Characterization of the activity of the spore cortex lytic enzyme CwlJ1

The underlying mechanism of bacterial spore germination: An update review

Bacterial spores are highly resilient and universally present on earth and can irreversibly enter the food chain to cause food spoilage or foodborne illness once revived to resume vegetative growth. Traditionally, extensive thermal processing has been employed to efficiently kill spores; however, the relatively high thermal load adversely affects food quality attributes. In recent years, the germination-inactivation strategy has been developed to mildly kill spores based on the circumstance that germination can decrease spore-resilient properties. However, the failure to induce all spores to geminate, mainly owing to the heterogeneous germination behavior of spores, hampers the success of applying this strategy in the food industry. Undoubtedly, elucidating the detailed germination pathway and underlying mechanism can fill the gap in our understanding of germination heterogeneity, thereby facilitating the development of full-scale germination regimes to mildly kill spores. In this review, we comprehensively discuss the mechanisms of spore germination of Bacillus and Clostridium species, and update the molecular basis of the early germination events, for example, the activation of germination receptors, ion release, Ca-DPA release, and molecular events, combined with the latest research evidence. Moreover, high hydrostatic pressure (HHP), an advanced non-thermal food processing technology, can also trigger spore germination, providing a basis for the application of a germination-inactivation strategy in HHP processing. Here, we also summarize the diverse germination behaviors and mechanisms of spores of Bacillus and Clostridium species under HHP, with the aim of facilitating HHP as a mild processing technology with possible applications in food sterilization. Practical Application: This work provides fundamental basis for developing efficient killing strategies of bacterial spores in food industry.

Novel spore lytic enzyme from a Bacillus phage leading to spore killing

Bacterial spores maintain metabolic dormancy and have high resistance to external pressure. Germination requires degradation of the spore cortex and the participation of germination-specific cortex-lytic enzymes (GSLEs). Previously reported GSLEs have been identified in bacteria and facilitate germination. In this study, we have characterized a novel spore lytic enzyme, Ply67, from Bacillus pumilus phage vB_BpuM_BpSp. Ply67 had a similar cortex-lytic activity to GSLEs but disrupted the inner membranes (IMs) of spores, leading to spore killing rather than germination. The amino acid sequence of the complete protein, Ply67^FL, exhibited 40% homology to the GSLE SleB. Domain prediction showed that Ply67^FL was composed of three domains: a signal peptide, N-terminal domain protein and C-terminal domain protein. Ply67^FL rapidly caused E. coli cells lysis when it was expressed in E. coli. The protein containing the C-terminal domain protein, Ply67^C, could kill B. pumilus spores. The protein containing the N-terminal domain protein, Ply67^N, could combine with the decoated B. pumilus spores, indicating that N-terminal was the binding domain and C-terminal was the hydrolase domain. The protein lacking the signal peptide but containing the N-terminal and C-terminal domain proteins, Ply67, had activity against spores of various Bacillus species. The surface of spores treated with Ply67 shrank and the permeability barrier was disrupted, and the inner contents leaked out. Immunoelectron microscopic observation showed that Ply67 was mainly acted on the spore cortex. Overall, Ply67 is a novel spore lytic enzyme that differs from other GSLEs not only in amino acid sequence but also in activity against spores, and Ply67 might have the potential to kill spores of pathogenic Bacillus species, e.g., B. cereus and B. anthracis.

N-Deacetylases required for muramic-δ-lactam production are involved in Clostridium difficile sporulation, germination, and heat resistance

Spores are produced by many organisms as a survival mechanism activated in response to several environmental stresses. Bacterial spores are multilayered structures, one of which is a peptidoglycan layer called the cortex, containing muramic-δ-lactams that are synthesized by at least two bacterial enzymes, the muramoyl-l-alanine amidase CwlD and the N-deacetylase PdaA. This study focused on the spore cortex of Clostridium difficile, a Gram-positive, toxin-producing anaerobic bacterial pathogen that can colonize the human intestinal tract and is a leading cause of antibiotic-associated diarrhea. Using ultra-HPLC coupled with high-resolution MS, here we found that the spore cortex of the C. difficile 630Δerm strain differs from that of Bacillus subtilis Among these differences, the muramic-δ-lactams represented only 24% in C. difficile, compared with 50% in B. subtilis CD630_14300 and CD630_27190 were identified as genes encoding the C. difficile N-deacetylases PdaA1 and PdaA2, required for muramic-δ-lactam synthesis. In a pdaA1 mutant, only 0.4% of all muropeptides carried a muramic-δ-lactam modification, and muramic-δ-lactams were absent in the cortex of a pdaA1-pdaA2 double mutant. Of note, the pdaA1 mutant exhibited decreased sporulation, altered germination, decreased heat resistance, and delayed virulence in a hamster infection model. These results suggest a much greater role for muramic-δ-lactams in C. difficile than in other bacteria, including B. subtilis In summary, the spore cortex of C. difficile contains lower levels of muramic-δ-lactams than that of B. subtilis, and PdaA1 is the major N-deacetylase for muramic-δ-lactam biosynthesis in C. difficile, contributing to sporulation, heat resistance, and virulence.

Genetic Environment of cry1 Genes Indicates Their Common Origin

Although in Bacillus thuringiensis the cry genes coding for the insecticidal crystal proteins are plasmid-borne and are usually associated with mobile genetic elements, several aspects related to their genomic organization, diversification, and transmission remain to be elucidated. Plasmids of B. thuringiensis and other members of the Bacillus cereus group (n = 364) deposited in GenBank were screened for the presence of cry1 genes, and their genetic environment was analyzed using a comparative bioinformatic approach. The cry1 genes were identified in 27 B. thuringiensis plasmids ranging from 64 to 761 kb, and were predominantly associated with the ori44, ori60, or double orf156/orf157 and pXO1-16/pXO1-14 replication systems. In general, the cry1 genes occur individually or as a part of an insecticidal pathogenicity island (PAI), and are preceded by genes coding for an N-acetylmuramoyl-l-alanine amidase and a putative K⁺(Na⁺)/H⁺ antiporter. However, except in the case of the PAI, the latter gene is disrupted by the insertion of IS231B. Similarly, numerous mobile elements were recognized in the region downstream of cry1, except for cry1I that follows cry1A in the PAI. Therefore, the cassette involving cry1 and these two genes, flanked by transposable elements, named as the cry1 cassette, was the smallest cry1-carrying genetic unit recognized in the plasmids. Conservation of the genomic environment of the cry1 genes carried by various plasmids strongly suggests a common origin, possibly from an insecticidal PAI carried by B. thuringiensis megaplasmids.

Metabolic engineering of Corynebacterium glutamicum for anthocyanin production

Background

Anthocyanins such as cyanidin 3-O-glucoside (C3G) have wide applications in industry as food colorants. Their current production heavily relies on extraction from plant tissues. Development of a sustainable method to produce anthocyanins is of considerable interest for industrial use. Previously, E. coli-based microbial production of anthocyanins has been investigated extensively. However, safety concerns on E. coli call for the adoption of a safe production host. In the present study, a GRAS bacterium, Corynebacterium glutamicum, was introduced as the host strain to synthesize C3G. We adopted stepwise metabolic engineering strategies to improve the production titer of C3G.

Results

Anthocyanidin synthase (ANS) from Petunia hybrida and 3-O-glucosyltransferase (3GT) from Arabidopsis thaliana were coexpressed in C. glutamicum ATCC 13032 to drive the conversion from catechin to C3G. Optimized expression of ANS and 3GT improved the C3G titer by 1- to 15-fold. Further process optimization and improvement of UDP-glucose availability led to ~ 40 mg/L C3G production, representing a > 100-fold titer increase compared to production in the un-engineered, un-optimized starting strain.

Conclusions

For the first time, we successfully achieved the production of the specialty anthocyanin C3G from the comparatively inexpensive flavonoid precursor catechin in C. glutamicum. This study opens up more possibility of C. glutamicum as a host microbe for the biosynthesis of useful and value-added natural compounds.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0990-z) contains supplementary material, which is available to authorized users.

Germination of Spores of the Orders Bacillales and Clostridiales

Dormant Bacillales and Clostridiales spores begin to grow when small molecules (germinants) trigger germination, potentially leading to food spoilage or disease. Germination-specific proteins sense germinants, transport small molecules, and hydrolyze specific bonds in cortex peptidoglycan and specific proteins. Major events in germination include (a) germinant sensing; (b) commitment to germinate; (c) release of spores' depot of dipicolinic acid (DPA); (d) hydrolysis of spores' peptidoglycan cortex; and (e) spore core swelling and water uptake, cell wall peptidoglycan remodeling, and restoration of core protein and inner spore membrane lipid mobility. Germination is similar between Bacillales and Clostridiales, but some species differ in how germinants are sensed and how cortex hydrolysis and DPA release are triggered. Despite detailed knowledge of the proteins and signal transduction pathways involved in germination, precisely what some germination proteins do and how they do it remain unclear.

In vitro gene expression-coupled bacterial cell chip for screening species-specific antimicrobial enzymes

Targeting infectious bacterial pathogens is important for reducing the evolution of antibiotic-resistant bacteria and preserving the endogenous human microbiome. Cell lytic enzymes including bacteriophage endolysins, bacterial autolysins, and other bacteriolysins are useful antibiotic alternatives due to their exceptional target selectivity, which may be used to lysins rapidly kill target bacteria and their high specificity permit the normal commensal microflora to be left undisturbed. Genetic information of numerous lysins is currently available, but the identification of their antimicrobial function and specificity has been limited because most lysins are often poorly expressed and exhibit low solubilities. Here, we report the development of bacterial cell chip for rapidly accessing the function of diverse genes that are suggestive of encoding lysins. This approach can be used to evaluate rapidly the species-specific antimicrobial activity of diverse lysins synthesized from in vitro transcription and translation (TNT) of plasmid DNA. In addition, new potent lysins can be assessed that are not expressed in hosts and display low solubility. As a result of evaluating the species-specific antimicrobial function of 11 (un)known lysins with an in vitro TNT-coupled bacterial cell chip, a potent recombinant lysin against Staphylococcus strains, SA1, was identified. The SA1 was highly potent against not only S. aureus, but also both lysostaphin-resistant S. simulans and S. epidermidis cells. To this end, the SA1 may be applicable to treat both methicillin-resistant S. aureus (MRSA) and lysostaphin-resistant MRSA mutants. Biotechnol. Bioeng. 2017;114: 1648-1657. © 2017 Wiley Periodicals, Inc.