The 100 trillion bacteria that populate fecal microbiome basically fall into six phyla: Actinobacteria, Bacillota (formerly known as Firmacutes), Bacteroidetes, Fusobacteria, Proteobacteria and Verrucomicrobia. 90% of the organisms though fall into just two of these phyla: Bacillota(Firmacutes) and Bacteroidetes. The ratio between these two has been referred to as the F/B ratio and represents a measure used to evaluate gut microbiome balance.

The phylum Bacteroidota is made up of a diverse set of facultative anaerobic Gram-negative organisms that play a variety of roles in the microbiome. Although some of its members, such as Bacteroides thetaiotaomicron, have been found to contribute to anti-inflammatory processes, most of the Bacteriodota, such as Bacteroides fragilis, have shown potentially harmful pro-inflammatory effects. Many of the gram negative bacteria that make up this phylum contain potent neurotoxins called lipopolysaccharides (LPSs) in their cell walls. LPSs have been linked to excessive immune and inflammatory responses, which can be particularly concerning in individuals with age or disease related disruption of the intestinal or blood-brain barrier.

The phylum Bacillota on the other hand are predominantly gram positive obligate anaerobic organisms and do not have LPS. In addition, they include many key commensal bacteria responsible for the breakdown of complex carbohydrates to produce vital short-chain fatty acids (SCFAs), which are fatty acids with 6 or fewer carbon atoms. The most common SCFAs are acetate, propionate and butyrate.

SCFAs have multiple healthy functions including:

There are three main SCFAs: acetate, propionate and butyrate. They each have different functions and are produced by different phyla of bacteria.

The production of SCFAs varies by the organism with most of the butyrate being produced by Bacillota (Firmacutes) and propionate by Bacteriodetes. Acetate is formed by all of them.

These SCFAs are critical for maintaining gut barrier integrity and decreasing inflammation, particularly the key SCFA butyrate. Bacillota, although, have also been found at higher levels in overweight and obese individuals. It is thought that Bacillota extract energy at a greater efficiency due to their propensity for carbohydrate consumption and SCFA production, potentially contributing to their association with obesity.
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Considering the factors outlined above, although a high F/B ratio is associated with a stronger gut barrier and lower inflammation; it is also associated with a tendency for obesity and metabolic disorders when carbohydrate intake is excessive. Despite a growing body of literature revealing the significance of the balance between these two major phyla, results across studies have been varied. There is not yet scientific consensus on the ideal F/B ratio for the human gut microbiome. The ratio can be influenced by many other factors including diet, lifestyle, gender, age and more. Evidence does not support using this ratio as a stand-alone indicator of health, but it can be used to understand the composition of one's microbiome at a glance. An argument can be made that maintaining a higher F/B ratio with carbohydrate rich, but low glycemic index foods while also managing weight with proper nutrition and exercise could maximize the benefits one receives from the microbiome, but more research into the topic is needed to reach consensus.

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