Are the bacteria in apples good or bad?

Flesh Apple on a branch

Does the doctor actually holding an apple away a day, as the saying goes? This is largely dependent upon what types of bacteria the apple bears, according to a new study.

All apples have millions of bacteria, but new research has found that not all of them are equally healthy.

Apples are amongst the world’s most popular fruits. According to some statistical estimates, the USA alone produced around 5.13 million tons of apples in 2018.

And in 2015, a study published in the journal Pediatrics found that 18.9 per cent of the total intake of a child’s fruit consisted of apples.

This much-loved fruit is an excellent food source, containing vitamin C, several B-complex vitamins, natural antioxidants, and many minerals. Apples also provide a good source of dietary fibre.

As with any other raw food, however, apples are also a source of micro-organisms that enter and colonize the gut. Although this exchange of bacteria typically is temporary, it can have health implications.

A team of researchers from the University of Technology in Graz in Austria recently decided to find out more about the bacteria that come with our “apple a day.”

More specifically, this were interested in finding out if there was a difference between the bacterial communities carried by organic, hand-grown apples and those present in typical store-bought apples, which have often had greater exposure to chemical fertilizers and pesticides.

Findings from the team, which appear in the Microbiology journal Frontiers, illustrate how many bacteria a typical apple holds. This will also demonstrate how the bacterial populations in organic apples can affect the taste of the fruit and the health of the customer.

Not all apples are equal

“The bacteria, fungi and viruses in our food colonize our gut transiently. Cooking kills most of these, so raw fruit and veg are particularly important sources of intestinal microbes,” explains senior author Prof. Gabriele Berg of the study.

“While recent studies have mapped the fungal content of[ apples],” Berg says, “less is known about the bacteria in apples,” and that is what has shaped the subject of the current study.

The researchers studied the bacterial content of an apple’s various constituents, including stem, peel, fruit pulp, seeds, and calyx. For organic apples as well as traditional store-bought apples, they did that. They then compared their findings regarding the two fruit types.

The team found that both apple styles harbored a similar amount of bacteria. “Applying the averages for each apple part, we estimate that a standard 240 g[ gram] apple contains around 100 million bacteria,” Prof. Berg says.

The team also found most of the bacteria in the seeds of the apples were gone, and most of the remaining bacteria had collected in the pulp.

There were clear differences in organic and normal store-bought apples between the bacterial populations present.

“Freshly harvested, organically controlled apples host a bacterial population that is considerably more complex, more even and distinct than conventional ones,” notes Prof. Berg.

“This variety and balance would be expected to reduce overgrowth of any one plant, and previous studies reported a negative correlation between the abundance of human pathogen and the diversity of fresh produce microbiomes,” she continues.

In regular store-bought apples, potentially harmful bacteria seemed to be more abundant. By contrast, organic apples appeared to contain a more significant number of healthful bacteria.

EscherichiaShigella — 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,” says the senior author.

According to Prof. Berg, different bacterial populations may also explain why organic apples may taste better than regular ones: “Methylobacterium, known to enhance the biosynthesis of strawberry flavor compounds, was significantly more abundant in organic apples; particularly on peel and flesh samples, which generally had a more diverse microbiota than seeds, stem or calyx.”

“Our findings are strikingly in line with a recent study on the apple fruit-associated fungal population, which demonstrated the specificity of fungal varieties to different tissues and management practices,” adds the lead author of the study, Birgit Wasserman, who is a PhD student at Prof. Berg’s lab.

Looking at the current results, the authors of the study propose that researchers will replicate this experiment for other types of fruit to find out whether other dietary staples still vary on the basis of cultivation methods in terms of their bacterial content.

“The microbiome and antioxidant profiles of fresh produce may one day become standard nutritional information, displayed alongside macronutrients, vitamins, and minerals to guide consumers,” says Wasserman.

“Here, a key step will be to confirm to what extent diversity in the food microbiome translates to gut microbial diversity and improved health outcomes.”

Birgit Wasserman


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