Scientists studied seven strains of a popular probiotic bacteria called L. rhamnosus that were found in people’s bloodstreams. Using advanced DNA sequencing technology, they discovered that these bacteria have different genetic traits than the most common probiotic strain used in supplements. Some of these bacteria had genes that could help your health, but others had genes that might cause problems. The research shows that not all L. rhamnosus bacteria are the same, and companies need to be careful about which strains they put in supplements and foods to make sure they’re safe.

The Quick Take

  • What they studied: Researchers looked at the genetic makeup of seven strains of L. rhamnosus bacteria that were found in patients’ blood to understand what makes them different from the probiotic strains used in supplements.
  • Who participated: Seven different samples of L. rhamnosus bacteria isolated from blood of patients (specific patient numbers and demographics not detailed in the abstract).
  • Key finding: The bacteria found in blood were genetically different from the most popular probiotic strain (L. rhamnosus GG) used in commercial products. Some strains had helpful genes, but others had genes that could potentially cause harm.
  • What it means for you: If you take L. rhamnosus probiotics, this research suggests manufacturers should test which specific strains they’re using to make sure they’re safe. Not all L. rhamnosus bacteria are equally beneficial or safe, so strain selection matters.

The Research Details

Scientists collected seven strains of L. rhamnosus bacteria that had been found in patients’ blood samples. They used two different advanced DNA-reading technologies (Illumina and Oxford Nanopore) to map out the complete genetic code of each strain. This hybrid approach allowed them to get very detailed and accurate genetic information. They then compared these blood-derived strains to other L. rhamnosus strains from different sources like dairy products, supplements, plants, and the human digestive system that were already documented in scientific databases. The researchers looked for specific genetic patterns, special defensive structures called CRISPR modules, and genes related to both helpful and harmful functions.

Understanding the genetic differences between probiotic strains is crucial because bacteria with the same species name can behave very differently. Some genes that help bacteria survive in your gut might also help them cause infections. By mapping the complete genomes of these blood-derived strains, scientists can identify which strains are truly safe for use in supplements and which ones might pose risks to vulnerable people.

This study provides detailed genetic information from seven real-world bacterial samples found in patients, which is valuable for understanding potential risks. However, the study is relatively small in scope (seven strains) and focuses on bacteria that were already in blood, which is unusual. The research doesn’t tell us how common these problematic strains are in commercial supplements or how often they actually cause illness. The findings are descriptive rather than experimental, meaning they show what genes are present but don’t prove those genes actually cause harm in people.

What the Results Show

The seven L. rhamnosus strains found in blood had complete genetic codes ranging from 2.96 to 3.13 million base pairs (the building blocks of DNA). These strains were genetically related to L. rhamnosus bacteria from many different sources worldwide, including food, supplements, and human body sites. Importantly, none of these blood-derived strains were genetically identical to L. rhamnosus GG, which is the most widely used probiotic strain in commercial products. This suggests that the bacteria found in blood are different from what most people are taking in supplements. The researchers found that these strains contained genes for producing bacteriocins (natural antibiotics made by bacteria), helping bacteria stick to body tissues, and promoting health—but they also found genes that could potentially cause problems.

The analysis revealed that some genes responsible for helpful probiotic functions were the same genes found in harmful bacteria that cause infections. This dual nature of certain genes is important because it means a gene that helps bacteria survive in your gut might also help it survive in your bloodstream where it shouldn’t be. The researchers also found that these blood-derived strains had different defensive genetic structures (CRISPR modules) compared to the popular GG strain, suggesting they evolved differently and may behave differently in the body.

Previous research has shown that L. rhamnosus is generally safe and beneficial for most people, which is why it’s so popular in supplements. However, rare cases of infection from L. rhamnosus have been reported, mostly in people with weak immune systems or serious underlying health conditions. This new research helps explain why some strains might be riskier than others—they have different genetic traits. The findings support the idea that not all probiotic bacteria are created equal, and the specific strain matters for both safety and effectiveness.

This study examined only seven strains of bacteria that were already found in blood, which is a very small sample. We don’t know how common these strains are in commercial supplements or how often they actually cause problems in real people. The research identifies which genes are present but doesn’t prove those genes actually cause harm—it’s like finding a tool in someone’s toolbox without knowing if they’ll use it. The study also doesn’t tell us about the actual risk to healthy people versus people with weak immune systems. Finally, the research doesn’t test whether these strains are more or less likely to cause infection than other bacteria.

The Bottom Line

Based on this research, probiotic manufacturers should identify and test the specific strains they use rather than just listing ‘L. rhamnosus’ on labels. Consumers, especially those with weak immune systems, should choose supplements that clearly identify the exact strain (like ‘L. rhamnosus GG’) and come from reputable manufacturers. Healthy people taking standard probiotic supplements are unlikely to be at risk, but this research suggests the industry needs better quality control. Confidence level: Moderate—this research identifies a potential concern but doesn’t prove it’s a major problem.

People with weak immune systems, serious infections, or critical illnesses should be especially cautious about probiotic supplements and should consult their doctor before using them. Healthy people taking commercial L. rhamnosus supplements are unlikely to be affected, but may want to choose products with clearly identified strains. Parents considering probiotics for children should also be aware of strain differences. Healthcare providers treating patients with infections should know that some L. rhamnosus strains can occasionally be found in blood infections.

This research doesn’t address how quickly probiotics work or how long benefits take to appear. Rather, it’s a safety and quality concern that affects which products you should choose. If you’re already taking a probiotic supplement, you don’t need to stop immediately, but you might want to check the label to see if it specifies the strain.

Want to Apply This Research?

  • If using a probiotic supplement, track the specific strain name and manufacturer. Note any digestive changes, energy levels, or illness episodes weekly. This creates a personal record to discuss with your doctor if any concerns arise.
  • When shopping for probiotic supplements, look for products that list the specific strain (like ‘L. rhamnosus GG’) rather than just the species name. Take a photo of the label with your phone and save it in your app for future reference.
  • Maintain a monthly log of your overall health, any infections or illnesses, and which probiotic product you’re using. This helps you and your healthcare provider identify any patterns. If you switch brands or strains, note the change and monitor for any differences in how you feel.

This research describes genetic characteristics of bacteria found in blood and does not prove these strains are dangerous or that commercial probiotics are unsafe. Most people taking L. rhamnosus supplements experience no problems. However, people with weakened immune systems, serious infections, or critical illnesses should consult their healthcare provider before taking any probiotic supplements. This article is for educational purposes and should not replace professional medical advice. If you have concerns about probiotic safety or are experiencing unusual symptoms, contact your doctor immediately.