Mind & body, in brief

Your gut, skin, nose and vagina are already fully occupied. Residents hold the food and the landing sites, so most arrivals never get a foothold. [3][4][10][13][17]

Scientists call it colonization resistance. The idea dates to the 1950s, when Marjorie Bohnhoff proposed that an antibiotic could leave a host more open to infection. [4]

The mechanism is competition, not defence. Early colonisers use up the growth-limiting resources, which holds back later arrivals that need the same things. [3][5]

They also make the place hostile. Fibre fermented into short-chain fats acidifies an invader from the inside, and bile acids suppress others. [3][4]

The gut is not one habitat but many. In mice, oxygen falls from 58 mmHg in the stomach to about 3 mmHg in the far colon, and each zone suits different residents. [3]

The lining helps by being restless. It renews itself every four to seven days, and a mucus layer several hundred microns thick keeps most bacteria off the cells. [3]

A course of antibiotics empties seats. Among 14,979 Swedish adults, those who had taken none in eight years carried about 350 gut species; users carried fewer. [1][2]

Clindamycin did the most damage. Each course in the previous year was linked to 47 fewer species detected, and shifts in nearly 300 of the 1,340 species measured. [2]

The mark lasts. Species abundances were still altered in people whose single course came four to eight years before the sample. [1]

Diversity rebounds fastest in the first two years and then flattens. The study's senior author, Tove Fall, put it plainly: it seems like you do not recover completely. [2]

Empty seats are what Clostridioides difficile needs. People are up to 10 times more likely to get a C. diff infection while on an antibiotic and in the month after. [6]

About 1 in 5 people who take antibiotics get diarrhoea from them. The severe form is usually C. diff inflaming the large intestine. [7][8]

The exposure is ordinary; the opening is not. C. diff waits outside as spores that survive months or years on surfaces, and finding them in a home is not unusual. [6]

Carrying C. diff without being ill is more common than the infection itself. Those people test positive and often have no symptoms at all. [46]

Refilling the room is now the treatment. Stool transplants work by restoring secondary bile acids and taking the free nutrients back off the pathogen. [9]

Why it matters. The next test is whether the damage a drug does to the resident community starts being weighed when it is prescribed, rather than only counted after an infection follows it. [34]

A study of about 1,100 people found seven types of nasal community. Persistent carriage went with more staph, and with less of three Corynebacterium species, Dolosigranulum pigrum and Staphylococcus epidermidis. [10]

Why it matters. Same rule as the gut, on a different surface: staph settles where nobody else is sitting.

One London orthopaedic hospital clears it on purpose before surgery. Patients use a nasal antibiotic ointment for five days and chlorhexidine cloths for two. [11]

Why it matters. When a knife is about to open a route inward, the harmless resident becomes the risk.

Three years of genome-level surveillance in a neonatal unit traced strains passing between infants through shared spaces and caregivers. The clusters that reached the most infants over time were also the ones associated with invasive infection. [12]

Why it matters. Being colonised comes first; the infection is what happens when the lodger moves.

Staphylococcus epidermidis was long filed as an opportunist that infects implants. A new review describes it as an active resident that calms inflammation and holds the skin barrier together. [13]

Why it matters. The same organism defends in one place and infects in another; the address decides which.

Residents eat the oils your skin makes and convert them into new compounds, including short-chain fats. Those products then shape which other microbes can grow there. [14]

Why it matters. Occupancy is chemical as well as physical: they change the room, not just fill it.

When skin breaks, the resident community reorganises through every phase of repair. [15] S. epidermidis and Lactobacillus plantarum prompt the skin to make its own antibacterial proteins, while S. aureus and Pseudomonas slow closure with biofilm. [16]

Why it matters. A cut is a vacancy, and what fills it partly decides how fast it shuts.

About 95% of the resident bacteria there are lactobacilli. They make lactic acid and hydrogen peroxide, holding the pH near 3.8 to 4.2, which most newcomers cannot grow in. [17]

Why it matters. The protection is a by-product of what lactobacilli do to feed themselves.

Bacterial vaginosis is not an invasion by one germ. It is the loss of hydrogen-peroxide-producing lactobacilli and the overgrowth of anaerobes already present. [18] The WHO names douching as a risk factor. [19]

Why it matters. Named for what arrives, caused by what left.

A community dominated by Lactobacillus crispatus protects against picking up chlamydia. Where those species are depleted, acquisition risk rises and the body clears the infection less well. [20]

Why it matters. The same defence and the same failure, measured against a specific infection.

Live biotherapeutic products are being designed to restore Lactobacillus dominance rather than only kill the overgrowth. [21] A review of fermented foods found 87% of food starter uses drew on only two of the 54 strains with clinical evidence. [47]

Why it matters. The strains that work in trials and the ones in the shops are mostly not the same strains.

129 patients with recurrent C. diff took freeze-dried capsules at two acute-care hospitals. 103 of them, or 80%, had no recurrence at three months, and follow-up ran a median of 182 weeks. [22]

Why it matters. Refilling the room is a standard treatment now, not an experiment.

The FDA approved the first two stool-derived therapies in 2022 and 2023. OpenBiome stopped shipping frozen preparations on 31 December 2024, which made conventional transplants harder to obtain. [23]

Why it matters. The licensed version arrived as the improvised version got scarce.

A phase 2 trial randomised 72 people newly diagnosed and not yet on medication. At 35 weeks, motor scores improved by 3.8 points in the donor group against 0.1 in those given back their own stool. [24]

Why it matters. The claim is spreading well past C. diff on much thinner evidence.

In a phase 1 trial, 20 people with advanced kidney cancer took donor capsules with immunotherapy. Half had serious immune side effects, and half of those assessable responded. [25]

Why it matters. Twenty people is a safety signal, not a result.

UCLA Health's physicians point to research showing probiotics after antibiotics can delay recovery. A few outside strains disturb the natives that are trying to regrow. [26]

Why it matters. Adding tenants is not the same as the room being full again.

A 91-day randomised trial gave a mixed synbiotic through a week of two antibiotics, and stool butyrate rose 119% against placebo. [27] A 32-patient postbiotic trial reported 40% higher diversity, and its authors include the product company's chief executive and co-founders. [28]

Why it matters. Both are real results; only one of them is at arm's length from the seller.

A meta-analysis of 34 trials in 2,390 adults found possible gains in cognitive scores, rated very low certainty. [29] Another pooled 29 trials in 1,633 older adults and measured bacteria, inflammation markers and fatty acids, not how people fared. [30]

Why it matters. Changing the roster is not the same as making someone better.

The NIH's Office of Dietary Supplements says there are no official recommendations covering probiotic use by healthy people. It also notes the FDA has reported that probiotics might cause infections or life-threatening illness in preterm infants. [31]

Why it matters. The safety case is thinnest exactly where the marketing is warmest.

Drug-resistant bacteria sit quietly in the gut and later cause bloodstream, surgical and urinary infections. Resistant infections were directly responsible for 1.27 million deaths in 2019. [32]

Why it matters. The reservoir is inside the patient long before the infection has a name.

Where bacteria of the Lachnospiraceae family were abundant, multidrug-resistant Enterobacterales were less likely to settle. Among liver transplant patients, lower diversity tracked with carrying them. [33]

Why it matters. Named residents, not a vague notion of good bacteria: occupancy is specific.

A review names the problem microbiotoxicity: unintended microbiome damage caused by non-antibiotic drugs, and by consumer, dietary and agricultural products. [34]

Why it matters. The bill is being run up in places nobody is counting.

In mice, amoxicillin caused long-lasting gut disruption and a long-term drop in one arm of lung immune defence, well after the drug had stopped. [35]

Why it matters. A local clearance with a distant, delayed cost.

A review of 25 studies in low- and middle-income countries found children's disruption ran deeper and lasted longer than adults'. Resistance genes persisted after the community itself had recovered. [36]

Why it matters. The room refills before the resistance leaves it.

Researchers tracked 134 people and 1,013 stool samples across three nurseries. Babies were passing strains to each other within a month, and antibiotic treatment was the condition that most increased the influx of new strains. [37]

Why it matters. Filling a room is a social process, not a private one.

507 samples from 195 mother-infant pairs showed milk and infant gut sharing strains, driven by Bifidobacterium longum. Antibiotic-resistance genes overlapped within pairs too. [38]

Why it matters. The seats are handed over, and so is what is written on them.

Bifidobacterium infantis given to breastfed US infants at two to four months colonised rapidly and was still there a month after the supplement stopped. [39]

Why it matters. Being first is powerful but not absolute; the right food still buys entry.

Breastfed infants tend to hold a stable community dominated by bifidobacteria. Formula-fed infants show more diversity earlier and move sooner toward an adult-like pattern. [40]

Why it matters. More diversity is not automatically better here, because stability was doing a job.

Solid food at around six months drives shifts that are known to persist into adulthood. The weaning window has had far less study than birth. [48]

Why it matters. A second handover, and much less watched.

Among 76,083 people having liver or pancreatic surgery, 1.1% got C. diff within 30 days, and nearly half of those cases came in the first week. After liver resection it more than tripled the odds of dying. [41]

Why it matters. Antibiotics, hospital and a cut stack the openings into one week.

A survey of 350 French infection-control staff found 70% used contact precautions for patients with symptoms. 71% cleaned with spore-killing products, and 5% used products that do not kill spores. [42]

Why it matters. Spores survive what ordinary cleaning leaves behind.

A Cell Host and Microbe review argues C. diff thrives on nutrients that only become available once the community is disrupted, including some enriched by modern diets. [9]

Why it matters. Not just an empty room, but an emptied room with food left out on the table.

Reviewers describe gut carriage of Klebsiella pneumoniae as the step before pneumonia, urinary and bloodstream infection. Resident bacteria are what normally hold it down. [43]

Why it matters. Another familiar hospital pathogen that begins as a lodger.

A study compared 88 patients infected with resistant organisms, 100 merely carrying them and 86 healthy controls. Both patient groups had lost Bacteroides, Faecalibacterium and Roseburia and gained Enterococcus. [49]

Why it matters. The thinned community and the resistant organism turn up in the same people.

Methods for identifying the keystone gut bacteria that hold a community together are inconsistent between studies, which is why diet advice built on them is shaky. [45]

Why it matters. We can watch a room empty without knowing which seats were load-bearing.

Researchers linked patterns of gut bacteria and short-chain fatty acids to different presentations of irritable bowel syndrome, in a field where earlier findings had failed to replicate. [44]

Why it matters. An honest example of evidence that is genuinely unsettled.