Fungal strain selection for protease production by solid-state fermentation using agro-industrial waste as substrates

Characteristics and application in cheese making of newly isolated milk-clotting enzyme from Bacillus megaterium LY114

Milk-clotting enzyme (MCE) is a crucial active agent in cheese making. It is necessary to find traditional MCE substitutes due to the limited production of traditional MCE (e.g., calf rennet) and increased cheese consumption. Bacillus megaterium strain LY114 with good milk-clotting activity (MCA) (448 SU/mL) and a high MCA/proteolytic activity (PA) ratio (6.0) was isolated and identified from agricultural soil in Laiyang (Shandong, China) through 16S rRNA sequencing of 45 strains. The Bacillus megaterium LY114 MCE had a remarkable specific activity (7532 SU/mg) and displayed a 4.83-fold purification yield with 34.17% recovery through ammonium sulfate fractionation and DEAE-Sepharose Fast Flow. The purified LY114 MCE was a metalloprotease with a molecular weight of 30 kDa. LY114 MCE was stable at pH 5.0-7.0 and temperature <40 °C. The highest MCA appeared at a substrate pH of 5.5 with 30 mM CaCl2. The Michaelis constant (Km) and maximal velocity (Vm) for casein were 0.31 g/L and 14.16 μmol/min, respectively. LY114 MCE preferred to hydrolyze α-casein (α-CN) rather than β-casein (β-CN) and had unique α-CN, β-CN and κ-casein (κ-CN) cleavage sites. LY114 MCE hydrolyzed casein to generate significantly different peptides compared with calf rennet and fungal MCE as determined by SDS-PAGE analysis. Chemical index analysis and sensory evaluation confirmed the usefulness of LY114 MCE in cheese making. LY114 MCE had the potential to be used in dairy processing and enriched traditional MCE substitutes.

Isolation and production optimization of a novel milk-clotting enzyme Bacillus velezensis DB219

The milk-clotting enzyme (MCE) is a crucial ingredient in cheese manufacture. Due to the limits of traditional MCE, finding viable substitute is a pressing issue. This study aims to isolate and identify a wild strain with high milk-clotting activity (MCA) and low proteolytic activity (PA) and optimize the fermentation conditions for MCE production. A strain of Bacillus velezensis DB219 with high MCA/PA value (9.2) was isolated from dairy soil (Wuchang, Heilongjiang, China) and identified through 16S rRNA from 40 strains. The optimal wheat bran, carbon, nitrogen, inoculum size, volume and initial pH were 60 g/L, soluble starch 12.5 g/L, corn steep liquor 3 g/L, 5%, 40 mL and 6.15, respectively for improving DB219 MCE production through single factor experiment. The wheat bran concentration, corn steep liquor concentration and volume were the most critical factor and their changed range was determined through Plackett–Burman design and the steepest ascent/descent experiments. The response surface analysis experiment of three factors and three levels was conducted by Box–Behnken design. The theoretical optimal fermentation conditions for DB219 MCE were as follows: wheat bran concentration 60.14 g/L, soluble starch 12.5 g/L, corn steep liquor 3 g/L, inoculum size 5%, volume 40.08 mL and initial pH 6.15. DB219 MCE achieved the maximal MCA (3164.84 SU/mL) that was 101.9% of the predicted value (3104.49 SU/mL) and 4.3-fold higher than the control.

Green synthesis of novel antifungal 1,2,4-triazoles effective against γ-irradiated Candida parapsilosis

This study reports the green synthesis of 11 novel 3-substituted-4-amino-5-mercapto-1,2,4-triazole derivatives using water as a readily available nontoxic solvent. Evaluation of their antimicrobial potential against several clinical pathogenic microorganisms was carried out. The newly synthesized cysteine derivative 6 showed promising antifungal activity against both γ-irradiated and nonirradiated Candida parapsilosis 216, with the lowest MIC (minimum inhibitory concentration) value of 3.125 µg/ml, probably through inhibition of 14α-demethylase. In addition, compound 6 showed complete inhibition of gelatinase, a virulence enzyme of C. parapsilosis. Also, scanning electron microscopy was carried out. Interestingly, compound 6 acted as a dual agent as it also showed good antibacterial activity against strains of Gram-positive bacteria used in the study. The synthesized compounds showed no cytotoxicity.

Evaluation of Laccase Activities by Three Newly Isolated Fungal Species in Submerged Fermentation With Single or Mixed Lignocellulosic Wastes

Three newly isolated fungal species, namely, Cerrena unicolor Han 849, Lenzites betulina Han 851, and Schizophyllum commune Han 881, isolated from their native habitats in Wulingshan National Nature Reserve of Hebei Province of northern China, were screened for laccase production with single or mixed lignocellulosic wastes. C. unicolor Han 849 was found to express the highest levels of laccase with single or mixed lignocellulosic wastes compared with L. betulina Han 851 and S. commune Han 881. The highest laccase activity from the mixed fungal culture of C. unicolor Han 849 and S. commune Han 881 or L. betulina Han 851 on Firmiana platanifolia was 1,373.12 ± 55.93 and 1,144.85 ± 34.97 U/L, respectively, higher than that from other tested conditions. L. betulina Han 851 or S. commune Han 881 mixed with other species was also helpful for accelerating laccase secretion due to reach maximum enzyme activity quickly. The treatment of mixing different species, including the mixture of two or three species, was obviously conducive to the improvement of laccase activity on Firmiana platanifolia. These results revealed that the fungal co-culture and the mixed lignocellulosic wastes contribute to the improvement of laccase activities and enhance laccase activities within a short period. These findings would be helpful for providing a new method for rapid production of low-cost laccase and for optimization of integrated industrial laccase production.