Inside the gut: Understanding microbes

Written by: Dr Simon Smale
Edited by: Kate Forristal

The human body is a bustling ecosystem, home to trillions of microorganisms that play pivotal roles in our health and well-being. Among these, the gastrointestinal tract serves as a thriving habitat, housing a diverse array of bacteria. While the gut is teeming with organisms, the distribution varies significantly throughout its length, with distinct concentrations found in different regions. In his latest online article, Dr Simon Smale gives us his insights.

At the forefront of this microbial landscape lie the mouth and the large intestine, where approximately 10 trillion bacteria reside, densely packed in concentrations of billions per millilitre. However, when it comes to the small bowel, the scene shifts. Here, the microbial population is relatively sparse, with only around 1000 bacteria per millilitre in the ileum and a mere 100 organisms per millilitre in the jejunum.


The bacterial community in the colon plays a crucial role in the breakdown and fermentation of undigested food residues, aiding in the extraction of nutrients and energy. However, when these bacteria proliferate excessively within the small intestine, a condition known as Small Intestinal Bacterial Overgrowth (SIBO) emerges. This overgrowth disrupts the delicate balance of digestion and absorption, leading to a cascade of symptoms including bloating, belching, abdominal pain, and loose stools.


SIBO often arises in conjunction with other abnormalities within the gut, particularly those affecting metabolism or motility. One common scenario involves the long-term use of proton pump inhibitors, medications that reduce gastric acid production. In the absence of sufficient acid, bacteria that would typically be neutralised in the stomach can survive and colonise the small bowel, particularly the jejunum and ileum.


Similarly, abnormalities in gut motility, such as those seen in conditions like scleroderma and connective tissue diseases like ehlers-danlos syndrome, can predispose individuals to SIBO. These motility disorders disrupt the coordinated movement of food through the intestines, creating an environment ripe for bacterial overgrowth.


Diagnosing SIBO typically involves hydrogen and methane breath testing, which detects the gases produced by bacterial fermentation in the small intestine. Once diagnosed, treatment often involves a multifaceted approach, combining dietary modifications and antibiotics. Dietary interventions focus on minimising foods that promote bacterial fermentation, while antibiotics target the overgrown bacterial population.


However, it's crucial to tread carefully with antibiotic therapy, as repeated or prolonged courses can disrupt the balance of the gut microbiome and contribute to antibiotic resistance. Therefore, treatment decisions should be made carefully, considering the potential long-term consequences on microbial diversity and antibiotic efficacy.


Dr Simon Smale is an esteemed consultant gastroenterologist. You can schedule an appointment with Dr Smale on his Top Doctors profile.

By Dr Simon Smale

Dr Simon Smale is an exceptionally well-regarded consultant gastroenterologist who specialises in irritable bowel syndrome, colonoscopy, abdominal pain, gastroscopy, diarrhoea, and constipation, amongst many other gastrointestinal-related conditions. He is currently practising at the Nuffield Health York Hospital

Impressively, Dr Smale, who has a strong interest and high level of expertise in heartburn, indigestion, rectal bleeding, and abdominal swelling, was awarded the York Press Healthcare Hero Award (for providing excellence in patient care) in 2009. Between 2014 and early 2016, he held the position as medical director of a digital health start-up which provided integrated behavioural change programs for patients suffering from chronic disease. 

Dr Smale is the current medical director of gastrointestinal medicine and surgery at Manchester Royal Infirmary, and has published extensively within peer-reviewed journals and publications. He undertook much of his specialist gastroenterology training at King’s College Hospital in London. 

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