Firmicutes: One of the Most Important Bacteria You've Never Heard About

July 04, 2022 8 min read

Firmicutes: One of the Most Important Bacteria You've Never Heard About - Layer Origin Nutrition

Firmicutes: What Are They and Why Are They So Important?

The human gut is home to a wonderful microscopic ecosystem called the gut microbiome. Within it resides trillions of tiny microorganisms that live in harmony with you, their host, without causing you any harm or instead, bringing you some benefit.

The Firmicutesphylum, for example, can be thought of as a bacterial tribe in your gut. It is renowned for producing the important substance, butyrate, a short-chain fatty acid vital for keeping you healthy.

Over 200 different types of bacteria belong to the Firmicutesphylum, and it is one of the two major groups that make up the human gut microbiome. Together (Firmicutes and Bacteroidetes) they represent over 90% of the entire community[i].

In this article, we will take a deep dive into what Firmicutes are, what they do, and the specific strains that have been identified as good by the PREDICT 1 study. So please sit back and relax as we take you on a microbial mystery tour.

What are Firmicutes?

Although the human gut microbiome consists of more than 100 trillion microbes, two main phyla make it up: Bacteroidetes and Firmicutes. The Firmicutes phylum consists of over 200 different genera, many of which you’ll already be familiar with, such as:

  • Bacillus
  • Clostridium
  • Enterococcus
  • Lactobacillus
  • Ruminococcus

Clostridium is the dominant genera in the Firmicutes phylum, making up 95% of it[ii]. Firmicutes are important for human health because many break down dietary fibre and resistant starch and transform them into important metabolites such as short-chain fatty acids.

Health-Promoting Properties of Firmicutes

The Firmicutes phylum is pivotal for human health, primarily because of the phylum’s involvement in producing butyrate. We like to talk about butyrate a lot here at Layer Origin Nutrition, and for good reason.

Despite the gut microbiota producing less butyrate than any other short-chain fatty acid, it is essential for maintaining intestinal homeostasis and has beneficial effects on energy metabolism[iii]. The Firmicutes phylum is home to many prolific butyrate producers, including:

  • Eubacterium
  • Faecalibacterium
  • Lactobacillus
  • Roseburia

As well as being renowned butyrate producers, many members of the phylum are also probiotics. According to the World Health Organisation (WHO), probiotics are “live microorganisms which when administered in adequate amounts confer a health benefit on the host”[iv].

It’s likely that you are already familiar with the term, even if you weren’t sure what it meant — because food, like some yogurts, contain probiotics.

For example, some yoghurt drinks contain Lactobacillusstrains, such as L. acidophilus.These strains are important because they help break down the sugars in milk (lactose) by producing the enzyme lactase.

Some research has shown that yogurt fortified with L. acidophilusand Bifidobacterium sp.could help treat lactose intolerance[v].

Probiotic cultures also produce other important substances and metabolites, like acetate, butyrate, and help to ward off nasty pathogens that can potentially make you unwell.

If you’ve ever been given antibiotics to treat an infection, you may also have been prescribed probiotics or advised to take some for the duration of your course.

Although antibiotics can be excellent at ridding your body of bacterial infections, they also clear the beneficial bacteria in your gut. So, taking probiotics and prebiotics with antibiotics can help to replenish the gut microbiome[vi].

Specific Firmicute Strains and Their Benefits

Specific strains of the Firmicutes phylum have been listed as ‘good’ gut microbes by the PREDICT 1 study, namely:

  • Firmicutes bacterium CAG 95
  • Firmicutes bacterium CAG 170
  • Oscillibacter sp PC13
  • Clostridium sp CAG 167 [vii]

The PREDICT program is the world's largest, in-depth nutritional research program (not associated with Layer Origin). PREDICT 1 was specifically designed to examine how genetics, metabolic differences, the gut microbiome, meal composition and individual characteristics affect postprandial meal responses. It comprised 1102 healthy participants, 1002 from the UK and the remainder from the US[viii].

Let’s look at some of these strains highlighted by the PREDICT program and their link to human health.

Firmicutes bacterium CAG 95

Of the four members of the Firmicutes phylum featured in this article, Firmicutes bacterium CAG 95is the one scientists seem to know most about. Unsurprisingly, this bacterium is a butyrate producer, hence its importance for human health.

Why is Firmicutes bacterium CAG 95 "Good"?

Firmicutes bacterium CAG 95was found in the guts of around half of PREDICT 1 participants[ix]. It is believed to be a good gut bug because of its ability to transform dietary fibre into butyrate, a molecule produced during microbial fermentation of fibre that’s been shown to reduce the risk of inflammatory bowel disease and colorectal cancer[x].

The PREDICT 1 study also found an association between the presence of Firmicutes bacterium CAG 95and lower insulin secretion and increased insulin sensitivity. This is particularly good for regulating blood sugar levels; high glucose or insulin levels in the blood can increase the risk of developing metabolic diseases, such as obesity and type 2 diabetes[xi].

 Firmicutes bacterium CAG 170

Similar to Firmicutes bacterium CAG 95, 170was found in 33% of PREDICT 1 participants’ gut microbiomes and has been listed as one of the 15 ‘good’ gut microbes.

Why is Firmicutes bacterium CAG 170 "Good"? 

Firmicutes bacterium CAG 170has only recently been discovered, so there is still lots more for scientists to learn about this microbe. However, the PREDICT 1 study found that Firmicutes bacterium CAG 170 was associated with increased insulin sensitivity, which is excellent for regulating blood sugar levels[xii].

The researchers also suggest that Firmicutes bacterium CAG 170is associated with a lower risk of cardiovascular disease, the leading cause of death in the US[xiii]. In the United Kingdom, ischaemic heart disease is the leading cause of death in men[xiv].

Oscillibacter sp PC13

Oscillibacter sp PC13 is a new strain of bacteria that scientists don’t know a lot about yet. The PREDICT 1 researchers, however, found this strain in the gut of more than half of the study’s participants.

Why is Oscillibacter sp PC13 "Good"?

Similar to Oscillibacter sp. 57_20, Oscillibacter sp PC13has been linked to increased levels of polyunsaturated fat and lower insulin secretion, both of which are advantageous. Polyunsaturated fats are considered ‘good’ fats. The most common examples are omega-3 and omega-6 fatty acids, essential for proper brain function[xv].

Clostridium sp CAG 167

Hearing Clostridiumand ‘good’ bacteria may seem a little alien. Still, there are many different strains, some of which can cause diseases or illnesses, like tetanus or food poisoning, but some are also beneficial for your gut health.

And this may be true of Clostridium sp CAG 167.The PREDICT 1 study showed that this microbe was found in approximately 40% of participants. So, it looks to be common in the guts of healthy people.

 Why is Clostridium sp CAG 167 "Good"?

According to PREDICT 1, Clostridium sp CAG 167in the gut is linked with lower insulin secretion and high insulin sensitivity. However, more research needs to be conducted to determine the actual effects of this bacterium on human health.

Some emerging research suggests that some beneficial Clostridiumspecies could be developed as probiotics in the future due to their ability to support intestinal homeostasis.

These strains love to break down the dietary carbohydrates we eat and transform them into SCFAs like butyrate. So, there are numerous advantages to using Clostridiumas potential probiotics in the future[i].

How to Keep Your Firmicutes Nourished

Firmicutes have numerous beneficial properties; being great butyrate producers is one of them, meaning they need to be well-nourished to increase their growth and activity. There are plenty of good sources that will help to improve the composition of your gut microbiome as well as ensure that it remains diverse.

Prebiotics are a great way to help boost the growth and activity of Firmicutes. Prebiotics are described as “a non-digestible food ingredient that beneficially affects the host by selectively stimulating the growth and activity of one or a limited number of bacteria in the colon, and thus improves host health”[xvii].

Prebiotics are different from probiotics (live cultures that, if eaten in adequate amounts, can provide their host with benefits). Prebiotics include foods such as dietary fibre, resistant starch and polyphenols, all of which your gut microbes love to have a good munch on.

Human milk oligosaccharides (HMOs) are great examples of prebiotics. They were first found in human milk and have been shown to boost immunity, keep your microbiome balanced, and improve your health. At Layer Origin, we have a wide range of prebiotics and HMOs to choose from so that you can proactively take care of your gut. Have a browse here.


In our opinion, Firmicutesare cool.

Most of them produce butyrate in the gut when they break down the indigestible carbs your body is ill-equipped to break down. Over 200 different genera are making up this impressive phylum.

Several bacterial strains that belong in this phylum have been identified as ‘good’ gut microbes by the PREDICT program, which serves to promote them a little bit more.

DISCLAIMER: Layer Origin Nutrition is NOT affiliated with the PREDICT 1 study or the PREDICT program. The knowledge of PREDICT 1 study can be accessed at


Written By: 

Leanne Edermaniger

Leanne is a science writer who specializes in human health and enjoys writing about all things related to the gut microbiome.

Join the Layer Origin Nutrition community: 






[i] Magne F, Gotteland M, Gauthier L, Zazueta A, Pesoa S, Navarrete P, Balamurugan R. The Firmicutes/Bacteroidetes Ratio: A Relevant Marker of Gut Dysbiosis in Obese Patients? Nutrients. 2020 May 19;12(5):1474. doi: 10.3390/nu12051474. PMID: 32438689; PMCID: PMC7285218.

[ii] Rinninella E, Raoul P, Cintoni M, Franceschi F, Miggiano GAD, Gasbarrini A, Mele MC. What is the Healthy Gut Microbiota Composition? A Changing Ecosystem across Age, Environment, Diet, and Diseases. Microorganisms. 2019 Jan 10;7(1):14. doi: 10.3390/microorganisms7010014. PMID: 30634578; PMCID: PMC6351938.

[iii] Hu Liu, Ji Wang, Ting He, Sage Becker, Guolong Zhang, Defa Li, Xi Ma, Butyrate: A Double-Edged Sword for Health?, Advances in Nutrition, Volume 9, Issue 1, January 2018, Pages 21–29,

[iv] Hill, C., Guarner, F., Reid, G. et al. The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nat Rev Gastroenterol Hepatol 11, 506–514 (2014).

[v] Masoumi SJ, Mehrabani D, Saberifiroozi M, Fattahi MR, et al. The effect of yogurt fortified with Lactobacillus acidophilusand Bifidobacterium sp.Probiotic in patients with lactose intolerance. Food science and Nutrition 9(3). 2021 January, p. 1704-1711,

[vi] Henriques M. Is it worth taking probiotics after antibiotics? BBC Future. 2019 January. Available at:

[vii] Berry, S.E., Valdes, A.M., Drew, D.A. et al. Human postprandial responses to food and potential for precision nutrition. Nat Med 26, 964–973 (2020).

[viii] ZOE. The PREDICT Program. 2020 October. Available at:

[ix] Hewings-Martin Y. What is Firmicutes bacterium CAG:95 and why is he a ‘good’ bug? ZOE 2022 February. Available at:

[x] Gonçalves P, Martel F. Regulation of colonic epithelial butyrate transport: Focus on colorectal cancer. Porto Biomed J. 2016 Jul-Aug;1(3):83-91. doi: 10.1016/j.pbj.2016.04.004. Epub 2016 Jul 1. PMID: 32258556; PMCID: PMC6806744.

[xi] Centers for Disease Control and Prevention. Insulin resistance and diabetes. CDC. 2021 August. Available at:

[xii] Hewings-Martin Y. What is Firmicutes bacterium CAG:170 and why is he a ‘good’ bug? February 2022. Available at:

[xiii] Centers for Disease Control and Prevention. Leading Causes of Death. January 2022. Available at:

[xiv] Office for National Statistics. Leading causes of death, UK: 2001 to 2018. March 2020. Available at:

[xv] Tan ZS, Harris WS, Beiser AS, Au R, Himali JJ, Debette S, Pikula A, Decarli C, Wolf PA, Vasan RS, Robins SJ, Seshadri S. Red blood cell ω-3 fatty acid levels and markers of accelerated brain aging. Neurology. 2012 Feb 28;78(9):658-64. doi: 10.1212/WNL.0b013e318249f6a9. PMID: 22371413; PMCID: PMC3286229.

[xvi] Guo, P., Zhang, K., Ma, X. et al. Clostridium species as probiotics: potentials and challenges. J Animal Sci Biotechnol 11, 24 (2020).

[xvii] Davani-Davari D, Negahdaripour M, Karimzadeh I, Seifan M, Mohkam M, Masoumi SJ, Berenjian A, Ghasemi Y. Prebiotics: Definition, Types, Sources, Mechanisms, and Clinical Applications. Foods. 2019 Mar 9;8(3):92. doi: 10.3390/foods8030092. PMID: 30857316; PMCID: PMC6463098.


1 Response


October 17, 2023

Great info!

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