Electrotransformation of Lactobacillus plantarum using linearized plasmid DNA

Determining the Role of UTP-Glucose-1-Phosphate Uridylyltransferase (GalU) in Improving the Resistance of Lactobacillus acidophilus NCFM to Freeze-Drying

Lactobacillus acidophilus NCFM is widely used in the fermentation industry; using it as a freeze-dried powder can greatly reduce the costs associated with packaging and transportation, and even prolong the storage period. Previously published research has reported that the expression of galU (EC: 2.7.7.9) is significantly increased as a result of freezing and drying. Herein, we aimed to explore how galU plays an important role in improving the resistance of Lactobacillus acidophilus NCFM to freeze-drying. For this study, galU was first knocked out and then re-expressed in L. acidophilus NCFM to functionally characterize its role in the pertinent metabolic pathways. The knockout strain ΔgalU showed lactose/galactose deficiency and displayed irregular cell morphology, shortened cell length, thin and rough capsules, and abnormal cell division, and the progeny could not be separated. In the re-expression strain pgalU, these inhibited pathways were restored; moreover, the pgalU cells showed a strengthened cell wall and capsule, which enhanced their resistance to adverse environments. The pgalU cells showed GalU activity that was 229% higher than that shown by the wild-type strain, and the freeze-drying survival rate was 84%, this being 4.7 times higher than that of the wild-type strain. To summarize, expression of the galU gene can significantly enhance gene expression in galactose metabolic pathway and make the strain form a stronger cell wall and cell capsule and enhance the resistance of the bacteria to an adverse external environment, to improve the freeze-drying survival rate of L. acidophilus NCFM.

High-efficiency transformation of Streptococcus thermophilus using electroporation

BACKGROUND

Streptococcus thermophilus, one of the most important lactic acid bacteria, is widely used in food fermentation, which is beneficial to improve food quality. However, the current genetic transformation systems are inefficient for S. thermophilus S-3, which hinders its further study.

RESULTS

We developed three electroporation transformation methods for S. thermophilus S-3, and optimized various parameters to enhance the transformation efficiency up to 1.3 × 10⁶  CFU/μg DNA, which was 32-fold higher than that of unoptimized. Additionally, transcriptional analysis showed that a series of competence genes in S. thermophilus S-3 were remarkedly up-regulated after optimization, indicating that improvement of transformation efficiency was attributed to the expression level of competence genes. Furthermore, to prove their potential, expression of competence genes (comEA, cbpD and comX) were employed to increase transformation efficiency. The maximum transformation efficiency was obtained by overexpression of comEA, which was 14-fold higher than that of control.

CONCLUSION

This is the first report of competence gene expression for enhancing transformability in S. thermophilus, which exerts a positive effect on the development of desirable characteristics strains. © 2021 Society of Chemical Industry.

Recent progress on n-butanol production by lactic acid bacteria

n-Butanol is an essential chemical intermediate produced through microbial fermentation. However, its toxicity to microbial cells has limited its production to a great extent. The anaerobe lactic acid bacteria (LAB) are the most resistant to n-butanol, so it should be the first choice for improving n-butanol production. The present article aims to review the following aspects of n-butanol production by LAB: (1) the tolerance of LAB to n-butanol, including its tolerance level and potential tolerance mechanisms; (2) genome editing tools in the n-butanol-resistant LAB; (3) methods of LAB modification for n-butanol production and the production levels after modification. This review will provide a theoretical basis for further research on n-butanol production by LAB.

Selection-free markerless genome manipulations in the polyploid bacterium Thermus thermophilus

A genome manipulation approach based on double-crossover homologous recombination was developed in the polyploid model organism Thermus thermophilus HB27 without the use of any selectable marker. The method was established and optimized by targeting the megaplasmid-encoded β-glucosidase gene bgl. When linear and supercoiled forms of marker-free suicide vector were used for transformations, the frequencies of obtaining apparent Bgl^- mutant were 10^- 5 and 10^- 3, respectively; while the frequency could reach 10^- 2 when transformation with concatemer form of the same vector. All randomly selected Bgl^- colonies from the transformations were found to be true bgl knockout mutants. Thus, markerless gene deletion mutants could be constructed in T. thermophilus by the direct selection-free method. The functionality of this approach was further demonstrated by deletion of one chromosomal locus (TTC_0340-0341) as well as by generation of a reporter strain for the phytoene synthase promoter (PcrtB), homozygous mutants of the both targets could also be detected with a frequency of approximately 10^- 2. During the genome modification process, heterozygous cells carrying two different alleles at a same locus (e.g., bgl and pyrE) could also be generated. However, in the absence of selection pressure, these strains could rapidly convert to homozygous strains containing only one of the two alleles. This indicated that allele segregation could occur in the heterozygous T. thermophilus cells, which probably explained the ease of obtaining homozygous gene deletion mutants with high frequency (10^- 2) in the polyploid genomic background, as after the mutant allele had been introduced to the target region, allele segregation would lead to homozygous mutant cells. This marker-free genome manipulation approach does not require phenotype-based screens, and is applicable in gene deletion and tagging applications.