Bacterial population structure and dynamics during the development of almond drupes

Abstract

To describe the bacterial populations and their dynamics during the development of almond drupes. We examined 16S rRNA gene libraries derived from the bacterial populations on almond drupes at three stages of development: (i) when the drupes were full sized, but before embryo development, (ii) when the drupe hulls first began to split and (iii) when the drupes were fully mature, but before harvesting. Our data revealed that the immature drupes were colonized by relatively few types of bacteria, belonging mostly to common phyllosphere-associated bacteria within the genera Pseudomonas, Pantoea, Methylobacterium and Sphingomonas. However, after the hulls first began to split, the level of bacterial diversity increased and continued to do so until the drupes were fully mature. At the last sampling period, we observed several sequences belonging to bacteria that are not usually associated with the phyllosphere, including some identical to Salmonella enterica. The bacterial populations on almond drupes before hull split were composed of relatively few types, most of which were commonly associated with the phyllosphere. However, after hull split, the level of microbial diversity increased, which was mostly due to increased levels of bacteria that are not normally associated with the phyllosphere, including Salm. enterica. This is the first report of the bacterial populations associated with almond drupes and their dynamics during development. Of specific significance is the observation that Salm. enterica was present on the drupes just prior to harvesting, which may represent a critical control point.