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"Lactobacillus plantarum strain maintains growth
of infant mice during chronic undernutrition"

Summary:
This paper (Schwarzer et al. 2016) explores the effects of gut microbiota on nutrition-dependent growth of infant mice. Specifically, the team investigates what impacts the presence and absence of L. plantarum strains have in promoting the growth of juvenile mice. They deduce that the bacterium interacts with growth-related hormones in the somatropic axis to allow weight and length growth in germ-free mice comparable to that of wild type mice in both normal breeding diets and nutrient depleted diets. They connect these physiological growth responses to changes in Growth Hormone (GH) and Insulin-Like Growth Factor-1 (IGF-1), and make the claim that the microbiota directly promotes growth by facilitating production and activity of these biological products.

Review:
While this paper investigates an interesting phenomenon of the microbiota's role in growth, it takes what starts as a discovery science paper and develops it into a validation science paper, by the use of the researchers' previously known bacterial strain, L. plantarum, which they had seen to function similarly in Drosphila. By doing so, the investigators limit their findings to validation, instead of looking for any other bacteria that might also impact this response. They didn't look into what bacterial populations were present in the mouse gut naturally, or investigate if other bacteria also caused similar GH and IGF-1 responses. I feel as though the investigators were not doing their responsibility as scientists to take an honest look at a system and explore seemingly unpredicted data, and instead chose to focus only on the data that would validate the idea that they had before beginning investigation. They also conclude that microbial interventions could buffer the adverse effects of undernutrition; however, the microbial interventions would just be towards a typical microbiome (WT mice) instead of no microbiome (GF), as the addition of an exclusively L. plantarum microbiome didn't improve from the WT condition experiencing undernutrition. The real best way to combat undernutrition is with increased nutrition.

Figure 1

The researchers begin by asking what effect the presence or absence of normal gut microbiota has on juvenile mouse growth post-weaning. They fed both wild type (WT) and germ free (GF) mice on a standard breeding diet, and measured their subsequent weight gain, longitudinal growth, and skeletal growth. They found that GF mice had significantly lower weights and smaller body lengths (including skeletal growth) than their WT counterparts, especially after weaning.

Figure 2

They then looked at what effect the presence or absence of gut microbiota had on the somatotropic axis. The researchers measured the circulating levels of growth hormone (GH), insulin-like growth factor-1 (IGF-1), and its binding protein (IGFBP-3), all major factors in regulating growth. They found that for both GF and WT mice, the GH levels peak around birth and gradually decline. However, IGF-1 and IGFBP-3 are present in significantly higher amounts in the sera of WT mice than in that of GF mice. They also saw that expression levels of the Igf1 and igfbp3 genes in the liver drop in the GF mice when their products peak in the WT mice. Phosphorylation of AKT in the WT mice showed that their IGF-1 receptors had signaling activity. Overall, they show that the somatotropic axis is more active in mice with gut microbiota.

Figure 3

They next wanted to see if recovery of typical growth could be achieved via introduction of L. plantarum into the GF mice. A subset of GF mice were born to parents (and therefore naturally colonized) with one of two strains of L. plantarum, one (LpWJL) which they had previously identified as a potent growth promoter, and the other (LpNIZO2877) which had a less profound effect in previous research. They performed the same analysis as in figure 1, and found that the LpWJL strain maintained WT-like growth in the mice, while the LpNIZO2877 also led to a recovery of growth, but not as much as LpWJL.

Then they took all four mice conditions and performed a similar experiment, except with a nutritionally depleted diet. While all mice exhibited less growth than in the breeding diet condition, they saw the same response of the mice to the Lp strains as before, the differential recovery of GF mice towards WT growth responses.

Figure 4

The researchers then looked at what the growth-related hormone and protien levels looked like in these four conditions of mice with different microbial compositions in undernutritional diets. They used the same analysis of sera as in Figure 2. They found that LpWJL mice tended to have the same relative concentrations of these factors as WT mice, while LpNIZO2877 trended with GF mice. They explain the data as demonstrating the reduced activity of the somatotropic axis in GF mice and mice with LpNIZO2887.

They next tried to simulate the lack of IGF-1 activity (as in the GF mice condition) in WT mice to see what their weight gain and body length responses would be, thereby directly linking that growth factor with mouse growth. They injected WT mice with an IGF-1R (IGF-1 receptor) inhibitor, picropodophyllin (PPP) and measured weight gain and length as in figure 1, and relative femur length. The control injection is DMSO. They found that the PPP led to lowered weight gain and body length gain in the breeding diet, and shorter femur length in both the breeding and depleted diets. Therefore, they conclude that the IGF-1 must bind and be active (as in the LpWJL and full microbiota conditions) in order for the mice to grow as normal.