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

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.

Figure 1

Rarefaction curves of the twenty-eight samples.

Figure 2

Flower diagram of the OTUs in the twenty-eight samples (A) the fresh samples in 2016 and (B) the samples in 2015 in different regions. The numbers inside the diagram indicate the numbers of OTUs.

Figure 3

Composition of bacterial community at phylum level. The top ten phyla are shown, and the other phyla are included as “Others”.

Figure 4

Composition of bacterial community at family level. The top ten families are shown, and the other phyla are included as “Others”.

Figure 5

Heat map of the top 35 genera for all the samples. The color intensity with the legend at the right of the figure indicates the relative values for each genus.

Figure 6

PCoA (A) and NMDS plot analysis (B) based on un-weighted Unifrac metrics for all samples.

Figure 7

UPGMA clustering tree based on un-weighted Unifrac distance. The relative abundances of the top ten phyla in all samples are indicated, and the rest phyla are indicated as ‘Others’.

Figure 8

Sampling sites of walnuts (the original figure was available at http://www.map1000.com/, and then updated with mapinfo8.5). Samples were obtained from fourteen regions including the fresh walnuts in 2016 and the stored walnuts harvested in 2015.