A study by Hehemann et al.  discovered a specific gene that digests a type of sulfonated carbohydrate that is only found in seaweed sushi wrappers. This gene is found in the gut microbes of Japanese people but not North Americans. The study concluded that this specific gene has transferred at some point in history from the algae itself to the bacteria living on it and then to the gut microbiome of a Japanese person by horizontal gene transfer. This study also suggests that, even though some bacterial group might live in our gut for our entire lives, they can gain new functionalities throughout our lives by picking up new genes depending on the type of food that we eat.
In this direction, a study in Alm’s Laboratory investigated around 2000 bacterial genomes published in  with the aim of detecting genes that are 100% similar but belong to bacteria in different taxonomic groups. Any gene that is exactly the same between different bacterial groups would indicate a horizontal gene transfer event. In this study, around 100000 such instances were discovered.
When looked at specific environments, it was discovered that the bacteria isolated from humans share genes mostly with other bacteria isolated from human sites. If we focus on more specific sites; we see that bacterial genomes isolated from human gut share genes mostly with with other bacteria that are isolated from gut, and bacterial genomes isolated from human skin shared gene mostly with other isolated from human skin. This finding suggests that independent from the phylogeny of the bacterial groups, ecology is the most important factor determining the amount of gene transfer instances between bacterial groups.
In Figure 6.9, we see that between different bacterial groups taken from human that has at least 3% 16S gene distance, there is around 23% chance that they will share an identical gene in their genome. Furthermore, there is more than 40% chance that they share an identical gene if they are sampled from the same site as well.
On the other hand, Figure 6.10 shows that geography is a weak influence on horizontal gene transfer. Bacterial populations sampled from the same continent and different continents had little difference in terms of the amount of horizontal gene transfer detected.
Figure 6.11 shows a color coded matrix of the HGT levels between various human and non-human environments; top-right triangle representing the amount of horizontal gene transfers and the bottom-left triangle showing the percentage of antibiotic resistance (AR) genes among the transferred genes. In the top-right corner, we see that there is a slight excess of HGT instances between human microbiome and bacterial samples taken from farm animals. And when we look at the corresponding percentages of antibiotic resistance genes, we see that more than 60% of the transfers are AR genes. This result shows the direct effect of feeding subtherapeutic antibiotics to livestock on the emergence of antibiotic resistance genes in the bacterial populations living in human gut.