Lactobacillus johnsonii

Lactobacillus johnsonii

1. Identity, Taxonomy, and Natural Sources

Scientific Name and Classification

Lactobacillus johnsonii is a species in the genus Lactobacillus identified in 1980 by John L. Johnson, an American microbiologist and his associates. Its type strain is ATCC 33200. The species belongs to the broader family Lactobacillaceae (order Lactobacillales), and it is part of what is known as the L. acidophilus group — a cluster of closely related, host-adapted lactic acid bacteria. Lactobacillus gasseri and Lactobacillus johnsonii are among each other's nearest phylogenetic relatives.

First formally described in 1992, it was often mistaken for L. acidophilus until scientists realized it deserved its own name, honoring microbiologist J.L. Johnson. Subsequent to early deposit under the L. acidophilus designation, a taxonomic study of the Lactobacillus acidophilus group resulted in recognition of Lactobacillus johnsonii sp. nov. as a distinct species, published in the International Journal of Systematic Bacteriology in 1992. In 2020, when the massive Lactobacillus genus was split into 25 new groups, many species were renamed; however, L. johnsonii retained its original name, keeping its identity as part of the "acidophilus group."

Lactobacillus johnsonii is a Gram-positive, homofermentative, non-spore-forming rod-shaped host-adapted bacterium, with lactic acid being its predominant end product from sugar metabolism. L. johnsonii is a Gram-positive, non-bacteriophage-forming, partially anaerobic lactic acid bacterium.

Natural Habitat and Host Range

Lactobacillus johnsonii is found as part of the natural microbiota of the gut, the oral cavity, and the vagina. L. johnsonii is typically found in the human and animal gastrointestinal tract, where it can obtain nutrients from its host. As an auxotrophic bacterium that lacks certain enzymes needed for the digestion of complex carbohydrates, it is unable to compete with other GI tract bacteria such as Bifidobacteria, which inhabit the colon. Therefore, L. johnsonii resides in the upper GI tract, which is rich in amino acids and peptides — specifically, it is one of the dominant microorganisms found at the junction between the ileum of the small intestine and the cecum of the colon.

Beyond its quirky taxonomic history, Lactobacillus johnsonii is best known as a friendly commensal that lives alongside its hosts. It has been found in the guts of humans, pigs, dogs, poultry, rodents, and even honeybees, each time adapting to the specific conditions of that host. Several strains of this species have been isolated from vaginal and gastrointestinal (GI) tracts of vertebrate hosts, including humans, rodents, swine, and poultry.

Natural Food Sources

L. johnsonii is naturally found in the human gut. It can also be obtained from dietary supplements and some fermented foods, including kefir and yogurt. It is a naturally occurring microorganism found in the human gastrointestinal tract (particularly in the small intestine) as well as in fermented foods and dairy environments. L. johnsonii enhances fermentation by converting sugars into lactic acid, contributing both flavor and microbial safety. Though not a mainstay in traditional yogurt starter cultures, it adds probiotic value when included.

Common Supplement Forms

L. johnsonii is available as a dietary supplement in various strengths and dosage forms. Some dietary supplements that contain L. johnsonii also contain many other ingredients, including other probiotics. It is available in various forms, including capsules and powders, often as part of probiotic blends. The L. johnsonii strain La1 was one of the first cultures to be proposed as a probiotic dairy supplement in 1995 at the Nestlé Research Center, Lausanne. Although yeast and bacteria had been used in dairy products for fermenting purposes for centuries, the investigation and choice of a microorganism as a fermenting agent based on its health benefits was novel at the time. Today, the probiotic culture is used in the LC1 yogurt products by Nestlé.

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2. Traditional and Historical Use

L. johnsonii as a distinct species is a relatively recent identification — emerging from advanced microbiological studies in the late 20th century. Consequently, the bacterium itself has no documented history of isolated traditional use; rather, its history is inseparable from the broader tradition of lactic-acid-bacteria-containing fermented foods.

Lactobacilli have a long history of safe use in traditional fermented foods and beverages, and some species and strains are considered important members of the healthy gut microbiota. In addition, many members of the genus Lactobacillus are Generally Recognized As Safe (GRAS) by the U.S. Food and Drug Administration, or included in the Qualified Presumption of Safety (QPS) list of the European Food Safety Authority (EFSA), which guarantees their use in food and safety in humans.

Lactobacilli have played a crucial role in the production of fermented products for millennia. A Russian-born biologist, Elie Metchnikoff, is well known for his study of lactic acid-producing bacteria. He attributed the long lifespan of many Bulgarians and of villagers in the Caucasus Mountains to their consumption of fermented milks — yogurt and kefir. He named the main microorganism used in yogurt making Lactobacillus bulgaricus and convinced many that its subsequent colonization of the intestine helped to normalize bowel movements and combat intestinal disease, thereby increasing longevity. L. johnsonii would have been present in some of these historically consumed fermented dairy preparations, though it was not identified or named as a distinct species until the modern era.

Fermented foods such as yogurt, kefir, sauerkraut, and traditional sour milk have long been staples in various cultures, praised for promoting digestive health, boosting immunity, and restoring balance after illness. These traditional remedies often contained a blend of Lactobacillus species, likely including strains similar to L. johnsonii, which contributed to their health-promoting reputation. Medicinally, people historically relied on these probiotic-rich foods to soothe gastrointestinal ailments, such as diarrhea, indigestion, and infections related to poor gut flora.

Interest in lactobacilli was rekindled in the 1990s following Füller's article on probiotics in 1989, defining "probiotics" as "live microbial feed supplements that beneficially affect the host animal by improving its intestinal microbial balance." The L. johnsonii strain La1 was one of the first cultures to be proposed as a probiotic dairy supplement in 1995 at the Nestlé Research Center, Lausanne — marking the beginning of deliberate, strain-specific use of L. johnsonii as a functional food ingredient.

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3. Key Constituents and Active Compounds

Lactic Acid and Short-Chain Fatty Acids

L. johnsonii makes a substance called lactic acid. Lactic acid can help improve the health of the microbiome by reducing the amount of harmful bacteria and increasing the amount of beneficial bacteria. Weak acids like lactic acid produced by L. johnsonii act together with hydrogen peroxide, inhibiting and killing enteric, vaginosis-associated, and uropathogenic pathogens like pathogenic E. coli, S. typhimurium, and Gardnerella vaginalis. Several cell surface structures of L. johnsonii L531 can produce high levels of short-chain fatty acids (SCFAs), such as butyric, acetic, and lactic acids, affecting the metabolic profile and gut resident microbiota.

Bacteriocins

The commercial probiotic strain Lactobacillus johnsonii LA1 produces bacteriocins. The nature of these antimicrobial compounds can be characterized by a narrow inhibitory spectrum (only active against closely related strains), loss of activity when treated with proteinases, and rather small molecular masses (around 6,000 Da or less). L. johnsonii strains have been shown to produce a bacteriocin, lactacin F, that inhibits the growth of both E. faecalis and other lactic acid bacteria through membrane-disrupting pore formation. Bacteriocins produced by L. johnsonii LA1 showed inhibitory activity against the human gastric pathogen Helicobacter pylori.

Bile Salt Hydrolases

Bile salt hydrolase (BSH), the enzyme releasing taurine or glycine from bile, is almost exclusively associated with GIT-colonizing bacteria and may impart a selective advantage in the GIT environment. The genome of L. johnsonii NCC 533 encodes three BSHs — the largest number found in any bacterial genome at the time of its sequencing. BSH activity is involved in bile acid deconjugation within the small intestine and has been linked to anti-Giardia activity, as well as modulation of host lipid metabolism and cholesterol dynamics.

Cell-Surface Proteins and Adhesins

To gain insight into its physiology, researchers sequenced and analyzed the 1.99-Mb genome of L. johnsonii NCC 533. Strikingly, the organism completely lacked genes encoding biosynthetic pathways for amino acids, purine nucleotides, and most cofactors. In apparent compensation, a remarkable number of uncommon and often duplicated amino acid permeases, peptidases, and phosphotransferase-type transporters were discovered. Genome analysis also predicted an abundance (>12) of large and unusual cell-surface proteins, including fimbrial subunits, which may be involved in adhesion to glycoproteins or other components of mucin — a characteristic expected to affect persistence in the gastrointestinal tract. Among these, elongation factor Tu (EF-Tu) has been identified as a cell-surface-associated protein that mediates attachment to human intestinal cells and mucins. The chaperonin protein GroEL has also been reported to be cell surface–associated and may play a role in interactions with the host and with H. pylori.

Exopolysaccharides and Biosurfactants

Genomic and functional studies indicate that some L. johnsonii strains produce bacteriocins and other antimicrobial metabolites and express surface-layer proteins that enhance mucosal adhesion. Notably, L. johnsonii can disrupt pre-formed biofilms via exopolysaccharide-mediated interference with quorum-regulated behaviors — an attribute relevant to persistent infections. Certain lactobacilli, including L. johnsonii, produce soluble metabolites such as bacteriocins, weak organic acids (lactic and acetic acids), and biosurfactants with anticandidal properties.

Phospholipids and Immunomodulatory Lipids

Research utilizing bioassay-guided fractionation methods with bone marrow-derived dendritic cells has shown that L. johnsonii N6.2 interacts with the innate and adaptive immune system, inducing a Th17 cell bias in mesenteric lymph nodes of diabetes-prone rats and modulating innate and adaptive immune cell populations in healthy volunteers. It has been demonstrated to delay the onset of type 1 diabetes in diabetic-prone rats, modulate host phospholipid dynamics under a high fat diet, and improve general wellness in healthy adults.

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4. Mechanisms of Action

Gut Microbiota Modulation

L. johnsonii is a lactic acid-producing probiotic possessing the potential to modulate intestinal microbiota balance, which can enhance immune function and reduce the risk of intestinal infections. The abundance of this bacterium in various niches is often influenced by external factors such as diet, antibiotic treatment, and invading microbes.

Pathogen Inhibition

Probiotic bacteria can perform inhibitory services via a number of mechanisms, including indirect competition for nutrients and binding sites in the host, and directly through the production of bacteriocins, acids, and other compounds. L. johnsonii NCC 533 has been shown to reduce gastritis induced by H. pylori and infection by the diplomonad G. intestinalis in gerbils, while L. johnsonii FI9785 inhibited C. perfringens colonization in chickens. L. johnsonii demonstrates considerable gastrointestinal stress tolerance: it remained viable at pH 1.5 and 2.0 for up to 1 hour, with survival extending up to 3 hours at pH 2.5, and viability was maintained after 3-hour exposure to 0.3% bile salts. When challenged with simulated gastric and intestinal fluids, L. johnsonii showed minimal reduction in colony-forming units after 3 hours, indicating strong resistance to pepsin and trypsin.

Gut Barrier Reinforcement

In preclinical models, prophylactic L. johnsonii N5 administration alleviated colitis symptoms (weight loss, colon shortening), reduced fecal and serum lipocalin-2 levels, and suppressed colonic pro-inflammatory cytokines (IL-1β, IL-6). N5 preserved microbial diversity, enhanced mucus secretion, and reinforced mucosal barrier integrity, preventing colitis onset. Research shows that L. johnsonii enhances gut resilience by regulating fucose residue metabolism, thereby promoting goblet cell differentiation and sulfomucin maturation. Sulfomucins, as essential components of the mucus barrier, play a critical role in maintaining mucosal integrity and limiting pathogen invasion.

Immunomodulation

Anti-inflammatory mechanisms mediated through TLR/STAT3 signaling and ER stress modulation have been established for L. johnsonii. Other substances made by L. johnsonii may also help activate the immune system. The supernatant of L. johnsonii culture has been shown to alleviate colitis and remodel gut microbiota, with increased SCFA production inhibiting the MAPK signaling pathway and M1 macrophage polarization. L. johnsonii GLJ001 prevented dextran sulfate sodium (DSS)-induced colitis in mice by inhibiting M1 macrophage polarization via the gut microbiota–SCFAs axis.

Indoleamine-2,3-Dioxygenase (IDO) Pathway Modulation

Lactobacillus johnsonii N6.2 mitigates the onset of type 1 diabetes (T1D) in biobreeding diabetes-prone rats, in part, through changes in kynurenine:tryptophan (K:T) ratios. This relates to modulation of the IDO enzyme pathway, which is a key regulator of tryptophan metabolism and immune tolerance.

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5. Scientific Evidence by Area of Use

5.1 Helicobacter pylori–Associated Gastritis

This is one of the best-studied areas for L. johnsonii. In clinical settings, Lactobacillus johnsonii La1 administration has been reported to have a favorable effect on Helicobacter pylori–associated gastritis, although the mechanism remains unclear. In a murine study, live La1 was administered continuously through the water supply to H. pylori–infected C57BL/6 mice, following colonization, the development of H. pylori–associated gastritis in the lamina propria, and the levels of proinflammatory chemokines MIP-2 and KC in the serum and gastric tissue over a period of 3 months. Researchers documented a significant attenuation in both lymphocytic (P = 0.038) and neutrophilic (P = 0.003) inflammatory infiltration in the lamina propria as well as in the circulating levels of anti-H. pylori immunoglobulin G antibodies (P = 0.003), although no suppressive effect of La1 on H. pylori colonizing numbers was observed.

Clinical observations further support its safety, including reduced H. pylori colonization in children receiving L. johnsonii preparations. A highly acid-resistant novel strain of Lactobacillus johnsonii No. 1088 has been reported to have antibacterial activity, including that against H. pylori, and inhibits gastrin-mediated acid production in mice.

Evidence strength: Preclinical (animal) evidence is moderately robust for anti-inflammatory effects in the context of H. pylori. Human clinical evidence is suggestive but limited in scale and design rigor. The mechanism by which L. johnsonii reduces gastritis without suppressing H. pylori colonizing load remains incompletely understood.

5.2 Inflammatory Bowel Disease and Colitis

Studies have revealed that L. johnsonii exhibits significant interventional potential in various cross-system diseases, including inflammatory bowel disease (IBD), intestinal infections, and Helicobacter pylori–associated gastritis. Lactobacillus johnsonii N5 prevents colitis by fortifying the mucosal barrier and alleviates disease by inhibiting NETosis and gut-liver inflammation, promoting mucosal repair. A strain from heat stress–resistant pigs has been shown to improve gut mucosal immunity and barrier in dextran sodium sulfate–induced colitis. L. johnsonii GLJ001 prevented DSS-induced colitis in mice by inhibiting M1 macrophage polarization via the gut microbiota–SCFAs axis. Research also suggests that butyrolactone-I regulates intestinal flora, promotes the proliferation of L. johnsonii, safeguards the integrity of the intestinal barrier, increases concentrations of butyric and propionic acids, and ultimately inhibits the activation of the MAPK signaling pathway, thus alleviating IBD.

Evidence strength: Current evidence is almost entirely from preclinical (murine) models. Robust human clinical trial data for L. johnsonii specifically in IBD are lacking as of the current literature base.

5.3 Immune Function and Type 1 Diabetes

In the BioBreeding rat model of type 1 diabetes, comparison of the intestinal microbial composition of diabetes-prone and diabetes-resistant animals found Lactobacillus species were negatively correlated with type 1 diabetes development. Two species — Lactobacillus johnsonii and L. reuteri — were isolated from diabetes-resistant rats. In a study where diabetes-prone rats were administered pure cultures of L. johnsonii or L. reuteri, rats administered L. johnsonii (but not L. reuteri) post-weaning developed type 1 diabetes at a protracted rate. Analysis of the intestinal ileum showed administration of L. johnsonii induced changes in the native microbiota, host mucosal proteins, and host oxidative stress response. A decreased oxidative intestinal environment was evidenced by decreased expression of several oxidative response proteins in the intestinal mucosa (Gpx1, GR, Cat).

In the key human pilot trial: Lactobacillus johnsonii N6.2 mitigates the onset of T1D in biobreeding diabetes-prone rats, in part, through changes in kynurenine:tryptophan (K:T) ratios. The goal of this pilot study was to determine the safety, tolerance, and general immunological response of L. johnsonii N6.2 in healthy subjects. A double-blind, randomized clinical trial in 42 healthy individuals with no known risk factors for T1D was undertaken. Participants received 1 capsule/day containing 10⁸ colony-forming units of L. johnsonii N6.2 or placebo for 8 weeks. In the L. johnsonii N6.2 group, serum tryptophan levels increased, resulting in a decreased K:T ratio. L. johnsonii N6.2 administration significantly decreased the occurrence of abdominal pain, indigestion, and cephalic syndromes. Immunophenotyping assays revealed that monocytes and natural killer (NK) cell numbers were durably increased significantly after washout (12 weeks). Moreover, an increase of circulating effector Th1 cells and cytotoxic CD8+ T cells was observed.

A completed, NIH-funded phase trial (ClinicalTrials.gov NCT03961347) evaluated L. johnsonii N6.2 supplementation in adults with type 1 diabetes, run by the University of Florida, with completion reported in 2026. Published results of this trial were not yet available in the literature base searched for this article.

Evidence strength: Animal model evidence is robust and mechanistically interesting. The one published human pilot RCT (n=42) demonstrated safety and immunological signal but was not powered to evaluate efficacy for T1D prevention. Larger human trials are underway or recently completed; definitive clinical conclusions cannot yet be drawn.

5.4 Gut Barrier Function and Intestinal Permeability

Increasing studies have demonstrated the positive impact of L. johnsonii and its metabolites on the health of multiple systems, including the stomach, intestine, liver, and brain. Systematic reviews have aimed to evaluate its specific mechanisms in regulating intestinal barrier function, immune response, and neuroendocrine signaling pathways. L. johnsonii enhances gut resilience by regulating fucose residue metabolism, thereby promoting goblet cell differentiation and sulfomucin maturation. Sulfomucins, as essential components of the mucus barrier, play a critical role in maintaining mucosal integrity and limiting pathogen invasion. The anti-inflammatory effect of certain bacterial metabolites is dependent on the presence of L. johnsonii, highlighting the synergistic relationship between microbial metabolites and beneficial bacteria in maintaining intestinal homeostasis. Through this pathway, L. johnsonii indirectly strengthens the mucosal immune barrier, attenuates colitis symptoms, and contributes to the host's defense against gut inflammation.

Evidence strength: Primarily preclinical. Mechanistic data from murine models are compelling, but human evidence for gut barrier–specific endpoints is limited.

5.5 Anti-Infective / Antimicrobial Properties

L. johnsonii has been reported to have potential effects on many pathogens, including Campylobacter jejuni, Salmonella typhimurium, Helicobacter pylori, and Clostridium perfringens. Viable enteropathogenic E. coli (EPEC) counts were markedly lower in the presence of both live and heat-killed L. johnsonii. The partial retention of inhibitory effect with heat-killed cells suggests that both secreted metabolites and structural cell components contribute to EPEC inhibition.

Evidence strength: Predominantly in vitro and animal model data. No large-scale, randomized human trials have specifically tested L. johnsonii as an anti-infective intervention in infectious gastroenteritis.

5.6 Anti-Giardia Activity

The probiotic strain Lactobacillus johnsonii CNCM I-4884 exhibits anti-Giardia activity in vitro and in vivo in a murine model of giardiasis. This activity has been attributed in part to bile-salt hydrolase activity, as deconjugated bile salts produced by L. johnsonii La1 have been demonstrated to inhibit Giardia intestinalis in preclinical work.

Evidence strength: In vitro and animal model evidence. No human clinical trials have been published specifically examining L. johnsonii for giardiasis in humans.

5.7 Non-Alcoholic Fatty Liver Disease and Metabolic Conditions

Studies have identified L. johnsonii as having significant interventional potential in non-alcoholic fatty liver disease (NAFLD). The focus of research has been on exploring its systemic regulatory role through the "microbiota-gut-organ axis." In preclinical animal models, L. johnsonii supplementation of feed had a positive effect on average daily gain and feed conversion ratio. In addition, supplementation decreased serum triglyceride and low-density lipid cholesterol levels, as well as abdominal fat deposition.

Evidence strength: Primarily preclinical (animal). Human clinical evidence is preliminary and indirect.

5.8 Gut–Brain Axis and Neurological/Psychiatric Conditions

Specific strains of L. johnsonii have shown anti-inflammatory, antipathogenic, and metabolic-modulating properties. These traits suggest potential therapeutic value in conditions such as IBD, intestinal infections, liver diseases, and gut-brain axis disorders. L. johnsonii BS15 has been reported to yield positive psychiatric effects in psychopathology through the brain–gut axis, with research mentioning intestinal barrier protective effects of this potential psychobiotic. BS15 positively regulated the hypothalamo-pituitary–adrenal axis by lowering the serum corticosterone level, and also reduced the levels of TNF-α, IFN-γ, and IL-15β in the mouse small intestine.

Evidence strength: Preliminary and largely confined to animal models. This is an emerging research area, and no robust human clinical trials on L. johnsonii for neuropsychiatric outcomes have been published.

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6. Body Systems and Health Areas Associated With L. johnsonii

• Gastrointestinal system: L. johnsonii can inhibit intestinal inflammation and produce organic acids such as lactic acid, which affect the gut pH making it less habitable for many pathogens. Research suggests that, through these benefits to gut health, L. johnsonii can help to prevent and alleviate symptoms of various diseases, including inflammatory bowel disease (IBD), intestinal infections, colitis, and E. coli–induced diarrhea.
• Immune system: Other substances made by L. johnsonii may also help activate the immune system. Clinical evidence from a pilot RCT in healthy adults demonstrated durable increases in monocytes, NK cells, and effector T-cell subsets following supplementation.
• Vaginal/urogenital microbiome: It is part of the healthy vaginal microbiota and has been identified as having probiotic properties. In people, it is part of the normal gut community and can also appear in the female urogenital tract.
• Endocrine/metabolic: Animal model evidence links L. johnsonii N6.2 to protection against type 1 diabetes via IDO pathway modulation and tryptophan metabolism.
• Hepatic: Evidence from preclinical research supports a potential role in NAFLD, mediated through the gut-liver axis.
• Neurological: Preliminary animal data link strain BS15 to gut-brain axis modulation via the hypothalamo-pituitary-adrenal axis, though human evidence is absent.

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7. Dosage Forms and Dosages Reported in Studies

There is not enough reliable information to establish what a universally appropriate dose of L. johnsonii might be. Dosages reported in specific published studies are listed below.

• A double-blind, randomized clinical trial in 42 healthy adults administered 1 capsule per day containing 10⁸ colony-forming units (CFU) of L. johnsonii N6.2 or placebo for 8 weeks.
• In a double-blind study in healthy volunteers, the dose was 5×10⁸ CFU per capsule, administered as one capsule per day for 8 weeks in a parallel design.
• In food and beverage products, L. johnsonii 456 would be present at concentrations from 10⁹ to 10¹¹ CFU per serving, according to a U.S. FDA GRAS notification.
• A study of fourfold excess consumption of a Lactobacillus johnsonii–containing milk drink (360 g/day) found no adverse side effects other than increased frequency of defecation.
• In an animal study, 180 chicks were fed diets supplemented with 1×10⁶ CFU of L. johnsonii BS15 per gram of feed throughout a 42-day experimental period.

Even at doses as high as 10¹² (one trillion) CFU per day, this level is described in the GRAS notification as well within levels known to be safe, as any excess cells are simply passed through the gut with normal defecation.

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8. Safety Considerations and Interactions

Regulatory Safety Status

Its strong safety record, with recognition as GRAS in the US and QPS in the EU, makes it an attractive candidate for therapeutic and dietary applications. Lactobacilli have a long history of safe use in traditional fermented foods and beverages. Many members of the genus Lactobacillus are GRAS (U.S. FDA) or included in the Qualified Presumption of Safety (QPS) list (EFSA), which guarantees their use in food and safety in humans.

Adverse Events in Healthy Populations

Lactobacillus johnsonii is generally recognized as safe (GRAS) for use as a probiotic. Common side effects are typically mild and gastrointestinal in nature, such as bloating or gas. L. johnsonii N6.2 administration did not modify the comprehensive metabolic panel (CMP) or complete blood count (CBC) of participants, suggesting general safety at the tested dose.

Risk in Immunocompromised Individuals

In a documented case report from Thailand, L. johnsonii bacteremia in an immunocompromised patient may have been linked to her ingestion of probiotic yogurt containing L. johnsonii La1 prior to admission. Although probiotic-associated bacteremia is rare, it can occur in severely immunocompromised patients, such as those with hematologic malignancies. There are rare reports of Lactobacillus infections, including bloodstream infections, in immunocompromised individuals. Lactobacillus infections linked to risk factors such as diabetes, heart disease, cancer, and medical treatments can manifest in various forms, including endocarditis, bacteremia, meningitis, dental caries, and pulmonary infections.

A few recent cases of bacteremia and/or sepsis associated with lactobacilli have been reported in patients with different underlying diseases, such as ulcerative colitis in pediatric or adult patients, suggesting that extensive damage of the colonic mucous membrane increases the risk of bacteremia. The scientific information available supports the need for careful evaluation for probiotics with special consideration in immunocompromised patients or during antibiotherapy.

Increasing reports on Lactobacillus bacteremia-associated morbidity and mortality in immunocompromised patients have raised safety concerns about its use in this group.

Probiotic Bacteremia: Population-Level Perspective

Lactobacillus bacteremia in Sweden was examined over a 6-year period, during which time there was increasing use of 3 commercial probiotic Lactobacillus strains. There was no change in the rate of lactobacillemia, and no case of Lactobacillus isolated from the bloodstream was identified as one being related to the probiotic strains.

Interaction With Antibiotics

Antibiotics are used to reduce harmful bacteria in the body. Taking antibiotics along with L. johnsonii can reduce the effects of L. johnsonii. To avoid this interaction, L. johnsonii products should be taken at least 2 hours before or after antibiotics.

Resistance Gene Considerations

The scientific information available supports the hypothesis of the existence of a gut resistance gene pool and the possible transferability of antibiotic resistance genes. Reports on possible in vivo transfer are very scarce but are an area requiring further study.

Central Venous Catheter Precaution

Because some cases of Lactobacillus bacteremia have occurred in an intensive care setting in the presence of a central venous catheter, experts recommend the use of scrupulous hand hygiene when manipulating central venous catheters after handling probiotic preparations.

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9. Evidence Summary and Research Gaps

Despite significant differences in the actions of different strains of L. johnsonii, available research strongly suggests that the strain as a whole holds core potential to improve digestive health and associated systemic diseases through multiple mechanistic modules, including flora modulation, barrier repair, immunomodulation, and metabolite production. The positive effects (e.g., anti-infective, anti-inflammatory, metabolic regulation, neuroprotection) demonstrated by many strains in specific models or clinical situations provide an important basis and direction for further research.

Despite established anti-inflammatory mechanisms mediated through TLR/STAT3 signaling and ER stress modulation, critical gaps persist in elucidating the bioactive metabolites (e.g., propionic acid, bacteriocins) and host–microbe interactions underlying L. johnsonii. Functional differences and dose-dependent responses among strains suggest that precise screening and validation are still needed. The large majority of mechanistic evidence derives from in vitro cell assays and murine models; well-powered, randomized human clinical trials — particularly for IBD, NAFLD, and gut-brain axis applications — remain a significant scientific gap.

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