Bacterial community diversity on in-shell walnut surfaces from six representative provinces in China

Lihui Zhang,Shaojin Wang

doi:10.1038/s41598-017-10138-y

Abstract

Walnuts (Juglans regia) have been associated with foodborne illness outbreaks in recent years. Thus, the purpose of this study was to investigate the distribution of bacteria on in-shell walnut surfaces from six representative provinces in China. The bacterial populations on walnut surfaces were investigated by high-throughput sequencing based on the bacterial 16 S rRNA hypervariable region V4. Twenty-eight samples were collected from fourteen regions in six provinces and harvested in different periods (the fresh in 2016 and the old in 2015). Proteobacteria was the most dominant phylum in all samples except for XJ1. In XJ1, and the most abundant phylum was Cyanobacteria, which also accounted for a large proportion of the abundance in YN1, YN11, XJ2 and SC11. In addition, Firmicutes and Actinobacteria were also the abundant phyla in the given samples. Some genera belonging to the opportunistic pathogens were detected, such as Pseudomonas, Acinetobacter, Burkholderia and Bacillus. The results revealed that the composition and abundance of bacterial consortiums on walnut surfaces varied among the geographical sites where they were harvested. Moreover, the storage time of samples also had impact on the abundance of bacteria. This study may provide a better understanding of the bacterial communities’ diversity on in-shell walnut surfaces.

Highlights

Walnuts (Juglans regia) contain many nutrients, including proteins, fatty acid, vitamin E, mineral, folate, melatonin, numerous polyphenols, etc.[1, 2]
A total of 1,867,565 effective tags obtained by high-throughput sequencing (HTS) were clustered into operational taxonomic units (OTUs) based on 97% identity
This study provides the diversity of bacterial community on the in-shell walnut surface based on the high-throughput sequencing method

Summary

Introduction

Walnuts (Juglans regia) contain many nutrients, including proteins, fatty acid, vitamin E, mineral, folate, melatonin, numerous polyphenols, etc.[1, 2]. Due to the unique harvesting methods, the walnut surface is unavoidably infected by a number of microorganisms in the environment, and most likely to be re-infected during the process of storage, transportation and sales. These microorganisms may include some foodborne pathogens, such as Aspergillus, Salmonella, Shigella, Listeria monocytogenes, and Escherichia coli O157:H710, which can cause foodborne illness and even death in humans. The molecule-based methods, such as PCR, qPCR, and mPCR, are frequently used because of time-efficient and labor-saving[21, 22] While these methods have limitations, they cannot reveal the diversity of overall pathogens in the samples. HTS technology has been applied in many fields to investigate the microbial diversity, including food samples[23, 24], plants[25], soil and water samples[26,27,28,29,30]

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