozens of archaeal and eukaryotic organisms revealed evolutionary conservation and variations of this pathway across different taxa. In the most prevalent pathway of proline synthesis, glutamate is phosphorylated to γ-glutamyl phosphate by γ-glutamyl kinase, reduced to γ-glutamyl semialdehyde by γ-glutamyl phosphate reductase, cyclized spontaneously to Δ(1)-pyrroline-5-carboxylate and reduced to proline by Δ(1)-pyrroline-5-carboxylate reductase. In higher plants and animals the first two steps are catalysed by a bi-functional Δ(1) -pyrroline-5-carboxylate synthase. Alternative pathways of proline formation use the initial steps of the arginine biosynthetic pathway to ornithine, which can be converted to Δ(1)-pyrroline-5-carboxylate by ornithine aminotransferase and then reduced to proline or converted directly to proline by ornithine cyclodeaminase. In some organisms, the latter pathways contribute to or could be fully responsible for the synthesis of proline. The conservation of proline biosynthetic enzymes and significance of specific residues for catalytic activity and allosteric regulation are analysed on the basis of protein structural data, multiple sequence alignments, and mutant studies, providing novel insights into proline biosynthesis in organisms. We also discuss the transcriptional control of the proline biosynthetic genes in bacteria and plants.

Nutritional requirements and media development for Lactococcus lactis IL1403

Lactic acid bacteria are extensively used in food technology and for the production of various compounds, but they are fastidious in nutrient requirements. In order to elucidate the role of each component precisely, defined multicomponent media are required. This study focuses on determining nutrient auxotrophies and minimizing media components (amino acids, vitamins, metal ions, buffers and additional compounds) for the cultivation of Lactococcus lactis subsp. lactis IL1403, using microtitre plates and test tubes. It was shown that glutamine and asparagine were the most important media components for achieving higher biomass yields while the branched-chain amino acids were necessary to increase specific growth rate. The amino acid and glucose ratio was reduced to achieve minimal residual concentration of amino acids in the medium after the growth of cells, whereas the specific growth rate and biomass yield of cells were not considerably affected. As the percentage of each consumed amino acid compared to initial amount is larger than measurement error, these optimized media are important for achieving more precise data about amino acid utilization and metabolism.

Bifidobacterium bifidum PRL2010 modulates the host innate immune response

Here, we describe data obtained from transcriptome profiling of human cell lines and intestinal cells of a murine model upon exposure and colonization, respectively, with Bifidobacterium bifidum PRL2010. Significant changes were detected in the transcription of genes that are known to be involved in innate immunity. Furthermore, results from enzyme-linked immunosorbent assays (ELISAs) showed that exposure to B. bifidum PRL2010 causes enhanced production of interleukin 6 (IL-6) and IL-8 cytokines, presumably through NF-κB activation. The obtained global transcription profiles strongly suggest that Bifidobacterium bifidum PRL2010 modulates the innate immune response of the host.

Ornithine cyclodeaminase-based proline production by Corynebacterium glutamicum

BACKGROUND

The soil bacterium Corynebacterium glutamicum, best known for its glutamate producing ability, is suitable as a producer of a variety of bioproducts. Glutamate is the precursor of the amino acid proline. Proline biosynthesis typically involves three enzymes and a spontaneous cyclisation reaction. Alternatively, proline can be synthesised from ornithine, an intermediate of arginine biosynthesis. The direct conversion of ornithine to proline is catalysed by ornithine cyclodeaminase. An ornithine overproducing platform strain with deletions of argR and argF (ORN1) has been employed for production of derived compounds such as putrescine. By heterologous expression of ocd this platform strain can be engineered further for proline production.

RESULTS

Plasmid-based expression of ocd encoding the putative ornithine cyclodeaminase of C. glutamicum did not result in detectable proline accumulation in the culture medium. However, plasmid-based expression of ocd from Pseudomonas putida resulted in proline production with yields up to 0.31 ± 0.01 g proline/g glucose. Overexpression of the gene encoding a feedback-alleviated N-acetylglutamate kinase further increased proline production to 0.36 ± 0.01 g/g. In addition, feedback-alleviation of N-acetylglutamate kinase entailed growth-coupled production of proline and reduced the accumulation of by-products in the culture medium.

CONCLUSIONS

The product spectrum of the platform strain C. glutamicum ORN1 was expanded to include the amino acid L-proline. Upon further development of the ornithine overproducing platform strain, industrial production of amino acids of the glutamate family and derived bioproducts such as diamines might become within reach.

A single point mutation in the listerial betL σ(A)-dependent promoter leads to improved osmo- and chill-tolerance and a morphological shift at elevated osmolarity

Betaine uptake in Listeria monocytogenes is mediated by three independent transport systems, the simplest of which in genetic terms is the secondary transporter BetL. Using a random mutagenesis approach, based on the E. coli XL1 Red mutator strain, we identified a single point mutation in a putative promoter region upstream of the BetL coding region which leads to a significant increase in betL transcript levels under osmo- and chill-stress conditions and a concomitant increase in stress tolerance. Furthermore, the mutation appears to counter the heretofore unreported “twisted” cell morphology observed for L. monocytogenes grown at elevated osmolarities in tryptone soy broth.

An efficient and reproducible method for transformation of genetically recalcitrant bifidobacteria

This study describes an efficient transformation system for the introduction of plasmid DNA into Bifidobacterium bifidum PRL2010 and Bifidobacterium asteroides PRL2011, for which to the best of our knowledge no transformation data have been reported previously. The method is based on electroporation of bifidobacterial cells, which were made competent by an optimized methodology based on varying media and growth conditions. Furthermore, the transformation protocol was applied in order to design a PRL2010-derivative, which carries antibiotic resistance against chloramphenicol and which was used to monitor PRL2010 colonization in a murine model.

Osmotically controlled synthesis of the compatible solute proline is critical for cellular defense of Bacillus subtilis against high osmolarity

Bacillus subtilis is known to accumulate large amounts of the compatible solute proline via de novo synthesis as a stress protectant when it faces high-salinity environments. We elucidated the genetic determinants required for the osmoadaptive proline production from the precursor glutamate. This proline biosynthesis route relies on the proJ-encoded γ-glutamyl kinase, the proA-encoded γ-glutamyl phosphate reductase, and the proH-encoded Δ1-pyrroline-5-caboxylate reductase. Disruption of the proHJ operon abolished osmoadaptive proline production and strongly impaired the ability of B. subtilis to cope with high-osmolarity growth conditions. Disruption of the proA gene also abolished osmoadaptive proline biosynthesis but caused, in contrast to the disruption of proHJ, proline auxotrophy. Northern blot analysis demonstrated that the transcription of the proHJ operon is osmotically inducible, whereas that of the proBA operon is not. Reporter gene fusion studies showed that proHJ expression is rapidly induced upon an osmotic upshift. Increased expression is maintained as long as the osmotic stimulus persists and is sensitively linked to the prevalent osmolarity of the growth medium. Primer extension analysis revealed the osmotically controlled proHJ promoter, a promoter that resembles typical SigA-type promoters of B. subtilis. Deletion analysis of the proHJ promoter region identified a 126-bp DNA segment carrying all sequences required in cis for osmoregulated transcription. Our data disclose the presence of ProA-interlinked anabolic and osmoadaptive proline biosynthetic routes in B. subtilis and demonstrate that the synthesis of the compatible solute proline is a central facet of the cellular defense to high-osmolarity surroundings for this soil bacterium.

A role for proline synthesis and transport in Listeria monocytogenes barotolerance

AIMS

To assess the contribution of proline biosynthesis to listerial barotolerance.

METHODS AND RESULTS

Using a Listeria monocytogenes proBA deletion mutant, incapable of synthesizing proline, together with a proline-overproducing strain, the contribution of proline synthesis to listerial barotolerance was determined. The ΔproBA strain does not survive as well as the wild type when subjected to treatment of 500 MPa in rich media and 400 MPa in minimal media (c. 1 log lower survival in both conditions). Betaine and carnitine decrease the ability of the wild type to survive at low pressures (300 MPa), but confer normal or slightly increased levels of protection at higher pressures (350 and 400 MPa).

CONCLUSIONS

A functional proline synthesis system is required for optimal survival of Listeria following treatment at high-pressure (HP) levels (500 MPa in brain heart infusion and 400 MPa in defined medium), particularly where other compatible solutes are absent or limiting.

SIGNIFICANCE AND IMPACT OF THE STUDY

  Given the potential of HP processing as an effective food processing/safety strategy, understanding how pathogens such as Listeria have evolved to cope with such stresses is an important food safety consideration. In this context, the work presented here may help to develop safer and more effective processing regimens.

Compatible solutes: the key to Listeria's success as a versatile gastrointestinal pathogen?

Recently we reported a role for compatible solute uptake in mediating bile tolerance and increased gastrointestinal persistence in the foodborne pathogen Listeria monocytogenes[1]. Herein, we review the evolution in our understanding of how these low molecular weight molecules contribute to growth and survival of the pathogen both inside and outside the body, and how this stress survival mechanism may ultimately be used to target and kill the pathogen.

Pyrroline-5-carboxylate synthase and proline biosynthesis: from osmotolerance to rare metabolic disease

Pyrroline-5-carboxylate synthase (P5CS) is a bifunctional enzyme that exhibits glutamate kinase (GK) and gamma-glutamyl phosphate reductase (GPR) activities. The enzyme is highly relevant in humans because it belongs to a combined route for the interconversion of glutamate, ornithine and proline. The deficiency of P5CS activity in humans is associated with a rare, inherited metabolic disease. It is well established that some bacteria and plants accumulate proline in response to osmotic stress. The alignment of P5CSs from different species and analysis of the solved structures of GK and GPR reveal high sequence and structural conservation. The information acquired from different mutant enzymes with increased osmotolerant properties, together with the position of the insertion found in the longer human isoform, permit the delimitation of the regulatory site of GK and P5CS and the proposal of a model of P5CS architecture. Additionally, the GK moiety of the human enzyme has been modeled and the known clinical mutations and polymorphisms have been mapped.

Compatible solutes: A listerial passe-partout?

Recently we reported a role for compatible solute uptake in mediating bile tolerance and increased gastrointestinal persistence in the foodborne pathogen Listeria monocytogenes.1 Herein, we review the evolution in our understanding of how these low molecular weight molecules contribute to growth and survival of the pathogen both inside and outside the body, and how this stress survival mechanism may ultimately be used to target and kill the pathogen.

Rational design of improved pharmabiotics

Herein we review the most recent advances in probiotic research and applications with particular emphasis on the novel concept of patho-biotechnology: the application of pathogen-derived (ex vivo and in vivo) stress survival strategies for the design of more technologically robust and effective probiotic cultures with improved biotechnological and clinical applications.

Enhancing bile tolerance improves survival and persistence of Bifidobacterium and Lactococcus in the murine gastrointestinal tract

Background

The majority of commensal gastrointestinal bacteria used as probiotics are highly adapted to the specialised environment of the large bowel. However, unlike pathogenic bacteria; they are often inadequately equipped to endure the physicochemical stresses of gastrointestinal (GI) delivery in the host. Herein we outline a patho-biotechnology strategy to improve gastric delivery and host adaptation of a probiotic strain Bifidobacterium breve UCC2003 and the generally regarded as safe (GRAS) organism Lactococcus lactis NZ9000.

Results

In vitro bile tolerance of both strains was significantly enhanced (P < 0.001), following heterologous expression of the Listeria monocytogenes bile resistance mechanism BilE. Strains harbouring bilE were also recovered at significantly higher levels (P < 0.001), than control strains from the faeces and intestines of mice (n = 5), following oral inoculation. Furthermore, a B. breve strain expressing bilE demonstrated increased efficacy relative to the wild-type strain in reducing oral L. monocytogenes infection in mice.

Conclusion

Collectively the data indicates that bile tolerance can be enhanced in Bifidobacterium and Lactococcus species through rational genetic manipulation and that this can significantly improve delivery to and colonisation of the GI tract.

Improving gastric transit, gastrointestinal persistence and therapeutic efficacy of the probiotic strain Bifidobacterium breve UCC2003

Given the increasing commercial and clinical relevance of probiotic cultures, improving their stress tolerance profile and ability to overcome the physiological defences of the host is an important biological goal. In order to reach the gastrointestinal tract in sufficient numbers to exert a therapeutic effect, probiotic bacteria must resist the deleterious actions of low pH, elevated osmolarity and bile salts. Cloning the listerial betaine uptake system, BetL, into the probiotic strain Bifidobacterium breve UCC2003 significantly improved probiotic tolerance to gastric juice and conditions of elevated osmolarity mimicking the gut environment. Furthermore, whilst stable colonization of the murine intestine was achieved by oral administration of B. breve UCC2003, strains harbouring BetL were recovered at significantly higher levels in the faeces, intestines and caecum of inoculated animals. Finally, in addition to improved gastric transit and intestinal persistence, this approach improved the clinical efficacy of the probiotic culture: mice fed B. breve UCC2003-BetL(+) exhibited significantly lower levels of systemic infection compared to the control strain following oral inoculation with Listeria monocytogenes.

Expression of Bacillus subtilis proBA genes and reduction of feedback inhibition of proline synthesis increases proline production and confers osmotolerance in transgenic Arabidopsis

Proline accumulation has been shown to correlate with tolerance to drought and salt stresses in plants. We attempt to introduce the wild-type, mutant, and fusion proBA genes derived from Bacillus subtilis into Arabidopsis thaliana under the control of a strong promoter cauliflower mosaic virus 35S (CaMV35S). The transgenic plants produced higher level of free proline than control and the overproduction of proline resulted in the increased tolerance to osmotic stress in transgenic plants. Besides, the mutation in proBA genes, which were proved to lead gamma-glutamyl kinase (gamma-GK) reduces sensitivity to the end-product inhibition and the fusion of proB and proA also result in increasing proline production and confer osmotolerance in transgenic lines.

Patho-biotechnology; using bad bugs to make good bugs better

Given the increasing commercial and clinical relevance of probiotic cultures, improving their stress tolerance profile and ability to overcome the physiochemical defences of the host is an important biological goal. Pathogenic bacteria have evolved sophisticated strategies to overcome host defences, interact with the immune system and interfere with essential host systems. We coin the term 'patho-biotechnology' to describe the exploitation of these valuable traits in biotechnology and biomedicine. This approach shows promise for the design of more technologically robust and effective probiotic cultures with improved biotechnological and clinical applications as well as the development of novel vaccine and drug delivery platforms.

Directed evolution of an artificial bifunctional enzyme, γ-glutamyl kinase/γ-glutamyl phosphate reductase, for improved osmotic tolerance ofEscherichia colitransformants

To produce the artificial bifunctional enzyme gamma-glutamyl kinase/gamma-glutamyl phosphate reductase, a mutant library of the proBA fusion gene from Bacillus subtilis was created by error-prone PCR. Selecting by functional complementation of the proline auxotroph Escherichia coli JM83 and NaCl tolerance, we isolated a mutant of the proBA fusion gene that improved the osmotolerance of host cells of E. coli JM83. A single amino acid replacement (Asn177Asp) located in a conserved domain in gamma-glutamyl kinase leads to overproduction of proline by host cells. The mutated gamma-glutamyl kinase/gamma-glutamyl phosphate reductase enzyme was rendered about 100-fold less sensitive to proline-mediated feedback inhibition than the control.

Oral inoculation of A/J mice for detection of invasiveness differences between Listeria monocytogenes epidemic and environmental strains

Four-week-old Harlan A/J mice were orally infected with six epidemic and six environmental strains of Listeria monocytogenes. Epidemic strains were significantly more invasive as a group than were environmental strains (P < 0.05), and the intestines of some mice infected with epidemic strains had extensive hemorrhage. Mice inoculated with epidemic strains were significantly more likely to become systemically infected than mice inoculated with environmental strains (P < 0.01).

Expression of cellular antigens of Listeria monocytogenes that react with monoclonal antibodies C11E9 and EM-7G1 under acid-, salt- or temperature-induced stress environments

AIMS

To study the expression of cellular antigens of Listeria monocytogenes that react with monoclonal antibodies (MAbs) C11E9 and EM-7G1 under acid-, salt- or temperature-induced stress environments.

METHODS AND RESULTS

The reaction patterns of antibodies to L. monocytogenes held in stressful environments for a short duration (3 h) or grown for extended periods (16-72 h) were investigated. During both short or prolonged exposure to stress environments of high temperature (45 degrees C) and NaCl (>1.5%, w/v), reactions of whole cells of L. monocytogenes to antibodies were severely affected as determined by ELISA and by the reduced expression of the antibody-reactive 66 kDa antigen in the Western blot assay. Conversely, cold (4-15 degrees C) or acid (pH 2-3) stress environments had very little effect on antigen expression or antibody reaction. Additionally, heat-killed cells showed reduced reactions to these antibodies when compared with unheated cells. Artificially created stress environments in hotdog slurry also affected the antigen expression in L. monocytogenes. Immunoelectron microscopy revealed that the antibody-reactive antigens were uniformly present on the surface of the cells. Morphological characteristics following growth in stressed environments revealed that heat stress at 45 degrees C caused L. monocytogenes cells to be elongated and to form clumps; whereas, osmotic stress (5.5% NaCl, w/v) caused filamentous appearance with multiple septa along the length of the cell.

CONCLUSIONS

These results indicated that MAb C11E9 or EM-7G1 could detect L. monocytogenes from cold or acid-stress environments; however, they may show weaker reactions with heat or osmotically stressed cells or cells grown at 4 degrees C.

SIGNIFICANCE AND IMPACT OF THE STUDY

Bacteria in food are routinely subjected to various stresses, induced by cold, heat, salt or acid during processing and storage. Whether stresses would modify the expression of cellular antigens of L. monocytogenes is of a great concern for immunodetections in food products.

Identification of regions of the tomato gamma-glutamyl kinase that are involved in allosteric regulation by proline

The first step of proline biosynthesis is catalyzed by gamma-glutamyl kinase (GK). To better understand the feedback inhibition properties of GK, we randomly mutagenized a plasmid carrying tomato tomPRO1 cDNA, which encodes proline-sensitive GK. A pool of mutagenized plasmids was transformed into an Escherichia coli GK mutant, and proline-overproducing derivatives were selected on minimal medium containing the toxic proline analog 3,4-dehydro-dl-proline. Thirty-two mutations that conferred 3,4-dehydro-dl-proline resistance were obtained. Thirteen different single amino acid substitutions were identified at nine different residues. The residues were distributed throughout the N-terminal two-thirds of the polypeptide, but 9 mutations affecting 6 residues were in a cluster of 16 residues. GK assays revealed that these amino acid substitutions caused varying degrees of diminished sensitivity to proline feedback inhibition and also resulted in a range of increased proline accumulation in vivo. GK belongs to a family of amino acid kinases, and a predicted three-dimensional model of this enzyme was constructed on the basis of the crystal structures of three related kinases. In the model, residues that were identified as important for allosteric control were located close to each other, suggesting that they may contribute to the structure of a proline binding site. The putative allosteric binding site partially overlaps the dimerization and substrate binding domains, suggesting that the allosteric regulation of GK may involve a direct structural interaction between the proline binding site and the dimerization and catalytic domains.

Bacterial osmoadaptation: the role of osmolytes in bacterial stress and virulence

Two general strategies exist for the growth and survival of prokaryotes in environments of elevated osmolarity. The 'salt in cytoplasm' approach, which requires extensive structural modifications, is restricted mainly to members of the Halobacteriaceae. All other species have convergently evolved to cope with environments of elevated osmolarity by the accumulation of a restricted range of low molecular mass molecules, termed compatible solutes owing to their compatibility with cellular processes at high internal concentrations. Herein we review the molecular mechanisms governing the accumulation of these compounds, both in Gram-positive and Gram-negative bacteria, focusing specifically on the regulation of their transport/synthesis systems and the ability of these systems to sense and respond to changes in the osmolarity of the extracellular environment. Finally, we examine the current knowledge on the role of these osmostress responsive systems in contributing to the virulence potential of a number of pathogenic bacteria.