All dog owners have experienced it: their pooch sheds hair everywhere, testing the patience of those who try to keep their home spotless. From his forays outdoors, Fido brings many microorganisms back inside. Their presence modifies the microbiota of the dust in our homes, potentially explaining the protective effect of dogs against asthma.

Fido’s influence on the dust microbiota...

A team of researchers thus carefully examined the dust of 182 rural-suburban homes in Finland (56 with a dog) and 284 urban homes in Germany (18 with a dog). The results were conclusive: Fido’s presence went hand in hand with greater bacterial diversity and the increased abundance of certain bacteria in house dust. Seven bacterial genera were found to be much more abundant in homes with dogs. Conversely, Fido’s influence on fungi in the dust microbiota appeared to be more limited: only one yeast, known to occur in water and soil, was more abundant in homes with dogs.

...and the risk of asthma?

At the same time, bacteria typically associated with humans are reduced by a third when a canine companion lives in the home. All the better, since a high abundance of human-associated microbes may be linked to a greater number of human pathogens and an increased risk of developing asthma. According to the researchers, this relative reduction in “human” bacteria and/or the increase in “canine” bacteria may explain the reduced risk of respiratory infections and asthma in children who grow up with one or more pets. One more argument to support those who dream of adopting a four-legged companion... and of easing up on the housekeeping.

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A major public health issue, geriatric depression may affect up to 25% of elderly people, with lower remission rates and higher relapse rates than in younger adults. This has led to a search for biomarkers predictive of responses to antidepressants. Unsuccessful to date, this search has now reached a turning point. Based on the fact that brain-gut-microbiota interactions modulate mood disorders and that gut dysbiosis appears in late life, researchers have hypothesized that the gut microbiota may predict responses to antidepressant treatments in geriatric depression. The secondary analysis of a randomized controlled trial (12 weeks of Levomilnacipran [LVM] or a placebo) in California involving seniors with major depressive disorder seems to support this hypothesis.

Bacterial genera capable of predicting remission

As no differences were observed in remission rate between the LVM group and the placebo group, the analysis combined the 4 treated patients and the 8 controls. Of these 12 seniors, 5 overcame their depression (score ≤ 6 on the Hamilton scale): they were younger (67 vs. 74 years) and all were male; but there were no differences in pretreatment gut microbiota alpha or beta-diversity between the two groups. However, the content of 9 bacterial genera in the baseline microbiota was accurate in predicting remission. A significant content of Faecalibacterium, and to a lesser extent Agathobacter and Roseburia, was associated with remission.

A gut microbiota that changes in the case of remission

The study also showed that the gut microbiota of remitters (but not that of non-remitters) changed during their recovery from depression. The researchers observed an increase in certain taxa, including Flavonifractor and DTU089. According to the authors, the increased presence of these bacteria may be the result of depression remission (better diet, increased physical activity, improved sleep, reduced stress, etc.), although they have no known antidepressant effects.

Personalized medicine to treat geriatric depression soon at hand?

Despite this, the relationship between the gut microbiota and depression in aging remains poorly understood. Does a slow decline in the immune system (immunosenescence) lead to a progressive increase in chronic inflammation? Does this increase alter the gut microbiota (loss of diversity)? While questions remain, this is the first study demonstrating that the gut microbiota can predict treatment response in geriatric depression, even for treatment via a placebo. If these results are confirmed by larger prospective studies, they could pave the way for personalized medicine capable of selecting the appropriate antidepressant based on the microbiota and predicted efficacy, or of treating depression by reinforcing certain beneficial taxa.

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Trastuzumab is the first therapeutic option for women with HER2-positive breast cancer and works by blocking the HER2 receptor. Recent studies have shown that it also has immunomodulatory properties. Although effective against HER2-positive breast cancer, a large number of patients present or develop resistance to this treatment. To try to understand this, the authors of the study focused on the gut microbiota, which has been implicated in the efficacy of both chemotherapy and immunotherapy in other types of cancer via the modulation of host immunity. The study involved experimental models and 24 women with HER2-positive breast cancer.

Efficacy mediated by the microbiota...

First the researchers demonstrated in murine models that the efficacy of the treatment was influenced by the gut microbiota. Mice exposed to antibiotics–either directly or via a fecal microbiota transplant from mice that had received antibiotic treatment–saw the complete suppression of trastuzumab’s tumor growth inhibition effect. At the same time, antibiotic exposure not only led to changes in the tumor immune microenvironment, but also altered the gut microbiota. More precisely, the study showed that the modification of the gut microbiota affected intestinal mucosal immunity and systemic cytokine circulation (impaired recruitment of CD4+ T cells and GZMB+ cells in tumors). For the authors, all of this ties in: alterations to the gut microbiota modify the tumor immune microenvironment, which in turn reduces the efficacy of treatment.

...with confirmation in patients with HER2-positive breast cancer

The study subsequently focused on 24 patients with HER2-positive breast cancer treated with trastuzumab. An analysis of their gut microbiota revealed a lower α-diversity and a lower abundance of certain bacteria in nonresponsive patients (NR) compared to responsive patients (R), similar to antibiotic-treated mice. In addition, the transplant of fecal microbiota from R and NR patients into mice resulted in the same response to trastuzumab observed in the patients. Lastly, fecal microbiota β-diversity distinguished patients according to their response to treatment, independently of tumor intrinsic subtype. For the authors, the direct involvement of the gut microbiota offers promising therapeutic strategies involving the manipulation of gut bacteria to improve the efficacy of anti-HER2 therapy. Furthermore, it presents a potential biomarker for treatment response.