How does a diverse microbiome actually block infections?
The primary mechanism is nutrient blocking: a diverse community of gut bacteria collectively consumes the same nutrients that invading pathogens need to establish themselves. A landmark 2023 study in Science demonstrated this by testing how well different human gut communities resisted colonization by two major bacterial pathogens (Escherichia coli and Klebsiella pneumoniae) both in lab dishes and in germ-free mice [3]. Single bacterial species had almost no protective effect, but as community diversity increased, colonization resistance grew dramatically. The key was that diverse communities contained species that together used up overlapping nutrients—when the pathogens arrived, there was nothing left for them to eat [3].
This explains why diversity matters: a single species can only consume a narrow set of nutrients, leaving plenty for a pathogen. A diverse community, however, creates a 'nutrient vacuum' that starves the invader. The study also showed that this principle could be used to predict which communities would resist a novel pathogen, suggesting it's a general rule rather than a one-off effect [3].
What does the evidence from actual infections show?
Multiple studies across different infections confirm that low gut microbiome diversity is consistently linked to worse infection outcomes. In HIV-1 patients, a 2016 study of 28 viremic patients found that lower bacterial diversity (measured by the number of species and the Shannon index) was strongly associated with lower CD4 T-cell counts—a key measure of immune function—and with higher markers of microbial translocation and immune activation [5]. For every additional bacterial species in the gut, CD4 count increased by 0.88 cells/μl (a statistically significant association), meaning more diverse microbiomes predicted better immune status [5].
The pattern holds for parasitic infections too. A 2022 study of diarrheal patients infected with the pathogenic Blastocystis ST7 parasite found that infected patients had significantly lower bacterial diversity than uninfected patients, along with a shift toward more Enterobacteriaceae (a family that includes many opportunistic pathogens) and fewer beneficial Bacteroides [7]. Similarly, in chickens infected with the coccidiosis parasite Eimeria maxima, infection caused a clear microbial imbalance (dysbiosis), with the normally dominant Firmicutes bacteria dropping below 60% of the community and being replaced by Campilobacterota and Proteobacteria [1]. These changes in diversity and composition were directly linked to the infection, not just coincidental [1].
However, the relationship isn't always straightforward. A 2024 study of hookworm-infected people treated with albendazole found that baseline gut microbiota diversity did NOT predict whether the drug would cure the infection—patients who were cured and those who weren't had similar diversity levels [4]. This suggests that while diversity protects against initial infection, it may not influence how well an existing infection responds to treatment.
Are there cases where diversity doesn't protect—or even makes things worse?
Yes. Diversity is not a magic shield; it only helps if the community contains the right species. The 2023 nutrient-blocking study made this clear: some high-diversity communities were not protective because they lacked the specific bacteria needed to consume the pathogen's preferred nutrients [3]. In other words, a diverse community of the wrong bacteria is no better than a low-diversity one.
Additionally, some infections actually increase diversity in ways that may be harmful. A 2022 study of diabetic retinopathy patients found that those with the condition had higher gut microbial richness than healthy controls, but this was accompanied by a shift toward potentially harmful bacteria like Bacteroides and Ruminococcus torques, and a drop in beneficial genera like Blautia and Collinsella [6]. So higher diversity isn't always good—it depends on which species are driving the increase.
The context also matters. In burn patients with Clostridioides difficile infection, a 2021 study found that infected patients had significantly reduced levels of beneficial bacteria like Faecalibacterium prausnitzii and increased levels of Akkermansia muciniphila and opportunistic pathogens like Enterococcus and E. coli [2]. Here, the loss of key protective species was more important than overall diversity numbers.
About These Sources
This answer is built on 7 peer-reviewed studies — published from 2016 to 2024, 1 from 2024 or later, 4 in Q1 journals, collectively cited 468 times — selected as the most relevant from 15 studies that passed quality screening, drawn from 84 papers retrieved from a database of over 500 million.
Sources used in this answer
Influence of Eimeria maxima coccidia infection on gut microbiome diversity and composition of the jejunum and cecum of indigenous chicken
In chickens infected with Eimeria maxima, infection caused clear dysbiosis: Firmicutes dropped below 60% of the community and were replaced by Campilobacterota in the jejunum and Proteobacteria in the cecum.
Gut microbiota in burned patients with Clostridioides difficile infection
In burn patients with C. difficile infection, beneficial bacteria like Faecalibacterium prausnitzii were significantly reduced while Akkermansia muciniphila and opportunistic pathogens (Enterococcus, E. coli) increased.
Microbiome diversity protects against pathogens by nutrient blocking
In the largest and most direct test of the question, human gut communities with high bacterial diversity strongly resisted two major pathogens (E. coli and K. pneumoniae) by collectively consuming the nutrients the pathogens needed, while single species had negligible effect.
Baseline gut microbiota diversity and composition and albendazole efficacy in hookworm-infected individuals.
In hookworm-infected people treated with albendazole, baseline gut microbiota diversity did not differ between those who were cured and those who were not, suggesting diversity does not predict treatment response.
Gut microbiota diversity predicts immune status in HIV-1 infection.
In HIV-1 patients, lower gut bacterial diversity was strongly associated with lower CD4 T-cell counts and higher markers of immune activation; for every additional bacterial species, CD4 count increased by 0.88 cells/μl.
Composition and diversity of gut microbiota in diabetic retinopathy
Diabetic retinopathy patients had higher gut microbial richness than healthy controls, but this was accompanied by increases in potentially harmful bacteria (Bacteroides, Megamonas) and decreases in beneficial genera (Blautia, Collinsella).
Infection with pathogenic Blastocystis ST7 is associated with decreased bacterial diversity and altered gut microbiome profiles in diarrheal patients
Diarrheal patients infected with pathogenic Blastocystis ST7 had significantly lower bacterial diversity and a shift toward Enterobacteriaceae and Escherichia-Shigella, with fewer Bacteroides and Parabacteroides.