No, you don’t have two kilos of bacteria in your body, and their imbalance doesn’t cause most diseases.

The study of the bacteria living in our gut, the so-called microbiome, is one of those cutting-edge areas of research that has come into vogue. So much so that some exaggerations and lies have crept into rigorous scientific information that could harm future research on the subject. This is what two British researchers, Alan Walker and Lesley Hoyles, are defending, writing an article this Monday in the journal Nature Microbiology illustrating the myths or misconceptions that dominate the human microbiome literature.

Bacteria are becoming more and more resistant


“While this is indeed interesting, the growing focus on microbiome research has unfortunately also brought with it a buzz and perpetuated certain misconceptions,” they write. “As a result, many unsubstantiated or insufficiently substantiated claims have become ‘facts’ by constant repetition.”

They note that some bacterial hoaxes are more common than others and some are relatively trivial, but collectively they highlight how widespread misinformation is around the human microbiome. And, “given the potential importance of human microbiomes to health, it is important that claims are based on evidence,” they stress.

Here are some of the main myths about the microbiome:

Our bacteria do not weigh from 1 to 2 kg.

This unreliable information even appears in some scientific studies, although it is often not indicated where it comes from. “Most of the human microbiota resides in the large intestine, and these microorganisms typically make up less than half the weight of solid feces,” the authors note. “It is much more likely that the total weight of the human microbiota is less than 500 g, and in some cases, perhaps even significantly less.”

There are not ten bacteria for every human cell.

This myth was buried when, in 2016, Sender, Fuchs, and Milo at PLOS Biology reworked the calculations and found the source of the overestimation in 1970s work. “More detailed analysis shows that the real figure, while still impressive, is probably closer to a 1:1 ratio,” the paper’s authors say. “It should be noted that the ratio likely varies from person to person and depends on factors such as host body size and the amount of fecal material carried in the colon.”

Altered microbiome does not explain obesity

From a series of studies in rodents and individual or narrow studies in humans, the claim has been made that there is a link between bacterial populations living in our gut and obesity. “However, as with many other studies reporting associations between specific microbiota profiles and diseases, reproducibility is poor,” the authors note. “In fact, there have been at least three meta-analyses to date reporting that this finding is inconsistent with human studies and that there are in fact no reproducible microbial taxonomic traits of obesity in humans.”

The elusive concept of “dysbacteriosis”

There is also an increasing amount of reading about alleged harmful interactions between microbial communities and their hosts that lead to various diseases, but in a significant proportion of cases it is not at all clear that this is the case. “Unfortunately, this term is too simplistic and misleading in its essence,” the article says. “Microorganisms and their metabolites are neither ‘good’ nor ‘bad’, they just exist.” They also highlight the increasing use of the concept of “dysbiosis”, “a vague term with limited clinical applicability”. “It is very likely that this may contribute to disease progression in some conditions, including inflammatory bowel disease,” they admit. However, such changes are rarely permanent, and the microbiota varies greatly between individuals in both healthy and diseased states. This makes it extremely difficult to identify gut microbiota samples with the specificity and reproducibility needed for clinical practice.

“Attempts to identify ‘tipping points’ at which changes in microbiome composition definitively influence disease progression have not yet led to a clear consensus due to lack of consistency between different studies,” the scientists note. Therefore, they conclude, it is a leap that is not yet based on evidence to conclude that a characteristic microbiome plays a role in “most” diseases.

Not inherited from mother

“The microbiota is inherited from the mother at birth” is another widespread misinformation, say Walker and Hoyles, who see it as a good example of how important nuance is in describing the human microbiome. “While some microbes are passed directly from mother to child during birth,” they note, “proportionately few microbiota species are truly “hereditary” and persist in offspring from birth to adulthood.” Every adult has a unique microbiota configuration, remember, even identical twins raised in the same home. Therefore, factors such as environment, diet, antibiotic use, and genetics carry much more weight than the alleged direct “inheritance” from the mother, which plays a secondary role.

It’s not that new science.

Although the media continues to talk about this science as something new, the truth is that it has a very long history. “The pace of research into the human microbiome has accelerated significantly over the past 15 years, but the field is not in its infancy,” Walker and Hoyles note. “Indeed, there is a rich history of human-related microorganism research since at least the late 19th century. Escherichia coli was first isolated in 1885, bifidobacteria were described in 1899, and Mechnikov was reflecting on the importance of beneficial intestinal microorganisms in the early 20th century. It is also not true, as often read in many sources, that Joshua Lederberg coined the term “microbiome” in 2011, as it has been in use in the field for at least a decade.

Against dogmas and simplifications

country rose, Research Fellow at the Ramon y Cajal Hospital and member of the Specialized Group for Human Microbiota (SEIMC-GEMBIOTA), believes the article is timely. She knows this firsthand, because she is one of those specialists who often encounter misconceptions about “dysbacteriosis”. “This is the first thing that patients learn, despite the fact that the “normal” composition of the microbiome has not been established,” he explained to “The article is as correct as possible, we have been talking about this for almost 20 years and there are not even standardized procedures. In fact, it was not included in the public health system, since the breakdown of the composition is not directly related to the patient.

There are no good or bad bacteria/microbiomes, it all depends on context and location.

The researcher recalls that studies on mouse models are necessary for science, but agrees that they often distort reality compared to humans. “What remains for me is that there are no good or bad bacteria/microbiomes, it all depends on the context and location (for example, we all have coli in the intestines, but when it enters the urinary tract, it causes an infection).

Tony Gabaldon, head of the comparative genomics group at the Institute for Research in Biomedicine (IRB Barcelona) and the Barcelona Supercomputing Center (BSC-CNS), believes that those working in the field have these clear ideas and it is more of a problem for those who works in other areas or the public at large, leading to confusion. “The microbiome field (like other rapidly evolving fields) suffers from dogmas and simplifications made early on from very limited information that we need to correct and modulate as we learn more,” he says in a statement. for Science Media. Center (SMC).

We live in the golden age of the microbiota, the stone age. But there is no doubt about the enormous impact our microbes have on health.

Rob Knight, director of the Center for Microbiome Innovation and professor at the University of California, San Diego, also believes that most of these errors and misunderstandings are well known to experts in the field, but “it’s helpful to bring them together in one review.” place, primarily because nowadays many people are joining the countryside,” he tells SMC. In his opinion, the irreproducibility of microbiome-disease associations is an important issue, but it is more subtle than described in the article. “To be useful as a clinical test,” he notes, “a sample of a microbe or microbiome must not cause disease, but simply act as an accurate marker of disease.”

Ignacio López-Goni, professor of microbiology at the University of Navarra, also believes the claims in the article are broadly correct. “But we must remember that we do not know more than what we know,” he says. “We are not actually living in the golden age of the microbiota, but in the stone age.” Despite the limitations, he assures, there is no doubt that today the study of the microbiota opens up new possibilities in personalized medicine. “We don’t fully understand the mechanisms,” he concludes, “but there is no doubt about the enormous impact our microbes have on health.”

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