Everyone’s talking about gut health these days. It seems that modern science (and social media) is starting to realise what Hippocrates, popularly regarded as the father of medicine, was teaching 300-400 years before Christ (it took them a while).

According to statistical trends analysts McKinsey Insights, gut health was one of the top seven areas of growth in the wellness space of 2024.[1] And the news outlets and social media aficionados are not stopping at the gut – the microbes which make up the human gut microbiome are making news too. The microbiome is hot property in scientific circles, as evidenced by the staggering growth in microbiome-related research over the last 20-25 years.

Figure 1. Trends in the number of microbiome publications per year indexed in Pubmed.gov.[2]

And below are a diverse selection of 2024 microbiome-related scientific study titles. As we can see, current research is delving into how the microbiome may influence everything from Alzheimer’s disease, to how humans adapt to spaceflight…

Then there is the marketing and social media aspect. Every man and his dog seems to be peddling the newest breakthrough in gut health, with probiotics being the super-hero amongst the predictable fermented foods, kombucha, and that old favourite, fibre. Beyond this, a more eclectic group of gut health suggestions includes getting more sleep, increasing your exercise output, drinking more water, avoiding alcohol, and managing your stress levels. Even hypnosis is muscling in on the act with gut-directed hypnotherapy claiming positive results in ameliorating  irritable bowel syndrome, and to a lesser extent, inflammatory bowel disease.[3]

So what is the microbiome, and what do we know about the bugs which live in our gut? How can swallowing live bacteria possibly be good for us…? Let’s find out!

The human gut microbiome

A microbiome is a community of microorganisms (such as bacteria, fungi, and viruses) that inhabit a particular environment, especially the collection of microorganisms living in or on the human body. In addition to this, the ‘-biome’ part of microbiome, refers to the collective genomes of the microorganisms inhabiting that particular environment.[4] 

Allow me to break this down a little: our gut harbours a complex and dynamic population of microorganisms, the gut microbiota. They’re also often called ‘good bacteria’, ‘friendly flora’, or ‘symbiotic bacteria’, amongst other popular monikers. Whatever you want to call them, these bugs have been shown to exert a marked influence on our health. Intestinal bacteria play a crucial role in maintaining immune and metabolic homeostasis (think healthy ‘self-regulation’), and they even protect us against pathogenic bacteria colonising our gut.[5]

How many are there? An estimate from 2018 suggested a ratio of 1.3 : 1, bacterial cells : human cells. That means that for every one human cell, there is 1.3 bacterial cells. Whilst that’s still a lot of bacteria, it is considerably less than the 10 : 1 which had previously been suggested. However, the current estimates don’t take into consideration the viruses and phage also present within and on the human body, and these could potentially equal bacterial estimates if not outnumber them!

Let’s touch on some of the important players that shape the microbiome and gut health in general…

• Commensal bacteria: another name for the bacteria that reside within our intestines. Both the commensal bacteria and the host (you, me, humans) have a ‘symbiotic’ or mutually beneficial relationship in which both derive benefit from the interaction.
• Diversity: microbiome diversity refers to the number of unique bacterial strains (types) that are present within our gut. Higher diversity is considered beneficial, i.e., it’s better to harbour many different species rather than be dominated by just a few types.
• Probiotic: live microorganisms that, when administrated in adequate amounts, confer a health benefit on the host. Probiotics are also often referred to as good bacteria or friendly flora, but the differentiator is that they are deliberately consumed as foods or supplements.
• Prebiotic: dietary substances that are selectively consumed and metabolized by gut microbes, and which confer a health benefit. Some dietary fibres (but not all)  are prebiotics, as are polyphenols, derived from many fruits and vegetables.
• Short-chain fatty acids (SCFAs): are produced from bacterial fermentation of prebiotics. They are the main energy source for epithelial cells, which line our gut (see next). They also provide many systemic health benefits, such as anti-inflammatory actions, to name but one.
• Epithelium: the thin layer of tightly packed cells which line the gut. The epithelium is not only an external body surface, it also forms the structure which houses the gut microbiome. Below this, a layer of connective tissue houses much of our body’s immune cells.

Let’s now take a closer look at the diverse range of characters purported to be able to bring balance to the health of our gut: probiotics.

Probiotics

Derived from the Latin “pro” meaning for, and the Greek “bios”, referring to life, the emergence of probiotics was obviously considered quite an important discovery back in the early 1900’s.Today, they encompass a variety of forms and are administered via foods, dietary supplements, infant formula, topical creams, intravaginal suppositories, even drugs and medical devices. Millions if not trillions of probiotics are consumed by humans and animals every year.

Typically, probiotics do not hang around for very long after they are consumed. Most studies show that within two weeks of stopping consumption, probiotic bacteria can no longer be isolated from human poo.[6] This lack of longevity, however, does not render a probiotic useless – in fact nothing could be further from the truth! As they traverse the gastrointestinal tract, probiotics interact with immune cells and other microbes residing in different sections of the gut. As can be seen in Table 1 below, these interactions can have far-reaching effects.

Table 1. Important actions related to or dependent upon probiotics, confirmed by multiple scientific studies:

Just as the discoverers of our intestinal buddies predicted over one hundred years ago, probiotics can achieve a lot of important beneficial actions in service of human health. It is little wonder therefore, that gut health and probiotics are hugely popular these days.

If you’ve recently gotten on board the gut health bandwagon, welcome! There is likely to be many more exciting discoveries to marvel over in the years to come. If you haven’t as yet, isn’t it time you joined the cool kids and embraced probiotics, the friends within us all? Your gut will thank you for doing so!

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