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Many times in science, a research track is followed after an unexpected observation. Sometimes this comes from results not fitting with the proposed hypothesis but other times, it is something not relevant to the study undertaken but seems to be an interesting side effect. The impetus for the PLOS ONE article by Levkovich et al. began with a simple observation: mice fed probiotic yogurt in gastrointestinal studies had thicker, shinier fur than those that did not.
Male and female C57BL/6 mice, which have brown fur, were fed probiotic yogurt starting at 20–24 weeks of age and examined 20–24 weeks later. Levkovich et al. noted that even a week on yogurt, there was a visual difference in fur on mice fed yogurt versus control-fed mice. Fur luster was assessed both by human observation as well as refractometry. The latter measurement showed a significant difference in fur for females fed yogurt compared to control diet but not for males.
This shiny fur had to have an underlying cause. Tissue sections of skin were stained and studied by microscopy. This revealed that both male and female mice fed yogurt had thicker skin compared to control-fed mice (457.1 pixels versus 314.7 pixels for females; 357.9 pixels versus 249.8 pixels for males, respectively).
Because skin thickening occurs during anagen (growth) phase of hair cycling, the authors assessed mouse hair follicles using stained skin sections. Mice fed yogurt had more hair in anagen phase compared to control-fed mice. For male C57BL/6 mice eating probiotic yogurt, the hair cycle profile was 70% anagen, 14% catagen (transitional) and 16% telogen (resting) phase while male mice on the control diet had 64% of hair follicles in telogen phase with only 36% in anagen and none in catagen phase. Female C57BL/6 mice showed a similar difference: yogurt fed had hair follicles in predominantly anagen (62%) phase while control diet-fed mice had 50% of hair follicles in telogen and only 30% in anagen.
Fur luster is more than a function of hair in growth phase; secretions of lipids make the fur appear shiny. When the skin sections were studied under the microscope, Levkovich et al. found more sebocytes in mice fed yogurt versus those on the control diet. In addition, using a protein marker for proliferation, Ki-67, the sebocytes in yogurt-fed mice had a higher index of cell growth compared to control-fed mice. Because there were more sebocytes with a higher proliferation index, the authors suggested more cells with more lipids result in shiner fur on mice.
Because acidic pH also contributes to shiny hair, this aspect was also studied. The authors hypothesized that an acidic pH could explain the shinier fur found in female mice fed yogurt compare to male mice fed yogurt. The pH of mouse skin, oral cavity, vaginal mucosa and rectum were taken after death showed more acidic pH in female mice eating yogurt compared with those on the control diet. Levkovich et al. referenced another article that studied vaginal pH and microbiome in women, and based on the article’s data, extrapolated that the peak of fertility in women (~25 years of age) correlated with acidic pH and the presence of Lactobacillus species. Levkovich et al. speculated that their results in mice plus this data in women show probiotic bacteria can alter an organism physiology, resulting in radiant skin and shiny hair, which indicate peak health and fertility.
Is yogurt necessary for these changes in mouse skin and fur? Mice fed Lactobacillus reuteri in water showed similar changes to those fed yogurt. Skin thickness increased, more hair follicles were in anagen phase, pH was more acidic and sebocytes had a higher proliferation index compared to mice on control diet. Furthermore, there was no significant difference in skin thickness, hair cycling, sebocyte proliferation or fur luster when comparing mice fed L. reuteri in water with mice fed probiotic yogurt. When a patch of fur was shaved, the mice consuming L. reuteri regrew the fur faster than mice fed a control diet.
Based on other studies, Levkovich et al. hypothesized these skin benefits seen in mice consuming probiotic bacteria may be due to anti-inflammatory mechanism and chose to study cytokine Interleukin (IL)-10. C57BL/6 mice lacking IL-10 were fed L. reuteri or water only but there was no difference in skin thickness, number of subcutaneous hair follicles, anagen phase of hair, sebocyte number or sebocyte proliferation index. For female mice, the pH was more alkaline. Thus, IL-10 is required for the skin and fur effects seen when mice eat probiotic bacteria.
IL-10 is known to downregulate proinflammatory cytokines like IL-17. Therefore, IL-17A was depleted in untreated aged wild type C57BL/6 mice, resulting in increased skin thickness, more numerous hair follicles, hair follicles predominately in anagen phase with more sebocytes and increased sebocyte proliferation index; the same results seen in mice fed yogurt or L. reuteri.
Levkovich et al. also examined the effects of feeding L. reuteri to white-furred Swiss mice. Similar to the brown-coated C57BL/6 mice, the skin was thickened, sebocytes proliferated, hair follicles were more numerous, IL-10 was upregulated and IL-17A was downregulated.
The observations made by Levkovich et al. for mice fed probiotic bacteria imply that yogurt and L. reuteri make older mice appear younger and more attractive. While I do not believe mice are going to be nudging each other saying “Check out the shiny fur on that one”, I would be interested to learn if older mice fed yogurt attract the same number of mice as younger mice to better confirm this “glow of health” as the authors refer to the skin and fur changes.
Levkovich T., Poutahidis T., Smillie C., Varian B.J., Ibrahim Y.M., Lakritz J.R., Alm E.J., Erdman S.E. & Kobinger G.P. (2013). Probiotic Bacteria Induce a ‘Glow of Health’, PLOS ONE, 8 (1) e53867. DOI: 10.1371/journal.pone.0053867.g006