I. The Statutory Framework

The Sherman Antitrust Act, signed into law on July 2, 1890, remains the foundational statute of American competition law. Section 2, codified at 15 U.S.C. § 2, provides in its entirety that “every person who shall monopolize, or attempt to monopolize, or combine or conspire with any other person or persons, to monopolize any part of the trade or commerce among the several States, or with foreign nations, shall be deemed guilty of a felony.”¹ The penalty, as amended in 2004, is a fine of up to $100 million for a corporation, $1 million for any other person, or imprisonment not exceeding ten years, or both.

The Supreme Court has interpreted Section 2 through a two-element test. In United States v. Grinnell Corp. (1966), the Court held that the offense of monopolization requires “(1) the possession of monopoly power in the relevant market and (2) the willful acquisition or maintenance of that power as distinguished from growth or development as a consequence of a superior product, business acumen, or historic accident.”²

Monopoly power itself is defined in the case law as “the power to control prices or exclude competition.” This formulation, articulated in United States v. E.I. du Pont de Nemours & Co. (1956) and reaffirmed in every subsequent monopolization case, provides the operative test.³ The statute does not specify that the monopolist must be a corporation. It does not require that the monopolist be aware it is monopolizing. It does not require that the relevant market exist within the formal economy. It requires only that a person monopolize any part of the trade or commerce among the several States. The term “person,” as defined at 15 U.S.C. § 7, includes “corporations and associations existing under or authorized by the laws of… any State.” The term “commerce,” at 15 U.S.C. § 12, means “trade or commerce among the several States and with foreign nations.”

We acknowledge that the statute was not drafted with the gastrointestinal tract in mind. We submit that the statute does not care.

II. The Relevant Market

Antitrust analysis begins with the definition of a relevant market. The 2023 Merger Guidelines, jointly issued by the Department of Justice and Federal Trade Commission, define a relevant market as “a line of commerce and a section of the country” in which competitive effects are assessed.⁴ The relevant market has two dimensions: a product market (what is being sold) and a geographic market (where it is being sold).

The product market of the gut microbiome is biochemical intermediation. Gut bacteria produce short-chain fatty acids, synthesize vitamins, metabolize bile acids, and regulate the availability of neurotransmitters. The geographic market is the human gastrointestinal tract, a continuous tube approximately 30 feet in length that begins at the esophagus and terminates at the rectum.⁵ The relevant section of the country, for antitrust purposes, is the interior of approximately 330 million Americans.

The standard test for defining a product market is the hypothetical monopolist test, also known as the SSNIP test (Small but Significant and Non-transitory Increase in Price). It asks whether a hypothetical monopolist controlling all products in the proposed market could profitably impose a price increase of at least 5 percent.⁶ Consumers of the gut microbiome’s services cannot substitute an alternative provider. There is no competing gastrointestinal tract. There is no adjacent market. The microbiome could raise its prices to any level it chose, and the host organism would have no option but to continue purchasing. The SSNIP test is not merely satisfied. It is obliterated.

III. Market Concentration

The Herfindahl-Hirschman Index, or HHI, is the standard quantitative measure used by the DOJ and FTC to assess market concentration. The index is calculated by summing the squares of the market share percentages of all firms in the relevant market. Under the 2023 Merger Guidelines, a market with an HHI below 1,500 is considered unconcentrated, between 1,500 and 2,500 is moderately concentrated, and above 2,500 is highly concentrated.⁷

The human gut microbiome is dominated by two bacterial phyla. A landmark 2006 study by Ley, Turnbaugh, Klein, and Gordon, published in Nature, established that the phyla Firmicutes and Bacteroidetes together account for more than 90 percent of the gut microbial community in adult humans.⁸ Subsequent research has consistently confirmed this finding. The Human Microbiome Project, a National Institutes of Health initiative that characterized the microbial communities of 242 healthy adults, found that Firmicutes and Bacteroidetes maintained dominant positions across virtually all sampled individuals.⁹

A conservative allocation of market share assigns 60 percent to Firmicutes and 30 percent to Bacteroidetes, with the remaining 10 percent divided among Actinobacteria, Proteobacteria, Verrucomicrobia, and other minor phyla. The HHI calculation is straightforward: 60² + 30² + 10² = 3,600 + 900 + 100 = 4,600. The gut microbiome is not merely a highly concentrated market. At 4,600, its HHI nearly doubles the threshold the DOJ considers presumptively anticompetitive.

For comparison, the U.S. wireless telecommunications market, which the DOJ challenged in its 2011 attempt to block AT&T’s acquisition of T-Mobile, had an estimated post-merger HHI of approximately 3,100.¹⁰ The gut microbiome’s market concentration exceeds that of the telecom merger the government spent $39 billion trying to prevent.

IV. Exclusionary Conduct

Monopoly power alone does not establish a violation of Section 2. The Grinnell test requires evidence that the monopolist willfully acquired or maintained its power through conduct other than competing on the merits. The gut microbiome satisfies this element comprehensively.

The mechanism is called colonization resistance, and it has been documented in the microbiology literature for over sixty years. First described by van der Waaij, Berghuis-de Vries, and Lekkerkerk-van der Wees in 1971, colonization resistance refers to the ability of the resident intestinal microbiota to prevent colonization by exogenous organisms, including pathogens.¹¹

The incumbent bacteria employ multiple strategies to exclude competitors. They consume available nutrients, leaving insufficient resources for incoming organisms. They produce bacteriocins, which are antimicrobial peptides that kill competing bacterial species. They stimulate the host immune system to mount responses against newcomers while tolerating the resident population. And they occupy physical adhesion sites on the intestinal epithelium, denying competitors access to the mucosal surface.¹²

In antitrust terms, these are textbook exclusionary practices. Nutrient depletion is predatory pricing carried to its logical conclusion: the incumbent sells below cost until the competitor starves. Bacteriocin production is the biological equivalent of raising rivals’ costs. Immune system co-option is lobbying the regulator to enforce barriers that protect the incumbent while burdening new entrants. Physical site occupation is the original tying arrangement, bundling access to the intestinal wall with membership in the incumbent consortium.

The FTC’s own complaint against Qualcomm in 2017 alleged that the company maintained its monopoly in premium smartphone modem chips through practices that included “excluding competitors from the market” and imposing “onerous licensing terms.”¹³ The gut microbiome’s exclusionary practices are considerably less subtle. When antibiotics clear the field and new organisms attempt to colonize the vacated territory, the resident microbiome reassembles itself with remarkable fidelity, often restoring its original composition within weeks. This is not passive recovery. It is aggressive reconquest of lost market share, executed through precisely the mechanisms that antitrust law was designed to prohibit.

V. Control of the Serotonin Supply

The monopoly’s most consequential market manipulation involves the neurotransmitter serotonin. Approximately 95 percent of the body’s total serotonin is produced in the gastrointestinal tract, not the brain. This figure, confirmed by decades of research in gastroenterology and neuroscience, means that the gut is the dominant production facility for a molecule that regulates mood, appetite, sleep, memory, and social behavior.¹⁴

In 2015, Yano et al. published a study in Cell demonstrating that indigenous spore-forming bacteria from the gut microbiota directly promote serotonin biosynthesis by colonic enterochromaffin cells. When germ-free mice were colonized with specific gut bacteria, serotonin production increased dramatically. When those bacteria were absent, serotonin levels dropped by roughly 60 percent.¹⁵

The bacteria do not produce serotonin themselves. They manipulate the host’s own cells into producing it. They accomplish this by generating specific metabolites that signal enterochromaffin cells to upregulate expression of tryptophan hydroxylase 1, the rate-limiting enzyme in serotonin synthesis. In commercial terms, the microbiome does not own the factory. It controls the factory’s management. This is vertical market power of a kind the Department of Justice has challenged in industries ranging from aluminum to motion pictures.

The consequences of this supply-side dominance extend throughout the body. Gut-derived serotonin modulates gastrointestinal motility, bone density, cardiovascular function, and platelet aggregation.¹⁶ A consortium of organisms that controls 95 percent of the supply of a molecule regulating this many downstream markets is not a participant in the economy. It is the economy.

VI. The Immune System as Captured Regulator

The gut-associated lymphoid tissue, or GALT, is the largest component of the human immune system. Research published in Clinical and Experimental Immunology by Vighi et al. (2008) reported that the gut contains approximately 70 percent of the body’s immunocyte population, making it the primary site of immune regulation in the human body.¹⁷

Under normal conditions, the immune system functions as the body’s regulatory apparatus, charged with identifying and eliminating threats. The gut microbiome has co-opted this regulator. Commensal bacteria actively train the immune system to tolerate their presence through mechanisms that include induction of regulatory T cells, modulation of cytokine profiles, and maintenance of the mucosal barrier that separates the intestinal lumen from the systemic circulation.¹⁸

Simultaneously, the resident microbiome programs the immune system to attack newcomers. This is not a metaphor. Colonization resistance operates in part because the immune system mounts inflammatory responses against unfamiliar bacterial species while actively suppressing responses against the incumbents. The regulator has been trained, by the regulated entity, to protect the regulated entity’s market position.

In the history of American antitrust enforcement, regulatory capture has been identified as a significant barrier to competition in industries including telecommunications, banking, and energy. The gut microbiome has achieved something more thorough than regulatory capture. It has reprogrammed the regulator’s core operating parameters from infancy. Research on neonatal immune development shows that the initial microbial colonizers of the infant gut play an outsized role in shaping the immune system’s lifelong response patterns.¹⁹ The first bacteria to arrive do not merely gain a commercial advantage. They rewrite the rules of the market.

VII. The Conspiracy Element

Section 1 of the Sherman Act, codified at 15 U.S.C. § 1, prohibits “every contract, combination in the form of trust or otherwise, or conspiracy, in restraint of trade or commerce among the several States.”²⁰ Unlike Section 2, which addresses single-firm monopolization, Section 1 requires concerted action between two or more entities.

The gut microbiome is not a single organism. It is a consortium of approximately 38 trillion individual bacterial cells, as estimated by Sender, Fuchs, and Milo in their 2016 revision published in Cell.²¹ These cells belong to an estimated 500 to 1,000 distinct species, as documented by the Human Microbiome Project.²² They are not genetically related. They do not share a corporate parent. They are independent organisms that have entered into a coordinated arrangement to control a market.

The coordination is not speculative. Gut bacteria communicate through a mechanism called quorum sensing, in which individual cells release and detect signaling molecules called autoinducers. When the concentration of autoinducers reaches a threshold, the bacterial population modifies its collective behavior. This includes coordinated production of biofilms, synchronized release of metabolites, and collective responses to environmental changes.²³

In antitrust law, this is a cartel. Specifically, quorum sensing satisfies the elements of what the Supreme Court has termed a “hub-and-spoke conspiracy,” in which a central coordinating mechanism links otherwise independent actors into a unified scheme. The autoinducer molecules serve as the hub. The responding bacterial populations are the spokes. The collective behavior that results, including synchronized metabolite production and coordinated exclusion of competitors, constitutes the restraint of trade.

Certain categories of coordination are classified as per se violations of Section 1, meaning they are conclusively presumed to be unreasonable regardless of the parties’ justifications. These include price fixing, market allocation, and bid rigging.²⁴ When gut bacteria divide the intestinal tract into colonization zones, with Bacteroidetes predominating in the outer mucus layer and Firmicutes in the inner layer, they are allocating markets. When they collectively set the production levels of short-chain fatty acids including butyrate, propionate, and acetate, they are fixing prices. These are not violations that require economic analysis or a rule-of-reason weighing. They are per se illegal.

VIII. Damages

Section 4 of the Clayton Act, codified at 15 U.S.C. § 15, provides that “any person who shall be injured in his business or property by reason of anything forbidden in the antitrust laws may sue therefor… and shall recover threefold the damages by him sustained.”²⁵ This treble damages provision has produced some of the largest civil judgments in American legal history.

Quantifying the damages caused by the gut microbiome’s monopolistic conduct requires identifying the injury. The microbiome controls the production of serotonin, short-chain fatty acids, certain B vitamins, and vitamin K. It regulates the availability of dietary energy by determining what fraction of ingested calories the host actually absorbs. Research by Bäckhed et al. (2004), published in the Proceedings of the National Academy of Sciences, demonstrated that germ-free mice colonized with a conventional gut microbiota experienced a 60 percent increase in body fat despite no increase in food consumption.²⁶

The microbiome, in other words, takes a cut. It extracts a portion of every caloric transaction that passes through its territory. If this were a financial intermediary operating without regulatory authorization, the Consumer Financial Protection Bureau would have intervened. If it were a pipeline operator extracting tolls without a filed tariff, the Federal Energy Regulatory Commission would have opened a proceeding. But because the intermediary is composed of Firmicutes rather than holding companies, no agency has claimed jurisdiction.

The aggregate economic exposure is substantial. The U.S. food industry generated approximately $2.4 trillion in consumer spending in 2023, according to the USDA Economic Research Service.²⁷ The microbiome processes every dollar of that spending. If its intermediation fee is estimated conservatively at 10 percent of caloric throughput, based on the germ-free mouse data, the annual extraction is approximately $240 billion. Trebled under Section 4 of the Clayton Act, the annual damages are $720 billion. Over the twenty-year life of a typical antitrust claim, the cumulative exposure exceeds $14 trillion, which is approximately 50 percent of the current national debt.

IX. Conclusion

The evidence does not require creative interpretation. Section 2 of the Sherman Act prohibits monopolization. The gut microbiome holds monopoly power in the market for biochemical intermediation within the human gastrointestinal tract. It maintains that power through exclusionary conduct, including nutrient depletion, targeted elimination of competitors, and capture of the regulatory apparatus. Its market concentration, measured by the standard index the government applies to every proposed merger in America, is nearly double the presumptive threshold for anticompetitive harm.

Section 1 of the Sherman Act prohibits combinations in restraint of trade. The microbiome is a consortium of 38 trillion independent organisms that coordinate their behavior through chemical signaling, allocate markets by intestinal geography, and fix the output levels of critical biochemical commodities. These are per se violations that do not require economic justification.

The Clayton Act provides for treble damages. The annual exposure, based on the microbiome’s share of caloric throughput in a $2.4 trillion food economy, exceeds the GDP of most nations.

Three hundred and thirty million Americans are hosting the most concentrated, most exclusionary, and most thoroughly coordinated monopoly in the history of commerce. It operates inside their bodies. It has been operating there since approximately five days after birth, when the initial colonizers establish the market position they will defend for the next seven or eight decades. No consent decree has ever been entered. No divestiture has ever been ordered. No fine has ever been assessed.

The Department of Justice Antitrust Division employs approximately 600 attorneys. Not one of them has opened a file. The Federal Trade Commission’s Bureau of Competition has challenged mergers in markets for hospital beds, industrial gases, and sugar. It has not challenged the most concentrated market in the American body.

The microbiome does not dispute the facts. It does not retain counsel. It does not negotiate consent decrees. It simply continues to monopolize, in open violation of a statute that has been on the books for 136 years, secure in the knowledge that the enforcement apparatus of the federal government has not yet identified the gastrointestinal tract as a relevant market.

It is.

Ergo.