Two yogurt starter cultures cut how much nanoplastic got inside gut cells, and the particles never touched the bacteria

What internalisation is

Internalisation is the step where a particle stops being something passing through the gut and becomes something inside a cell. Cells pull material in by folding their own membrane around it, and once a particle is inside it can be handed onward into the tissue and the bloodstream rather than continuing along the intestine and out.

Drawn from the paper's introduction and background cell biology.

Why they ran it

The authors start from a gap they state directly: plenty is known about nanoplastics being taken into cells and causing oxidative stress and cell death, and nothing has been proposed for preventing that uptake in the first place. Because both strains were already documented as repairing barrier damage in the small intestine, the hypothesis was that they would also blunt what the particles do.

Hypothesis The authors hypothesised that Lactobacillus delbrueckii subspecies bulgaricus 2038 and Streptococcus thermophilus 1131 can protect against the adverse effects of nanoplastic exposure.

Drawn from the paper's introduction.

In a differentiated human intestinal cell monolayer, polystyrene nanoplastics were taken into the cells, entering by macropinocytosis and the clathrin route. Blocking either one reduced uptake.

Both yogurt starter strains significantly reduced uptake, Lactobacillus delbrueckii subspecies bulgaricus 2038 at p equals 0.002 and Streptococcus thermophilus 1131 at p below 0.001.

Heat killed bacteria worked as well as live ones, with no significant difference between them. Adding the bacteria first and then washing them away before the plastic arrived also worked.

The team tested whether the particles were simply sticking to the bacteria by passing the mixture through a filter fine enough to trap the bacteria. No fluorescence came off the trapped bacteria and the particles stayed in the flow through, so the two were not aggregating.

Both strains also reduced how much plastic crossed the monolayer to the far side, at p below 0.001 and p equals 0.02.

Gene set analysis showed plastic exposure raising an oxidative stress response and lowering DNA base excision repair. Neither strain significantly restored either of those two gene sets.

The numbers

Reduction in uptake	p equals 0.002 for strain 2038 and p below 0.001 for strain 1131
Heat killed bacteria	also effective, p equals 0.002 and p equals 0.009, no significant difference from live
Pre treatment then washout	still effective, p equals 0.01 and p equals 0.003
Aggregation test	no fluorescence recovered from filtered bacteria, particles remained in the flow through
Transcellular permeability	reduced, p below 0.001 and p equals 0.02
Oxidative stress gene set	enriched by plastic at p equals 0.011, not significantly restored by either strain at p equals 0.190 and p equals 0.796
Funding and affiliation	all four authors are employees of Meiji Holdings, which owns the commercial starter strains tested

Why this might happen

Proposed by the authors This is the explanation the authors offer in their discussion. This study did not test it.

Three explanations were tested and eliminated. Dead bacteria worked as well as live ones, so it is not metabolism. Adding bacteria and then washing them away before the plastic arrived still worked, so it is not physical crowding at the cell surface. And the filter test found no particles stuck to the bacteria, so it is not cargo.

What is left, in the authors' reading, is signalling. Their proposal is that surface components of these bacteria are recognised by a receptor on the gut cell, and that this dials down the machinery the cell uses to pull material in.

They are explicit that this last step is a hypothesis assembled from other people's work and that it needs testing.

Drawn from the paper's discussion.

What this does not show

This does not show the cells were protected from damage. Uptake fell, but the gene sets that moved with plastic exposure, an oxidative stress response and DNA base excision repair, were not significantly restored by either strain. Less plastic getting in did not translate into the damage signature going away.
This does not show the bacteria carry plastic out. The opposite. The authors tested for aggregation between the bacteria and the particles and found none, which rules out the cargo explanation for this result.
This does not identify the mechanism. The authors say the route remains unknown and offer a signalling hypothesis they did not test. What they established is a list of explanations that do not fit.
This does not involve a living gut. A cell monolayer in a dish has no mucus layer worth the name, no resident microbial community, no food and no transit. The authors state that only in vitro work was done and that human studies are the plan.

Where this leaves us

The field now contains a direct contradiction rather than a consensus. One group reports plastic being carried out by bacteria that hold it, and this group reports plastic being kept out of cells by bacteria that demonstrably do not hold it.

Human gut cells in a dish, one plastic type, two commercial starter strains.

Testing the proposed receptor route directly, and repeating the aggregation test in the other groups' systems to find out whether the two sets of results are describing different organisms or different assays.

Caveats worth holding

All four authors are employed by the company that owns the strains, and the paper's stated conclusion favours consuming them. The negative results reported against the company's interest are the reason this study earns its place.
One particle type only, carboxylate modified polystyrene, which the authors list as a limitation.
In vitro only. The authors state that human clinical studies are planned.