Nanoparticles may influence gut bacteria (which are shown here under the electron microscope).
Researchers from the University Medical Center of Mainz in Germany and colleagues from other centers in Germany, Austria, and the United States have discovered that the ultra-tiny particles can bind to gut bacteria.
In a study paper about their work — which now appears in the journal npj Science of Food — the authors explain how attachment to nanoparticles can alter the life cycle of gut bacteria and their interactions with their host's body.
The results should be useful to both medicine and the food industry. They could, for instance, lead to research into the use of nanoparticles in probiotics.
One example of this is the scientists' observation that synthetic nanoparticles can prevent infection by Helicobacter pylori.
"Prior to our studies," says study senior author Roland H. Stauber, a professor in the Department of Otolaryngology, Head, and Neck Surgery at Mainz University Medical Center, "nobody really looked whether and how nano-additives directly influence the gastrointestinal flora."
Use of nanoparticles is growing rapidly
Nanotechnology manipulates materials at the nanometer scale, which is around the same scale as that of atoms and molecules. One nanometer is 1 billionth of a meter, which means that there are 25,400,000 of them in 1 inch.
In their study background, Prof. Stauber and colleagues describe how the use of nanoparticles is rising rapidly in many fields. These range from medicine and agriculture to the manufacture of personal care products and food processing.
The food industry, for example, uses synthetic nanoparticles to lighten and color food, deliver nutrients, and prevent infection.
All of these can enter the human gut "as part of nano-enabled foods and beverages," report the study authors.
Nanoparticles are of interest not just because they are very small, but also because the materials that comprise them have unique properties at the nanoscale.
Compared with bigger particles derived from the same materials, nanoparticles have a much larger surface area relative to their size, have "greater Brownian motion," and are able to cross biological barriers. These barriers include the mucus layer that lines tissues such as the gut.
For these reasons, their fate in the human gut is likely to differ greatly from that of larger-scale counterparts derived from the same materials.
According to the study authors, "It is, therefore, important to ensure that any nano-enabled food ingredients are safe for application in foods."
The human gut and its microbiome
The human gut, or gastrointestinal tract, digests around 60 metric tons of food during the average lifespan. Over millennia, the human gut and the huge colonies of microbes that occupy it have developed a relationship that is both complex and mutually beneficial.
As the partnership has evolved, gut microbes have come to play a key role in human health and disease.
Gut microorganisms comprise mostly bacteria; they also include fungi, viruses, and single-celled organisms called protozoa.
Scientists use the term gut microbiome to refer to the sum of all the genomes of the trillions of microorganisms in the gut.
The 3 million genes in the gut microbiome vastly outnumber the 23,000 in the human genome. They also produce thousands of small molecules that carry out many functions in the human host.
In this way, gut bacteria help digest food, harvest energy, control immunity, and protect against pathogens.
However, imbalances in the gut microbiome can disturb these crucial functions to either trigger disease or fail to protect against it.
Studies have linked imbalance in the microbiome to cardiovascular diseases, allergies, cancer, obesity, and psychiatric conditions.
All nanoparticles bind to gut bacteria
Prof. Stauber and his colleagues set up experiments in which they could examine the effects of a wide range of synthetic nanoparticles.
These experiments simulated the journeys that the different particles might make as they travel through the different parts of the gut and encounter various bacteria.
The main result was that all the "currently used or potential future nanosized food additives" showed ability to bind to bacteria in the gut.
The nanoparticles bound to all kinds of bacteria, including the "probiotic" species that can breed in milk products such as yogurt.
While all the synthetic nanoparticles that they tested attached to bacteria, the researchers noticed differences in their binding properties.
When bound to nanoparticles, the bacteria altered their behavior in some ways that might prove beneficial and in other ways that might not.
A potential outcome that could be of benefit is the inhibition of infections, for example by H. pylori. The team made this discovery when experimenting with silica nanoparticles in cell cultures.
However, a potentially disturbing prospect that came up in other experiments was that binding to nanoparticles could render some unfriendly bacteria less visible to the immune system. Such a result could increase inflammation responses, for instance.
An important point the authors make is that food also contains naturally occurring nanoparticles — some of which can enter the food during preparation.
The team also ran experiments on natural nanoparticles and was surprised to find similar results to the experiments with synthetic nanoparticles.
"It was puzzling that we were able to also isolate naturally occurring nanoparticles from food, like beer, which showed similar effects."
Prof. Roland H. Stauber