Besides containing fibre and vitamins, an apple also carries about 100 million bacteria, and whether these microbes are good or bad for health may depend on how the fruit was grown, according to a study.
The study, published in the journal Frontiers in Microbiology, shows that organic apples harbour a more diverse and balanced bacterial community -- which could make them healthier and tastier than conventional apples, as well as better for the environment.
"The bacteria, fungi and viruses in our food transiently colonise our gut," said Professor Gabriele Berg, of Graz University of Technology, Austria.
"Cooking kills most of these, so raw fruit and veg are particularly important sources of gut microbes," Berg said.
To help us choose our colonic colonists wisely, the team analysed the microbiome of one of the world's favourite fruits: the apple.
"83 million apples were grown in 2018, and production continues to rise. But while recent studies have mapped their fungal content, less is known about the bacteria in apples," said Berg.
The researchers compared the bacteria in conventional store-bought apples with those in visually matched fresh organic ones.
Stem, peel, flesh, seeds and calyx— the straggly bit at the bottom where the flower used to be—were analysed separately.
Overall, the organic and conventional apples were occupied by similar numbers of bacteria.
"Putting together the averages for each apple component, we estimate a typical 240 gramme apple contains roughly 100 million bacteria," said Berg.
The majority of the bacteria are in the seeds, with the flesh accounting for most of the remainder. So, if the core is discarded, the intake falls to nearly 10 million, researchers said.
"Freshly harvested, organically managed apples harbour a significantly more diverse, more even and distinct bacterial community, compared to conventional ones," said Berg.
"This variety and balance would be expected to limit overgrowth of any one species, and previous studies have reported a negative correlation between human pathogen abundance and microbiome diversity of fresh produce," he said.
Specific groups of bacteria known for health-affecting potential also weighed in favour of organic apples.
"Escherichia-Shigella -- a group of bacteria that includes known pathogens -- was found in most of the conventional apple samples, but none from organic apples. For beneficial Lactobacilli -- of probiotic fame -- the reverse was true," Berg said.
"Methylobacterium, known to enhance the biosynthesis of strawberry flavour compounds, was significantly more abundant in organic apples; here especially on peel and flesh samples, which in general had a more diverse microbiota than seeds, stem or calyx," he said.
The results mirror findings on fungal communities in apples, researchers said.
"Our results agree remarkably with a recent study on the apple fruit-associated fungal community, which revealed specificity of fungal varieties to different tissues and management practices," said Birgit Wasserman, lead author of the study.
The studies show that across both bacteria and fungi, the apple microbiome is more diverse in organically grown fruits, researchers said.