In the stomach, the majority are the Lactobacilli, making up about 10^2 to 10^3. In the Duodenum, there are Streptococci and Lactobacilli, with a concentration of 10^4 or 10^5. As we descend past the jejunum and into the ileum, the concentration of bacteria increases dramatically - up to ten million bacteria reside here. Bacteria such as Enterobacteria, Enterococcus
Microbial density in the gut. Overall, there are 10^14 microorganisms residing in the human gut, with over 500 unique species. In the stomach, the majority are the Lactobacilli, making up about 10^2 to 10^3. In the Duodenum, there are Streptococci and  Lactobacilli, with a concentration of 10^4 or 10^5.  As we descend past the jejunum and into the ileum, the concentration of bacteria increase dramatically - up to ten million bacteria reside here. Bacteria such as Enterobacteria, Enterococcus, Faecalis, Bacteroides, Bifidobacteria, Peptococcus, Peptosteptococc, Ruminococcus, Clostridia, and Lactobacilli. These bacteria are also common in the colon and appendix, but the microorganism concentrations increase yet again, to 10^9 to 10^12. There is a general trend that complexity and concentration of bacteria increases as we descend the GI tract.