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Herein the effect of low intensity ultrasound on the fermentation of skim milk medium by Lactobacillus paracasei were investigated to obtain optimum ultrasonic conditions for the highest yield of yoghurt peptides. The results showed that the fermented skim milk medium treated with ultrasound with its seed culture without ultrasonic treatment was an optimum scheme. In this scheme with the ultrasonic conditions of 28 kHz, ultrasonic pulsed model of on-time 100 s and off-time 10 s, 100 W/L for the treatment time of 30 min after the fermentation time of 9 h, the peptide content in the fermented skim milk media increased by 49.5% and the viable cells in the same media increased by 43.5% compared with those in the untreated samples. By response surface methodology (RSM) analysis and its verification experiments, a reasonably accurate empirical model was established for investigating and predicting the relationship between skim milk concentration, ultrasonic treatment time, power and the yield of yoghurt peptides. The former two parameters 12.6% w/v and 35 min were taken in the verification experiments in which the peptide content of the fermented media reached 5.9 mg/mL with an increase by 64.23% and the peptide yield was 14.2%, similar to its theoretical value of 14.6% according to the empirical model. The comparison of extracellular enzyme activities in the fermented skim milk media between with and without ultrasonic treatment under the conditions in the optimum scheme indicated that the mechanism of the ultrasound-activated peptide content increment might be the extracellular enzyme activities immediately activated by the ultrasound, effect of which would disappear in the progress of fermentation after the ultrasound was removed.

BACKGROUND Surfactin, a good biological surfactant, is derived from the metabolites of microorganisms. However, the ability of natural strains to produce surfactin is low, and so the presented study aimed to use a novel mutagenesis technology to increase their yields. RESULTS Atmospheric and room temperature plasma (ARTP) was used to conduct mutation breeding of Bacillus subtilis CICC 10721, and a mutant strain M45 with a higher surfactin yield of 34.2% and a stable subculture was screened out. From the fermentation kinetics study, it was found that the maximum cell dry weight, maximum growth rate and surfactin synthesis parameters of the mutant strain M45 were all greater than that of the original strain. Scanning electron microscope and laser scanning confocal microscope observations showed that the spore morphology changed after ARTP treating, and the intracellular Ca2+ concentration of the mutant increased. Genome resequencing analysis showed that 66 single nucleotide poymorphism non-synonymous mutation sites occurred in M45, and the identification results of the fermentation broth extract from M45 showed that it is composed of C12 -C16 surfactin. CONCLUSION ARTP mutagenesis was found to change the morphology of bacteria, membrane permeability and genes related to the synthesis and secretion of surfactin. The present study provides a basis for industrial production of surfactin and an understanding of the mutagenesis mechanism. © 2021 Society of Chemical Industry.

Herein Bacillus tequilensis was chosen as a representative strain isolated from aerogenic vinegar produced by solid-state fermentation (SSF) for the purpose of this research to elucidate the bacterial inactivation mechanisms of intense pulsed light (IPL) at cellular and molecular levels. The optimum sterilization conditions of IPL for this strain were obtained by using response surface methodology (RSM), resulting in an average lethality of 99.98%. The results showed that IPL at the optimum sterilization conditions caused a destructed structure of the B. tequilensis strain. Its intracellular nucleic acid and proteins seeped out of the IPL-treated strain showing a positive relationship between the concentration of nucleic acid and proteins in the culture medium and the bacterial lethality. Extracellular calcium ions flowed in the IPL-treated strain, resulting in a higher concentration of calcium ions. Comparative proteomic analysis showed that there were 46 up-regulated proteins and 58 down-regulated proteins in the IPL-treated strain culture compared with the IPL-untreated strain culture. Western blot analysis confirmed the up-regulation of four proteins catalase 1, catalase 2, chitosanase and gamma-glutamyltransferase. It indicated that there existed stress reacting processes in the IPL-treated strain during its sterilization process.

To reduce the high cost caused by steam sterilization of fermentation substrates and equipments in mesophilic solid-state fermentation of rapeseed meal (RM), a thermophilic protease-producing strain RM-2 from RM was isolated and employed in thermophilic solid-state fermentation of unsterilized RM for polypeptide preparation. The isolate was identified as Geobacillus stearothermophilus by appearance of colonies and microscopic observation as well as 16S rDNA sequencing. The highest yield of polypeptide (9.67%) was obtained by RM-2 at 50% moisture content, 55 °C and 24 h fermentation time. Analysis of microbial community diversity in fermented RM revealed Geobacillus stearothermophilus as dominant strain inhibiting most mesophilic microorganisms. Thermophilic solid-state fermentation on RM improved the yield of rapeseed polypeptide, as well as the utilization rate of substrate. The application of this technology (thermophilic solid-state fermentation) would be environmentally friendlier and energy-saving to industry.

Nowadays, ultrasound is widely used in many aspects. In the last few years, many papers have concentrated on the applications of ultrasound in engineering, chemistry, medicine, physics and biology, but few in biological effects such as the acceleration effects on proliferation of microbial cells, the inactivation effects on microorganisms and the influences on the activities of enzyme. Thus, the objective of this review is to investigate the biological effects of ultrasound on these aspects.