Lactobacillus and Bifidobacterium
Our results showed that the abundance of Lactobacillus (p = 0.016) and Bifidobacterium tend to decrease in the infant gut fed on GDM BM. We found, additionally, that Lactobacillus abundance is decreased in the GDM mother gut and BM. Lactobacillus is a gram-positive genus that encompasses approximately 140 species. The species associated to human fecal samples are L. acidophilus, L. casei, L. crispatus, L. gasseri, L. paracasei, L. plantarum, L. reuteri, and L. ruminis. It is believed that this genus play a role maintaining the microbiota homeostasis [9, 22].
Species within the Bifidobacterium genus, including B. bifidum, B. breve and subspecies of B. longum, such as B. longum subsp. longum and B. longum subsp. infantis are human-specialized and coexist in the human gut . In the infant, Bifidobacterium participate in the metabolism of human milk oligosaccharides (HMOs) and mucin glycosylated proteins . It is known that Bifidobacterium is the dominant microorganism in the breast-fed infant gut microbiota under normal conditions. Since, during the breast-fed period, the infant does not have additional sources of colonizing Bifidobacterium, metabolism of main HMOs such as 2-fucosyllactose (2-FL) and lacto-N-neonatrose (LNnT) could be compromised affecting the bacterial strains community in the gut .
In this study, we found presence of the gram-positive Flavonifractor sp. that is highly associated with the gut bacteriome of GDM-fed infants (p = 1.06E-5). The resolution obtained did not allow to identify it to species level. Interestingly, we have not found evidence of Flavonifractor in the mothers’ group. However, it is known that Flavonifractor plautii is a human gut microbiome species able to degrade naturally produced flavonoids present in fruit and vegetables commonly consumed in diet, specifically the most abundant, quercetin. Previously, Flavonifractor was considered a possible biomarker since it is reported as one of the microorganisms heavily associated with colorectal cancer in two recent studies [25, 26]. Flavonifractor sp. was also found in high RA in diabetes mellitus affected mice . In contrast appears to be associated with lower GDM risk when diet is rich in polyphenols and flavonoids . The high RA and strong association of Flavonifractor in GDM infants could be effectively linked and serve as a potential marker for gestational hyperglycemia.
It is noteworthy that Paenibacillus spp. reads are present in very high RA across the analyzed samples (Supplementary table 3). Paenibacillus is a facultative anaerobic, rod-shaped gram-positive. This species has been barely described in previous studies. To date, only two species have been reported as isolated from human gut, P. faecis and P. phocaensis [29, 30]. Our results showed Paenibacillus in a high RA in GDM maternal gut and their children. However, further analyses are required to determine the species found in this study.
Vitamin K producers
It is known that gut bacteria have an essential role in human metabolism, including the production of essential nutrients such as vitamin K2, complex B vitamins, and short chain fatty acids (SCFA). Vitamin K2 are menaquinones (MKs) (2-methyl-3-multipreny-1,4-naphtoquinone), a fat-soluble vitamin produced by human gut microbiota synthesized during bacterial anaerobic respiration. There are several forms of MKs that vary in the number of their isoprene-5-carbon prenyl units ranging from 2 to 15 repeats . MKs biological importance resides in the fact that they serve as electron transport chains during the prokaryotic respiration, carrying electrons to the cytoplasmic membrane, functioning as antioxidants, protecting cellular membranes from lipid oxidation . Major MKs in human gut are produced by members of the genera Bacteroides (MK-10, MK-11), Enterobacteria (MK-8), Serratia (MK-4), Lactococcus (MK-7, MK-8, MK-9) and Lactobacillus (MK-4) [31, 33–36]. We found Enterobacteriaceae, Serratia sp., and Lactobacillus sp., in a low RA in gut samples of GDM-fed infants. Remarkably, Lactococcus had increased RA in control samples, within BM samples, GDM BM had significantly low RA of Serratia (p = 0.009). Singularly, Lactococcus had a low RA in GDM BM samples in contrast to the observed results in the GDM infants’ gut. Here, we hypothesized that the high levels of glucose occurring under GDM could affect the abundance of this species in the mother breast milk. However, their number increases again once they are established in the infant colon. Our findings suggest that GDM infants could have a diminished supplementation of MKs at least during the first months of life. However, further research measuring MKs concentration under hyperglycemic conditions is needed to elucidate the impact of GDM in women and their infants.
Complex B producers
It is known that besides niacin (vitamin B3), host mammalians do not synthesize vitamin B de novo, and, in consequence, these compounds should be obtained either from diet or synthesized by the gut microbiota .
Thiamine (vitamin B1) function as cofactor for several important enzymatic reactions including the citric acid cycle reactions catalyzed by the pyruvate dehydrogenase and α-ketoglutarate dehydrogenase . Thiamine is produced by several gut and probiotic bacterial species such as Bacteroides fragilis, Ruminococcus lactaris, Prevotella copri and some Lactobacillus spp., Bifidobacterium spp., and Fusobacterium spp. According to our findings, at least Lactobacillus spp. and Bifidobacterium spp. are in low RA in GDM women and infants, and this could lead to a reduced amount of bacterial synthesized thiamine. In consequence, the competition for thiamine between gut bacteria and the host could be increased.
Riboflavin (vitamin B2) is a precursor of the redox coenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) essential in TCA and fatty oxidation pathways. Although diet is an important source of riboflavin, it is also synthesized by several members of the human gut microbiota, including Lactobacillus spp., Lactococcus spp., and Weisella spp. [38, 39]. In this study Lactobacillus sp. RA is reduced in GDM BM-fed infants (p = 0.028) while Lactococcus sp. and Weissella sp. increase (p = 0.009).
Folates (vitamin B9) are anionic hydrophilic carrier molecules that are not naturally synthesized by human cells; thus, they must be acquired through diet components or produced by the gut microbiota. Bacterial folate biosynthesis in the form of tetrahydrofolate (THF) is carried out mainly by members of the phyla Bacteroidetes (Bacteroides spp. and Prevotella spp.), Firmicutes (Lactobacillus spp. and Streptococcus spp.) and Actinobacteria (Bifidobacterium spp.) [38, 40]. We found that Bifidobacterium spp. is a bacterial genus affected by GDM. Its low RA in the GDM BM fed infant could be traced to maternal BM and to the gut in hyperglycemic women (Fig. 3). Additionally, there is a low count of Lactobacillus and Streptococcus in GDM individuals could lead to stress levels of THF in affected infants since BM is the only folate source at that stage, and folate levels in BM are maintained by the BM microbiota [41, 42]
Cobalamin (Vitamin B12) is produced exclusively by microorganisms, animals, plants, and fungi that do not have the ability to biosynthesize it. In the human gut, Lactobacillus reuteri is the indigenous bacteria that supply the daily requirements . A study by Boran, Baris  compared the gut microbiota between healthy vitamin B12 sufficient and insufficient infants and found no difference between the two groups. However, vitamin B12 insufficiency could be related to several factors, including GDM. Our results suggest that low RA of Lactobacillus spp. could be linked to low RA of L. reuteri.
The bacteriome structure in the infants analyzed here suggests that GDM infants have a reduced RA of members of the genera Lactobacillus, Streptococcus, and Bifidobacterium compared with control samples and could have a reduced biosynthesis of several complex B vitamins.
Inflammatory bowel disease
Bacterial dysbiosis of gut species has been associated with inflammatory bowel disease (IBD). Several studies revealed that most common pattern associated with IBD is a reduced amount of Firmicutes and Bacteroides and an increase of Enterobacteriaceae species . Our results revealed an increased RA of Ruminococcus spp. and a significantly decreased abundance of Sutterella spp., Clostridium XVIII, Lactobacillus spp., and a non-statistically significant but reduced RA of Bifidobacterium spp. These results are similar to those presented by Joossens, Huys  and Gevers, Kugathasan .
Overall findings of this study suggest that gut bacteriome profiles vary between healthy and GDM women, leading to gut bacterial dysbiosis in their infants. A hypothesis for the entero-mammary translocation of internal bacteria hypothesis was previously proposed by Fernández, Langa . Consistently, we observed low RA of several bacterial genera in the infants’ gut (Fig. 4), such as Bifidobacterium, Sutterella, and Serratia, that can be traced to the maternal gut, and breast milk from GDM affected women.