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Assignment #2

Review: Lactobacillus plantarum strain maintains growth of infant mice during chronic undernutrition

        Schwarzer et al. study the role of microbiota in juvenile growth of infant mice when fed either standard diet or nutritionally depleted diet. They discover that postnatal systemic growth depends on the interaction between intestinal microbiota and the somatotropic hormone axis. The somatotropic hormone axis (Figure S1) contains the pituitary gland that produces Growth Hormone (GH), which induces the production of insulin-like Growth Factor-1 (IGF-1) and IGF-1 binding protein-3 (IGFBP-3)
. By feeding wild-type (WT) and germ-free (GF) infant male mice with standard breeding diet, they discovered that GF mice weighed less and were shorter than WT mice; moreover, both IGF-1 and IGFBP-3 concentrations and Igf1 and Igfbp3 expression were reduced in GF mice. In addition, by treating the WT mice with picropodophyllin (PPP), a noncompetitive inhibitor of IGF-1R, growth was delayed so the researchers concluded that IGF-1 activity is necessary for postnatal growth. Thus, gut microbiota, by facilitating the somatotropic activity, sustains postnatal somatic tissue growth, leading to increase in mass and longitudinal growth.
        When both WT and GF mice were introduced to nutritionally depleted diet, the group analyzed how the gut microbiota influenced the somatotropic axis and discovered that WT mice resumed growth in both body weight and length, although to a lesser extent than mice fed with the standard diet. They also identified specific Lactobacilli strains that supported juvenile growth even during chronic undernutrition. They tested two Lactobacillus plantarum strains, LpWJL and LpNIZO2877, and both strains produced higher weight and body length than GF mice under the condition of chronic undernutrition.


Figure S1. The somatotropic axis and associated molecular markers.


        Overall, I thought this paper is easy to follow and relatively compelling based on the results presented. This experiment demonstrates another important function of gut microbes, which have been studied by many to be essential in maintaining life on earth. The paper has a logical flow: I understand why each experiment was conducted and each experiment addresses a specific question that explains the effect of microbiota in juvenile growth. The data clearly shows the relationship between gut microbiota and the somatotropic axis and how they affect postnatal systemic growth in infant mice. Most importantly, this paper has great implication for public health because malnutrition still affects many children in developing countries, and malnutrition is not always remedied by improvement in nutrition. This study provides insights into how the introduction of gut microbiota can potentially mediate some of the pathology. Therefore, I would like to see what species of microbiota are in the WT mice since they outperformed the two Lactobacillus strains monocolonized mice in growth as measured by both body weight and length.

        However, there can be improvements in data visualization in this paper. I found the order of the figures in this paper rather confusing. For example, figure 4 was referenced in the paper before figure 3 so it might be easier for the readers to follow if they rearrange the order. Also, while panel A to C in Figure 4 addresses how the somatotropic axis activity was reduced in GF mice during chronic undernutrition, panel D to G discusses that the somatotropic axis is required for juvenile growth; the two are not closely related, so it might be better to have them as two separate figures. Furthermore, in Figure 4 panel A, they switch the position of GF and WT as opposed to Figure 1 where WT is on the left and GF is on the right; it will be nicer if they can be consistent throughout the paper.


(All figures below courtesy of Schwarzer et. al., 2016)

Figure 1