Are there any beneficial anaerobic bacteria which are found in the human gut?

There’s several species of Lactobacillus that live in the human gut that are so good for you that people buy little yoghurt drinks full of them.

The bacteria of the GI tract varies depending on which part of the GI tract you are talking about. The actual stomach and small intestine have very little bacteria due to the pH of that area. The colon has a large number of anaerobic organisms. The ileum has a good number of both aerobic and anaerobic organisms. How beneficial each of them are depends on what’s going on with them. In normal circumstances, the mere presence of the numerous bacteria in our GI tract is beneficial because the competition for space/nutrients keeps any one type of bacteria from over producing and making us sick (such as what happens when someone gets sick from clostridium difficile after taking antibiotics that kill of the other bacteria in our gut). Overall, the bacteria of our GI tract help prevent over production of one organism over the others, digestion of the food we eat, conversion of what we eat into nutrients we can use, and maintaining normal intestinal homeostasis. Here are a few examples:

E. coli can be very harmful to us, but it is also very helpful when it is kept at a level that it is not making us sick. It helps with vitamin K production. Vitamin K plays a very important role in our clotting mechanisms and bone health.

Bifidobacterium help with regulation of intestinal homeostasis, some production of vitamins, bioconversion of dietary compounds, and occupy space and provide that competition to help prevent other microorganisms from over production.

Enterococcus faecalis has probiotic capabilities and ferments glucose without gas production, but can cause very bad infections if give the chance to over produce. It’s mere presence helps stop other organisms from over producing as well.

Lactobacillus (as @Lightlyseared mentioned) help with the conversion of lactose and other sugars into lactic acid. By producing lactic acid, they make the area more acidic which inhibits over production of other organisms that can’t thrive in the acidic environment. It also may have anti-inflammatory and anti-cancer properties (this is still being studied).