Improving nitrogen source utilization from defatted soybean meal for nisin production by enhancing proteolytic function of Lactococcus lactis F44

Jiaheng Liu,Jianjun Qiao,Jianjian Zhou,Zhiqiang Ge,Zelin Ma,Lihong Wang,Guangrong Zhao,Hongji Zhu

doi:10.1038/s41598-017-06537-w

Abstract

Nisin, one kind of natural antimicrobial peptide, is produced by certain Lactococcus lactis strains, which generally require expensive high-quality nitrogen sources due to limited ability of amino acids biosynthesis. Here we use defatted soybean meal (DSM) as sole nitrogen source to support L. lactis growth and nisin production. DSM medium composition and fermentation conditions were optimized using the methods of Plackett-Burman design and central composite design. The highest nisin production of 3879.58 IU/ml was obtained in DSM medium, which was 21.3% higher than that of commercial medium. To further increase the utilization ability of nitrogen sources, we enhanced the proteolytic function in L. lactis through rationally expressing the related enzymes, which were selected according to the compositions of amino acids and molecular weight of peptides in DSM medium. Significantly, an artificial proteolytic system consisting of a heterologous protease (NprB), an oligopeptides transporter subunit (OppA) and two peptidases (PepF and PepM) was introduced into L.lactis. The constructed strain BAFM was capable of achieving efficient biomass accumulation and nisin yield with 30% decreased amount of DSM hydrolysates, which further reduced the cost of nisin production. The strategy described here offers opportunities for low-cost L. lactis fermentation and large-scale nisin production in industry.

Highlights

Nisin, one kind of 34-amino-acid-long natural antimicrobial peptide, could effectively inhibit pathogenic and spoilage microorganisms in food without resistance and allergic reactions[1, 2]
Due to its high consistency, nutrient availability, desirable amino acid content and low cost[21,22,23], Defatted soybean meal (DSM) has great potential to serve as nitrogen source for L. lactis fermentation
A new strategy to improve the efficiency of nitrogen source utilization via enhancing proteolytic function was proposed, which could significantly reduce the amount of DSM hydrolysates in the medium

Summary

Introduction

One kind of 34-amino-acid-long natural antimicrobial peptide, could effectively inhibit pathogenic and spoilage microorganisms in food without resistance and allergic reactions[1, 2]. L. lactis generally requires large amounts of expensive high-quality nitrogen sources such as tryptone, peptone, yeast extract, corn steep liquor or meat extract In most cases, these nitrogen sources must be incorporated as a formulation, which dramatically increased the costs of large-scale nisin production[10, 11]. The problem of inefficient absorption and utilization is encountered by commercial nitrogen sources such as peptone and yeast extract, which are generally excessive and remain large amount of residues in L. lactis fermentation. To address this issue, a new strategy to improve the efficiency of nitrogen source utilization via enhancing proteolytic function was proposed, which could significantly reduce the amount of DSM hydrolysates in the medium. An artificial proteolytic system consisting of the most effective genes was constructed and transferred into L. lactis F44 to realize more efficient utilization of DSM proteins

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Conclusion