Abstract
The human gut microbiota is comprised predominantly of bacteria, and also includes archaea, fungi, and viruses. The gastrointestinal epithelium, mucosal barrier, and mucosal immune system balance protection against infection at mucosal entry points with symbiosis and tolerance to non-harmful organisms and antigens. Aging is associated with notable changes in both gut microbiota and mucosal immunity, including reduced microbial diversity, increased proportion of pathobionts relative to commensals, immunosenescence, and chronic inflammation. These changes may disrupt gastrointestinal function and homeostasis and increase susceptibility to infection and inflammatory conditions. Multiple drug classes are also associated with disruption of the gut microbiota and mucosal immunity, including antibacterials, proton pump inhibitors (PPIs), metformin, and steroidal and non-steroidal anti-inflammatory agents. This review describes the mechanisms by which these drugs affect the gut microbiota and mucosal immunity to provide perspective of the concurrent effects of drugs and age-related changes.
Introduction
Overview of gut microbiota
The human gut microbiota functions as an intricate and diverse ecosystem composed of bacteria, archaea, fungi, and viruses, which collectively impact host digestion, metabolism, and immunity. The gut microbiota along with mucosal immunity provide colonization resistance to protect the host against pathogens at the mucosal surfaces. This relationship between the gut microbiota and host is critical for maintaining a commensal or mutualistic relationship, as well as host nutritional and immune homeostasis (). While there is significant variability, bacteria of the human gut microbiota are predominantly of the phyla Firmicutes, Bacteroidetes, Actinobacteria, Pseudomonadota (Proteobacteria), and sometimes Verrucomicrobia, and the population stays relatively constant over time for a given individual. The population can be disrupted or changed based on age, diet, antimicrobial exposure, and the immune response of the host ().
Firmicutes
Gut Firmicutes, primarily Lactobacilli, Clostridia, and Enterococci in, are Gram-positive bacteria that are essential for the fermentation of carbohydrates into short-chain fatty acids (SCFAs). These SCFAs serve a primary role in preserving and maintaining intestinal integrity along with regulating mucosal immunity. Numerous Firmicutes are probiotic organisms, often found in dietary supplements. The relative abundance of Firmicutes may be greater in obese individuals (). They are susceptible to alterations by beta-lactams, glycopeptides, and fluoroquinolones, which can lead to dysbiosis and opportunistic infections ().
Bacteroidetes
Bacteroidetes, primarily Bacteroides and Prevotella, are significant in the fermentation of complex polysaccharides and the synthesis of SCFAs. Though essential for gut homeostasis, certain species can become pathogenic in dysbiotic conditions. Bacteroidetes are primarily targeted by broad-spectrum antibiotics such as carbapenems and fluoroquinolones, affecting the microbial balance ().
Actinobacteria
Actinobacteria, such as Bifidobacterium, are important for the digestion of dietary fibers and immune response regulation. This bacterium is commonly found in probiotics and, in moderation, is known to promote intestinal health (). They are susceptible to macrolides, penicillins, vancomycin, and other agents that cover gram positive bacteria ().
Verrucomicrobia
Verrucomicrobia, primarily Akkermansia muciniphila, is one of few gut bacteria able to utilize mucin as a primary energy source. Mucin, a major component of the intestinal mucus layer, is known to contribute to the mucosal barrier integrity and metabolic health of the host. However, degradation by A. muciniphila is beneficial as it promotes mucus turnover and the release of short-chain fatty acids (). This bacterium is sensitive to broad-spectrum antibiotics and is being studied for use in treatment of metabolic and inflammatory diseases ().
Pseudomonadota (formerly Proteobacteria)
Bacteria of Pseudomonadota, such as Escherichia and Helicobacter, include beneficial and potentially pathogenic species, which may be associated with disease and inflammatory conditions. Pseudomonadota overgrowth, often associated with frequent antibiotic exposure or an immunosuppressive state, enhances gut inflammation (). Selected genera, role in gut health, and antibacterial therapy considerations for these phyla are contained in Table 1.
TABLE 1
| Phylum | Genera | Role in gut health | Dietary influence | Associated gut pathogenicity | Colonization resistance (mechanisms) | Antibacterial coverage | Probiotics use |
|---|---|---|---|---|---|---|---|
| Firmicutes | Lactobacilli Clostridia Faecalibacteria Streptococci Peptostreptococci Eubacteria Bacillus | SCFA production, carbohydrate fermentation, immune tolerance | Fiber promotes growth, high-fat diet increases abundance | Obesity Diabetes Inflammatory bowel disease (IBD) Mental health | Strong (SCFA production, biofilm, immune modulation, nutrient competition) | Generally susceptible to agents that cover Gram positive bacteria | Lactobacillus Bacillus, Streptococcus, Enterococcus spp. |
| Bacteroidetes | Bacteroides Prevotella Porphyromonas | Carbohydrate metabolism, SCFA production, regulation of mucin secretion | Fiber-rich diet enhances presence, high-fat diet decreases abundance | IBD Inflammatory bowel syndrome (IBS) Appendicitis Intra-abdominal abscess | Strong (SCFA production, immune modulation, bacteriocins, bile acid modification) | Generally susceptible to agents that cover anaerobic bacteria, high potential for acquired resistance | Uncommon |
| Actinobacteria | Bifidobacteria Actinomyces | Immune modulation, SCFA production, Gut permeability modulation | Fiber and prebiotics promote growth | Uncommon | Moderate (SCFA production, immune modulation, nutrient competition) | Generally susceptible to agents that cover Gram positive bacteria | Bifidobacterium, Streptomyces spp. |
| Pseudomonadota | Escherichia Helicobacter Salmonella Citrobacter Proteus Enterobacter Klebsiella Vibrio Moganella Serratia Yersinia | Diverse, may be beneficial or pathogenic, inflammation | Diet-dependent, high-fat diet may increase abundance | Nosocomial Enterocolitis Shanghai fever Necrotizing disease | N/A (Increased abundance associated with pathogenicity) | Antibacterial resistance is common | Uncommon |
| Verrucomicrobia | Akkermansia | Mucosal integrity, metabolic regulation | Fiber-rich diet supports growth | Uncommon | Moderate | May be susceptible to agents that cover Gram negative bacteria | Uncommon |
Gut microbiota characteristics and functions Table 1. Major bacterial phyla of the gut microbiota and their primary roles in health and disease.
The five predominant phyla—Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria, and Verrucomicrobia—have distinct metabolic and immune functions. Firmicutes are the main short-chain fatty acid (SCFA) producers and maintain barrier integrity (). Bacteroidetes play a critical role in carbohydrate metabolism and SCFA production (). Actinobacteria, particularly Bifidobacterium, ferment carbohydrates and modulate host immunity (). Proteobacteria include opportunistic pathobionts such as E. coli that expand during dysbiosis and inflammation (). Verrucomicrobia, mainly Akkermansia muciniphila, support mucosal integrity and metabolic regulation ().
Mucosal immunity
The mucosal immune system serves as the primary defense against intestinal pathogens while also maintaining tolerance to commensal or mutual microbes and dietary antigens. Many cells, substances, and processes are associated with maintaining this balance. Gut-associated lymphoid tissue (GALT), secretory immunoglobulin A (sIgA), epithelial cells, dendritic cells, toll-like receptors (TLRs), interleukins (ILs), inflammasomes, macrophages, and other chemical messengers collectively form an immunological barrier to pathogens while regulating the response of the adaptive immune system to microbiota (). For example, sIgA may bind to, neutralize pathogens, and prevent microbial adhesion to epithelial surfaces, or present components of commensal bacteria to tolerogenic dendritic cells to reduce response. The diversity of organisms and need to determine their pathogenicity therefore presents a unique challenge with potential for dysbiosis either by unregulated bacterial growth and pathogenesis or excessive response with inflammation and damage to microbiota.
Drugs and the aging gut microbiota
Immunosenescence and chronic inflammation (“inflammaging”) during aging contribute to microbial dysbiosis, reduced SCFA production, and weakened epithelial defenses, increasing susceptibility to infections (). Additionally, pharmaceutical interventions, including proton pump inhibitors (PPIs), metformin, nonsteroidal anti-inflammatory drugs (NSAIDs), corticosteroids, and antibiotics, further influence the gut microbiota and mucosal immune landscape. Risk of colorectal cancer (CRC), dysregulated growth and proliferation of cells of the inner lining of the colon or rectum, is higher in the aging population, although the incidence has increased in younger adults since the 1990s (). Microbial dysbiosis is a known risk factor for CRC, and the microbiota of CRC patients are distinct by overall composition and decreased diversity. Known bacterial associations include increased relative abundance of F. nucleatum, E. faecalis, E. coli, P. anaerobius in CRC patients, and decreased beneficial bacteria such as butyrate and lactate producers (). These associations occur at the population level, there is not one specific bacterial species or phylum is specifically associated with oncogenesis (the listed species are of members of Fusobacteriota, Firmicutes, Pseudomonadota, and Bacillota respectively), and further study is necessary to better understand the role of dysbiosis. Understanding the interactions between gut microbiota, mucosal immunity, aging, and medication use is crucial for developing targeted interventions that mitigate dysbiosis-related disorders. This review addresses drug-induced alterations in the gut microbiota impact mucosal immunity and aging-related changes, providing insights into strategies to preserve microbial and immune homeostasis.
The aging gut microbiota and mucosal immunity
One of the most notable alterations in the aging gut microbiota is the reduction in microbial diversity. The decrease of beneficial bacterial species leads to a general decline in overall colonization resistance, making the gut more susceptible to inflammation and dysbiosis. Reduced production of short-fatty acid chains (SCFAs), which are essential for the epithelial integrity and mucosal immune response of the gut barrier, is a key change implicated in decreased colonization resistance (). Among the SCFA-producing gut bacteria, reduced abundance of Bifidobacterium (Actinobacteria) and Clostridium (Firmicutes) are particularly associated with age-related decline. The reduction of these microbes leads to compromised mucosal barrier function and increased gut permeability, which can promote systemic inflammation and age-related metabolic disorders like metabolic syndrome and type 2 diabetes. SCFA’s role in increasing colonocytes and immune response modulation is well known, while their reduction is linked to chronic low-grade inflammation, a hallmark factor of aging (). Akkermansia muciniphila (Verrucomicrobia), a key bacterium associated with gut barrier integrity and metabolic regulation, tends to decrease with age (). The loss of this bacterium is associated with an increase in gut permeability and enhanced inflammatory responses (). Lactobacillus (Firmicutes), which is responsible for microbial homeostasis and the production of antimicrobials, also decreases with age, adding to gut dysfunction ().
The aging gut is also associated with an increase in pathobionts; microbes that have the potential to induce inflammation and disease in specific circumstances (). One such bacterium that shows an increase in the aging gut is Escherichia coli (Proteobacteria). An overgrowth of this microbe is often associated with antibiotic use, reduced immune surveillance, and shifts in gut pH, all of which are common in aging populations (). These conditions lead to enhanced gut inflammation, increased intestinal permeability, and a greater risk of infections. These changes are depicted in Figure 1.
FIGURE 1
In conjunction with changes in microbial composition, immunosenescence, a decline in the efficiency of the immune system, is also associated with aging. For example, aging is associated with reduction in secretory immunoglobulin A (sIgA), which is involved with neutralizing pathogens, preventing their adhesion to epithelial cells, and maintaining microbial hemostasis. Reduction of this immunoglobulin can cause a weakening of the mucosal defense system and an increase in the risk of infections and dysbiosis (
Drug-induced alterations in gut microbiota and mucosal immunity
Drugs may contribute to or exacerbate age-related changes in gut microbiota and may impact mucosal immunity. Proton pump inhibitors, metformin, anti-inflammatory agents such as NSAIDS and steroids, and antibacterials play a significant role in disruption of gut mucosa. Additionally, 24% of marketed drugs inhibit at least one common intestinal microbiome bacterial strain, including nonantibacterial agents such as statins, angiotensin converting enzyme (ACE) inhibitors, atypical antipsychotics, and cholinesterase inhibitors. Polypharmacy, the use of multiple drugs, is also common in elderly patients with an average daily use of more than seven drugs by nursing home patients (
TABLE 2
| Drug class | Examples (generic name) | Common indications | Microbiota association |
|---|---|---|---|
| Proton pump inhibitors | Omeprazole Pantoprazole Lansoprazole | Gastroesophageal reflux disease (GERD) | Increase in pathogenic bacteria and reduced production of SCFAs |
| Biguanide | Metformin | Type II diabetes mellitus | Increase in beneficial bacteria and modulation of gut permeability |
| NSAIDs | Ibuprofen Naproxen Indomethacin | Arthritis Headache Gout | Reduce mucus production, increase gut permeability, modulation of microbiota composition |
| Corticosteroids | Prednisone Dexamethasone Budesonide Methylprednisolone | Inflammatory conditions (e.g., infection, multiple sclerosis) | Increase in pathogenic bacteria, reduce mucus production, increase gut permeability |
| Antibacterials | Doxycycline Azithromycin Amoxicillin Clindamycin Meropenem | Bacterial infections | Variable based on spectrum, greatest risk of opportunistic pathogen overgrowth with broad spectrum agents |
| Statins | Atorvastatin Simvastatin Rosuvastatin | Dyslipidemia | Unknown, potentially shift microbiota composition toward beneficial bacteria |
| Antihypertensives | Lisinopril Losartan Amlodipine | Hypertension Heart failure | Unknown, may affect or be affected by hypertension or some drugs used to treat hypertension |
Drug Classes Associated with Alterations of Microbiota and/or Mucosal Immunity Proton pump inhibitors (PPIs) (
Proton pump inhibitors (PPIs)
These drugs decrease stomach acid production by irreversibly binding to H+/K+-ATPase (proton pump) in gastric parietal cells, preventing the ultimate step of gastric acid secretion. This inhibition leads to a significant and sustained increase in gastric pH, which disrupts the microbiota balance. This higher pH facilitates the overgrowth of microbes such as Streptococcus and Rothia in the lower gastrointestinal tract and pathogenic bacteria like Clostridium difficile (
Metformin
Metformin is another agent that may disrupt the gut microbiota through direct epithelial and indirect microbial pathways. While it is used therapeutically to reduce hepatic glucose production by activating AMP-activated protein kinase (AMPK), a key regulator of energy metabolism, it also affects the gut microbiota by altering microbial composition and increasing mucus production. Metformin increases the abundance of Akkermansia muciniphila, a beneficial bacterium that strengthens the gut lining, by stimulating mucin secretion by goblets cells and enhancing mucin genes such as MUC2 through AMPK activation. This leads to a thicker and more diverse mucus layer that favors the increase of Akkermansia muciniphila. (
Non-steroidal anti-inflammatory drugs (NSAIDs)
This class of drugs, which includes both prescription and over-the-counter (OTC) agents, inhibits either or both cyclooxygenase-1 and -2 (COX-1,2) enzymes with variable selectivity, thereby blocking the conversion of arachidonic acid into prostaglandins. COX-1 is constitutively active and generates prostaglandins that maintain gastrointestinal mucosal integrity (by stimulating mucus/bicarbonate secretion and mucosal blood flow), whereas COX-2 is inducible and produces prostanoids during inflammation. By inhibiting both isozymes, these drugs reduce prostaglandin synthesis, particularly prostaglandin E2 and prostacyclin, which are gastroprotective (
Corticosteroids
Corticosteroids, primarily prescribed for their anti-inflammatory effects, impact the gut microbiome by binding to the intracellular glucocorticoid receptor (GR), which displaces into the nucleus and modifies gene transcription, leading to a decreased production of pro-inflammatory cytokines such as TNF-α, IL-1, and IL-6. This suppression leads to a reduction in secretory immunoglobulin A (sIgA), an essential defense mechanism in mucosal immunity. SIgA aids in the neutralization of pathogens and maintenance of microbial homeostasis (
Antibacterials
Antibacterial antibiotics target fundamental bacterial processes such as cell wall or DNA synthesis. They are not inherently selective for pathogenic bacteria, and therefore indiscriminately deplete both pathogens and commensals (
Cardiovascular drugs
Medications for conditions such as dyslipidemia and hypertension may be associated with gut microbiota in either or both directions (i.e., the medication affects the microbiota or the microbiota affects the activity of the medication). Statins modify the composition and diversity of the gut microbiota, and their lipid-lowering effects may in turn be affected by gut bacteria. While not fully understood, potential mechanisms of bacteria-associated hypolipidemic activity may include drug metabolism, modulation of protein expression of enzymes associated with bile acid synthesis, or modification of drug transport. The mechanisms of effects of statins on gut bacteria are also not well understood and may involve an increase proportion of anti-inflammatory bacteria with a corresponding decrease in pro-inflammatory bacteria (
FIGURE 2

Age-related changes, gut health, and impact of drugs on gut microbiota. The horizontal axis depicts the general change in relative occurrence with aging, while the vertical axis lists major functions in gut health. (
Discussion
The dynamic relationship of gut microbiota, mucosal immunity, aging, and pharmaceutics interventions has a significant impact on overall physiological functions and disease susceptibility. Aging is associated with changes in the gut microbiome including decreased microbial diversity, reduced short-chain fatty acid (SCFA) production, and elevated pathobiont proportions (
Medications such as proton pump inhibitors (PPIs), metformin, nonsteroidal anti-inflammatory drugs (NSAIDs), corticosteroids, and antibacterials also influence gut microbiota function. PPIs may alter microbial colonies and induce overgrowth of pathogenic bacteria, which compromises the mucosal defenses and in severe cases, the resulting infections may cause ulcers (
To address drug-induced dysbiosis, probiotics and prebiotics products may be helpful to restore microbial balance, enhance SCFA production, and reinforce mucosal defenses (
Future research should address the long-term effects of pharmacological agents on gut microbiota and mucosal immunity in aging populations, as well as identification of connections between microbiota, immune function, and the effects of medications. Integrating microbiome-conscious approaches into clinical practice could allow healthcare providers to optimize patient care (
Conclusion
The connection between gut microbiota, mucosal immunity, aging, and pharmaceutical medications is critical to maintaining health and reducing the disease susceptibility of the aging population for maintenance of health and to decrease the risk of diseases such as CRC. Medications such as PPIs, NSAIDs, corticosteroids, metformin, and antibiotics alter the composition of the gut microbiota through complex and sometimes bidirectional mechanisms. Preservation of microbial diversity and mucosal barriers through the usage of probiotics, prebiotics, and microbiota-sparing drug therapies may amend such effects and warrant further investigation. Gut microbiota profiling and genomic analysis could individualize therapies to both improve microbial diversity and minimize the risk of dysbiosis complications for treatment of other conditions. Further research is needed to explore strategies that maintain or improve microbial balance and immune competency to improve treatment outcomes and the long-term health of the aging population.
Statements
Author contributions
LT: Methodology, Writing – review and editing, Investigation, Writing – original draft, Data curation, Resources, Visualization, Conceptualization. JT: Investigation, Writing – review and editing, Writing – original draft, Visualization, Resources, Data curation. DA: Methodology, Writing – original draft, Supervision, Writing – review and editing, Conceptualization.
Funding
The author(s) declare that no financial support was received for the research and/or publication of this article.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
aging, gut microbiota, mucosal immunity, antibacterials, PPIs, metformin, anti-inflammatory agents, corticosteroids
Citation
Totleben L, Thomas J and Austin D (2025) Drug-mediated disruption of the aging gut microbiota and mucosal immune system. Front. Aging 6:1603847. doi: 10.3389/fragi.2025.1603847
Received
01 April 2025
Accepted
18 September 2025
Published
13 October 2025
Volume
6 - 2025
Edited by
Jenna M. Bartley, University of Connecticut Health Center, United States
Reviewed by
Rachel McMahan, University of Colorado Anschutz Medical Campus, United States
Rekha Jalandra, Maharshi Dayanand University, India
Radhika Kumar, University of Southern California, United States
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Copyright
© 2025 Totleben, Thomas and Austin.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Daniel Austin, daustin@lecom.edu
Disclaimer
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