II. 2 Antibiotics and colonisation resistance

You understand why it is precisely the antibiotic that opens the door to C. difficile: it empties out the protective flora that had kept the pathogen in check. This protection is called colonisation resistance.

The invisible protective shield – colonisation resistance

Picture your bowel as a crowded, well-functioning city, where every spot is taken and every resource has an owner. The many kinds of beneficial bacteria^[G] occupy the surface of the bowel wall, use up the available nutrients, and produce substances that maintain precisely this existing order, as long as no intervention comes from outside. In this way no single bacterium can proliferate or turn into a pathogen. Potentially pathogenic bacteria harbouring such ambitions – C. difficile, for example – simply have nowhere to settle in larger numbers and nothing to multiply from.

This natural protection is called colonisation resistance^[G]. It is not a drug, not a treatment – simply one of the most important "side effects" of a healthy, diverse gut flora^[G]. As long as this shield is intact, C. difficile spores^[G] may even be present in the bowel and still cause no harm: they have no room to germinate and multiply.

Colonisation resistance is therefore not the merit of a single bacterium but of variety, of diversity^[G]. The more kinds of beneficial inhabitants your bowel has, the more stable and resistant the protection. This explains why a varied, fibre-rich diet is so important in the later weeks of the book: it nourishes this protective community and makes it diverse.

What happens when you take an antibiotic?

The antibiotic is an indispensable medicine that saves lives in many infections – this is not about it being bad. The problem is that most antibiotics cannot be selective: while they destroy the bacterium they were prescribed for, they also thin out or destroy a large part of the gut flora's beneficial inhabitants. Part of the crowded, well-protected city suddenly empties out. The selectivity of antibiotics – their supposed ability to destroy only certain pathogens – is in reality overstated.

In this emptied-out, thinned-out flora, colonisation resistance collapses: space and food are freed up. If C. difficile spores are present at this point, they get a free hand – they germinate, multiply, and begin to produce the toxins^[G] that cause the complaints. This is the typical scenario: someone receives an antibiotic for an infection, and then a few days or weeks later develops C. difficile-caused diarrhoea.

This is why the DiffBiome course emphasises so strongly that the flora must be restored, not merely the pathogen overcome. And this is why it is an essential rule that if you are still taking an antibiotic, it must be finished before starting the course – otherwise the antibiotic would also destroy the freshly delivered, healthy ecosystem (Cammarota 2017 [016]). The details of taking the capsules are covered in a later chapter, but it is good to keep this rule in mind already.

While you were waiting: the cautious antibiotic policy during the course

During and after the course it is particularly worth handling antibiotics consciously. This does not mean that you may never take an antibiotic again – there are situations where it is unavoidable and life-saving, and therefore right. But every unnecessary, not strictly justified antibiotic course is a risk: after repeated courses the diversity of the gut flora falls, and the pre-treatment state is often only incompletely restored even months later (Dethlefsen & Relman 2011 [033]) – and antibiotic exposure is the most important modifiable risk factor for C. difficile infection (McDonald LC 2018 [023]).

That is why, if in the coming weeks and months you should need an antibiotic for another reason, always mention to the doctor that you have had a C. difficile infection and are undergoing a microbiota transfer (FMT). That way they can weigh up whether the antibiotic is truly necessary, and if so, can choose one that damages the gut flora less (Kelly 2021 [005]). If you need to start an antibiotic while the course is running, tell your treating physician: pausing the capsules is then warranted, and they will tell you when it can be resumed. Prevention and antibiotic policy are discussed in detail in a later phase of the programme.

🍽️ Eating during these days

During these three days you understand how the antibiotic empties out the protective flora – and your eating now serves to let your bowel settle while this protection rebuilds. Since the danger of diarrhoea and dehydration still remains, the goal is a gentle, firming diet: plenty of fluids, easily digestible foods and soluble fibre. Soluble fibre forms a gel in water, which slows the passage of bowel contents and supports a firmer stool, while plenty of fluids replace the amount lost with the looser stool. Your concrete tasks: on all three days take the daily dose according to the usual routine, drink an extra glass of water after every looser stool, and do not start a further antibiotic or probiotic course on your own that would once again disturb the flora that is now rebuilding.

In this early phase there is not yet a daily plant – the Plant Calendar only starts from day 15. For now stay with easily digestible, boiled or steamed foods, and avoid raw, coarse, strongly gas-forming foods; the gradual building-up of a varied, fibre-rich diet comes later, as the symptoms settle.

Why does this matter?

If you understand that the antibiotic opens the door to C. difficile by thinning out the protective flora, then you also understand why it is not enough merely to attack the pathogen: for durable recovery, colonisation resistance must also be rebuilt. This is exactly what the DiffBiome course does – and it is helped if you spare your flora from unnecessary further antibiotics.

References

[003] Reed AD, Theriot CM. Contribution of Inhibitory Metabolites and Competition for Nutrients to Colonization Resistance against Clostridioides difficile by Commensal Clostridium**. Microorganisms. 2021. Link

This review examines how commensal *Clostridium* species mediate colonization resistance against C. difficile. Commensal *Clostridia* modify primary bile acids into secondary bile acids that suppress C. difficile spore germination and vegetative outgrowth. They additionally produce antimicrobial peptides and short-chain fatty acids that directly inhibit C. difficile and compete for limiting nutrients such as proline, important for C. difficile growth via Stickland fermentation. Loss of commensal *Clostridia* after broad-spectrum antibiotics is a key mechanistic step toward CDI susceptibility. The authors conclude from this that new therapies against CDI are urgently needed; the clinical validation of defined *Clostridium* consortia comes not from this review but from the VE303 phase 2 trial [56].

[005] Kelly C, Fischer M, Allegretti J, LaPlante K, Stewart D, Limketkai B, Stollman N. ACG Clinical Guidelines: Prevention, Diagnosis, and Treatment of Clostridioides difficile Infections. The American journal of gastroenterology. 2021. Link

According to the 2021 ACG guideline, FMT is part of standard care for rCDI; strongly recommended after ≥2 recurrences — American College of Gastroenterology's latest CDI guidelines: FMT strongly recommended after ≥2 CDI recurrences; capsule and colonoscopic administration are equivalent; detailed donor screening and storage protocol; COVID-era updates regarding FMT safety also incorporated.

[016] Cammarota G, Ianiro G, Tilg H et al. European consensus conference on faecal microbiota transplantation in clinical practice. Gut. 2017. Link

European consensus conference developing evidence-based recommendations on FMT for clinical practice, with 28 experts from 10 countries collaborating in working groups. Statements were generated through evidence-based review, evaluated electronically via a Delphi process, and finalized in a plenary consensus session. Recommendations cover FMT indications, donor selection, faecal material preparation, clinical management, faecal delivery, and minimum requirements for establishing an FMT centre. Provides the European standardization framework for safe and governed FMT delivery.

[023] McDonald LC, Gerding DN, Johnson S, Bakken JS, Carroll KC et al. Clinical Practice Guidelines for Clostridium. difficile Infection in Adults and Children: 2017 Update by the Infectious Diseases Society of America (IDSA) and Society for Healthcare Epidemiology of America (SHEA). Clinical Infectious Diseases. 2018. Link

Comprehensive IDSA/SHEA clinical practice guideline on the diagnosis, treatment and prevention of C. difficile infection in adults and children. It defines severity categories (non-severe, severe, fulminant) and characterises fulminant disease by hypotension or shock, ileus or toxic megacolon — findings that require inpatient care, intravenous therapy and surgical consultation. For multiply recurrent infection in which antibiotic therapy has repeatedly failed, faecal microbiota transplantation is recommended. This document provides the international frame to which the book's red flags and hospital-referral signs are aligned.

[033] Dethlefsen L, Relman DA. Incomplete recovery and individualized responses of the human distal gut microbiota to repeated antibiotic perturbation. Proceedings of the National Academy of Sciences of the United States of America. 2011. Link

Stanford-based longitudinal study tracking the gut microbiota of three individuals over 10 months across two consecutive courses of ciprofloxacin, with deep 16S rRNA sequencing. Loss of bacterial diversity was profound and rapid, occurring within 3-4 days of antibiotic initiation, and recovery toward the pre-treatment state was often incomplete months after cessation. Repeated antibiotic exposure produced incremental, non-recoverable shifts in community composition. With over 2,000 citations, this paper is the canonical reference establishing that antibiotic-induced dysbiosis is not a self-correcting disturbance but can leave a lasting ecological imprint – central to the case for MTT in patients with cumulative antibiotic exposure history.

[044] Wang S, Xiang L, Li F, Deng W, Lv P, Chen Y. Butyrate Protects against Clostridium. difficile Infection by Regulating Bile Acid Metabolism. Microbiol Spectr. 2023. Link

Mechanistic CDI study showing that butyrate, a short-chain fatty acid produced by commensal Firmicutes, exerts a protective effect against CDI through multiple synergistic pathways: enhanced gut barrier integrity, anti-inflammatory action, and modulation of bile acid metabolism via bile salt hydrolase regulation and FXR activation. CDI patients have markedly reduced fecal butyrate compared to controls. The findings establish a metabolic mechanism by which a healthy, diverse microbiota resists CDI colonisation – and by which a dysbiotic microbiota becomes permissive. This underpins the SCFA-related reasoning in Section 2.2 about depleted SCFA synthesis in chronic dysbiosis.

[048] Weingarden A, González A, Vázquez-Baeza Y, Weiss S, Humphry G, Berg-Lyons D, Knights D, Unno T, Bobr A, Kang J, Khoruts A, Knight R, Sadowsky MJ. Dynamic changes in short- and long-term bacterial composition following fecal microbiota transplantation for recurrent Clostridium. difficile infection. Microbiome. 2015. Link

Longitudinal microbiome characterisation following FMT in four patients with recurrent CDI, sampling daily for 28 days and weekly to 84 days post-treatment, over a total of 151 days. The recipient microbiota rapidly normalised from a markedly dysbiotic state to a healthy-range composition within days. Composition continued to change thereafter, diverging from the original donor implant material and fluctuating dynamically over both the short and the long term – while remaining throughout within the cloud of healthy microbiota. The paper supports the framing in this Guide that successful MTT produces a self-sustaining recipient ecology, not a permanent donor-tracked imprint.

[610] Jernberg C, Löfmark S, Edlund C, Jansson JK. Long-term ecological impacts of antibiotic administration on the human intestinal microbiota. ISME J. 2007. Link

This 2-year longitudinal study tracked the faecal microbiota of four healthy subjects exposed to 7-day clindamycin therapy and four controls at nine time points. Polyphasic analysis showed highly significant disturbances persisting for the entire follow-up. Clonal diversity of Bacteroides isolates declined sharply by rep-PCR, with long-term persistence of highly resistant clones. The Bacteroides community never returned to its original composition by T-RFLP fingerprinting. The findings document multi-year ecological consequences of a single short course of clindamycin.