Lactobacillus reuteri

Lactobacillus reuteri is a species of Gram-positive bacteria found in the digestive tracts of mammals and birds, and can also be detected in breast milk of humans, pigs, and dogs.^[4] Some strains are being investigated for usage as probiotic supplements.^[4] As with other species of the Lactobacillus genus, reuteri is a lactic acid-producing bacterium.^[5]

Lactobacillus refers to the genus of bacteria, which includes species such as acidophilus and reuteri. Beyond the species, differing strains may exist that are denoted by codes after the species name such as Lactobacillus reuteri ATCC 55730.

Probiotic supplements work through several mechanisms, such as enhancing the immune response of the host, synthesizing antimicrobial chemicals, and competing with pathogenic bacteria for binding and colonization sites.^[6] Some species of Lactobacillus have been shown to be effective in displacing pathogenic bacteria from intestinal cells in vitro.^[7][8]

The locations of the mammalian body where L. reuteri have been found include vaginal fluid, the gastrointestinal tract, and breast milk.^[9][10] Similar to many bacteria, they are predominantly located in the large intestines.^[4] L. reuteri is also present in the oral cavity, with studies noting that it is present in around 8-13% of participants before supplementation,^[11][12] which can be increased with bacteria laced chewing gum.^[11]

Different strains of this bacteria that have been successfully used with oral ingestion include:

• L. reuteri ATCC PTA 6475; shown to have aesthetic benefits in mice^[13] and genomically is somewhat similar to the popular strain ATCC 55730^[14]

• L. reuteri ATCC PTA 4659^[15]

• L. reuteri ATCC 23272;^[16] which may still have a therapeutic effect when administered heat-killed^[17]

• L. reuteri ATCC 55730 (SD2112^[18]); used in treatment of H. pylori^[19] and confirmed to colonize the intestines after oral ingestion in humans^[18]

• L. reuteri DSM 17938;^[20] a daughter strain derived from ATCC 55730 (slightly increased acid resistance and improved growth in vitro^[21]) that has been confirmed to colonize the intestinal tract.^[22] This daughter strain was created over concern regarding two genes found in ATCC 55730 that may confer resistance to tetracycline and lincomycin that were removed in the creation of DSM 17938^[21]

• L. reuteri NCIMB 30242; formerly known as Cardioviva^TM[23][24], now known as LRC^TM, is implicated in cholesterol reduction^[23] and increasing serum Vitamin D;^[24] this appears to require microencapsulation to survive digestion via food products^[23]

• L. reuteri RC-14; used alongside L. rhamnosus GR-1 for vaginal health (bacterial vaginosis^[2] and UTIs^[3]) due to these strains being able to colonize the vagina following oral administration^[25]

• L. reuteri CF48-3A^[15]

• L. reuteri DSMZ 17648 (Pylopass^TM); used in treatment of H. pylori^[26]

• L. reuteri GMNL 263 (Lr263); used in rats for treatment of fatty liver^[27]

• L. reuteri LTH 2584^[28]

• L. reuteri LTH 5531^[29]

• L. reuteri l5007; which appears to uniquely synthesize exopolysaccharides^[30]

• L. reuteri 100-23^[31]

The alphanumerical strain designations are used to refer to the company which safeguards and distributes the strain (e.g. ATCC being an acronym for American Type Culture Collection and JCM referring to Japan Collection of Microorganisms) and the numerical designation is used to differentiate between strains of the same location.

There are numerous different strains of Lactobacillus reuteri which, although similar, have some minor differences that make them unique. Three particular strains that are bolded above tend to be most commonly used in dietary supplements and are also the most researched.

Lactobacillus and lactic acid producing bacteria in general are present in many fermented foods, including:

• Bread products although highest in sourdough,^[32] possessing LTH 2584 (high antimicrobial activity amongst sourdough^[28]) and LTH 5531^[29]

It is possible for a bacterial colony to be killed yet still show benefits related to reducing H. pylori infection following oral administration, as has been seen with the heat-killed product Pylopass^TM (DSMZ 17648 strain)^[26] and has been noted with other probiotics besides L. reuteri.^[33][34]

When testing the differences between freeze-dried living and spray-dried dead L. reuteri, one study using the heat-killed strain known as DSMZ 17648 failed to find any significant difference between products, which indicates that live cultures are not needed for benefits seen related to H. pylori reduction.^[26]

Reuterin (3-hydroxypropionaldehyde) is a small molecule produced by Lactobacillus reuteri which has antimicrobial properties^[35] due to its ability to induce oxidative stress in microbes.^[36] This molecule is an intermediate made from glycerol and prior to its conversion to 1,3-propanediol,^[37] is synthesized in a particular bacterial compartment called the metabolosome.^[38] Reuterin is produced in higher levels when Lactobacillus reuteri are cultured in a medium containing high levels of glycerol.^[36]

Lactobacillus reuteri can use glycerol to produce reuterin, which is a molecule that has antimicrobial effects.

Reutericyclin is an antibiotic produced from L. reuteri which is a cyclical tetramic acid^[39] active primarily on the bacterial membrane of Gram-positive organisms^[40] including methicillin-resistant Staphylococcus aureus (MRSA)^[41] and Clostridium difficile, with the latter occurring at physiologically relevant concentrations.^[42] This particular metabolite may be produced in higher levels by the strain LTH 2584, since this strain possesses stronger antibacterial effects not accounted for by the weaker antimicrobial effects attributed to reuterin and small organic acids produced by other strains.^[28]

Reutericyclin is another antibacterial metabolite unique to L. reuteri, although it does not appear to be as ubiquitous as reuterin which is produced in most if not all strains of L. reuteri; the strain LTH 2584 seems to produce most reutericyclin.

By producing histamine,^[43] L. reuteri ATCC 6475 exerts a number of antiinflammatory actions, including suppression of TNF-α. Histamine is produced from L-histidine via histidine decarboxylase, which is expressed via a histidine decarboxylase gene cluster present in a few Lactobacillus species.^[44][45] Inhibiting any single gene in this cluster reduced but did not ablate the inhibitory effects of L. reuteri on TNF-α signalling, suggesting that multiple genes in this cluster are responsible.^[43] Histamine suppression of TNF-α occurs via activation of the histamine H2 receptor^[43][46]

L. reuteri ATCC 6475 has also been shown to reduce levels of the pro-inflammatory cytokine IL-17A after oral ingestion in mice,^[47][13] which is mediated by increased production of the anti-inflammatory cytokine IL-10 via histamine-activation of the H2 receptor.^[48] Upon binding to the IL-10 receptor, IL-10 suppresses IL-17 producing T cells.^[49][50]

L. reuteri appears to produce histamine which then exerts antiinflammatory actions.

Nuclear factor kappaB (NF-κB) is a transcription factor that controls a large number of physiological processes including inflammation, stress response, and cell survival.^[51] The NF-κB signaling system consists of a number of proteins from the rel family that, once activated, combine to form heterodimers that translocate to the nucleus, regulating gene expression.^[51][52] NF-κB is inhibited by the protein IκBα, which sequesters NF-κB dimers in the cytosol, preventing them from entering the nucleus and turning on gene expression.^[53] When NF-κB is activated, IκBα is degraded, allowing NF-κB to translocate to the nucleus.^[54]

By preserving the function and integrity of IκBα, L. rhamnosus^[55] as well as L. reuteri^[56] have been shown to inhibit NF-κB signaling. L. reuteri may also inhibit activity upstream of NF-κB, via selective modulation of MAPK proteins involving a suppression of ERK1/2 and an enhancement of JNK and p38.^[56]

L. reuteri DSM 17938 has been confirmed to survive acidic conditions both in vitro^[21] and in vivo following oral administration at the dose of 1x10⁹ CFU via a food product.^[20] The parent bacterium from which it is produced, ATCC 55730,^[21]) is also known to survive when consumed in chewable tablets or as a freeze-dried powder in doses between 4x10⁸ and 1x10¹⁰ CFU.^[57][18][58]

L. reuteri SD 2122 has been noted to survive and increase colony count in feces when ingested at 5x10⁹ CFU daily for three weeks via adding it to a cooled beverage (less than 37°C).^[58]

In humans in good health who are not supplementing probiotics, the entire genus of Lactobacillus appears to account for less than 2% total fecal bacteria.^[59] There is high variation between all individuals in colony count,^[59][60][61] even with multiple measurements in the same individual.^[62] When quantifying the total Lactobacillus levels, colony counts have been noted in the range of 1x10⁴ to 1x10⁸ CFU per gram.^[62][20]

One study examining Lactobacillus populations in humans failed to detect any reuteri, instead detecting acidophilus, fermentum, crispatus, rhamnosus, and plantarum.^[61] Lack of reuteri was confirmed in another study.^[20] This may be due to individual differences as it has been estimated that only 10% of the population has detectable L. reuteri in fecal analysis.^[18][10]

Lactobacillus tends to have a low quantitative presence in the large intestines of humans, and Lactobacillus reuteri levels are often too low to be detected or outright absent in many people.

One study noted that supplementing a food product containing at minimum 1x10⁹ CFU L. reuteri for three weeks can increase total Lactobacillus CFU in feces.^[20] Colonization has been noted elsewhere with 1x10¹⁰ CFU^[58] and 1x10¹¹ CFU^[57] taken as a daily supplement, with colonization being detected within one week in one study^[57] and within 2-4 days elsewhere.^[22]

Colonization induced by 1x10¹¹ CFU for four weeks appears to return to normal after two months after cessation of supplementation.^[57] Another study noted supplementation of DSM 17938 at 1x10⁸ every other day for one week was successful in colonizing the intestines (a similar potency to daily supplementation) but did not last for more than four days following supplement cessation.^[22]

Lactobacillus reuteri ingestion increases fecal concentrations of L. reuteri, and alternate day dosing may be as effective as daily dosing. Upon cessation of supplementation, L. reuteri concentrations readily return to presupplementation levels.

In rats, a high fructose diet appears to greatly increase Clostridia bacteria to 130.1% of control values , with no significant changes in Bifidobacteria or Lactobacilli counts.^[27] Feeding 2x10⁹ CFU L. reuteri (GMNL-263) during this period prevented the increase in Clostridia while increasing both Bifidobacteria (126.7%) and Lactobacilli (135.4+/-7.8%) concentrations.^[27] Similar suppression of Clostridia have been noted elsewhere in rats with prebiotics and different lactic acid producting probiotics (bifidobacterium and streptococcus thermophilus) as well as with fructooligosaccharides.^[63]

L. reuteri NCIMB 30242 is a strain that appears to be able to deconjugate bile acids in vitro as it expresses the bile salt hydrolase (BSH) enzyme.^[23] Deconjugation of bile acids is hypothesized to reduce circulating cholesterol by preventing its absorption from the intestinal tract.^[64] While atypical, this capacity to deconjugate bile acids has also been reported in L. reuteri CRL 1098 when tested in vitro.^[64]

Supplementation of L. reuteri NCIMB 30242 in hypercholesterolemic subjects at a dose of 5x10⁹ CFU twice daily via yogurt reduced circulating LDL-C by 8.92% and total cholesterol 4.81% over the course of six weeks; other parameters including HDL-C and apolipoprotein B100 were not significantly affected.^[23] In another study, 2.9x10⁹ CFU L. reuteri NCIMB 30242 taken twice daily for six weeks reduced both LDL-C (11.64%) and total cholesterol (9.14%) while additionally reducing apolipoprotein B100 (8.41%).^[65] All other parameters in these hypercholesterolemic adults were unaffected including apolipoprotein A1.^[65]

This hypocholesterolemic effect was not accompanied by any changes in deconjugated bile acids in the feces in one study,^[23] although an increase in serum deconjugated bile acids has been noted (45%) alongside reductions in serum plant sterols including campesterol (41.5%), sitosterol (34.2%), and stigmaterol (40.7%).^[65] It has been hypothesized that this reflects an increased efflux of sterols from the blood to the intestines to compensate for increased BSH activity.^[65]

A number probiotics have been found to have a potent protective effect against diabetes by positively affecting insulin sensitivty.^{[66][67][68][69]} The Lactobacillus species in particular has demonstrated a potent effect in diabetic animal models, ameliorating metabolic dysfunction by normalizing blood glucose and insulin levels^[66] in addition to preventing the development of glucose intolerance, dyslipidemia, and oxidative stress that are often associated with ths disease.^[70] 

L. reuteri GMNL-263 has recently emerged as a potentially promising therapeutic for type II diabetes, demonstrating potent anti-diabetic effects in rats fed a high fructose (insulin-resistance promoting) diet. When a daily dose of 2x10⁹ CFU L. reuteri GMNL-263 was administered to high-fructose fed rats over a period of 14 weeks, animals receiving the L. reuteri were protected from metabolic abnormalities such as the development of fatty liver, and also gained less weight and adipose tissue.^[27]

Among the many metabolic issues associated with diabetes, insulin resistance promotes increased blood glucose levels. Because glucose is a natural reducing agent (a reactive molecule that donates electrons in chemical reactions) levels need to be precisely controlled. When blood glucose levels creep out of range and become sufficiently high, it becomes covalently bound to proteins, lipids, and other cellular structures in a glycation reaction. Importantly, increased glycation may be a major cause of metabolic dysfunction that has been attributed to many chronic diseases including cancer, cardiovascular disease, and neurodegenerative diseases such as Alzheimer’s.^[71] Due to the direct relationship between blood glucose levels and glycation, glycated hemoglobin levels (HbA1C) are often used as a diagnostic to assess blood glucose concentration over long periods of time.

Consistent with the ability of L. reuteri GMNL-263 to promote increased insulin sensitivity under diabetic conditions, (2x10⁹ CFU) GMNL-263 daily for 14 weeks significantly limited high-fructose diet induced increases in HbA1c, making the probiotic group not significantly different than control.^[27]

Adipose tissue is an important glucose disposal site that plays a critical role in regulation of glucose homeostasis. Insulin resistance decreases the ability of adipose tissue to absorb/utilize glucose in part by decreasing expression of the adipogenic proteins PPARγ and GLUT4. The decrease in the expression of PPARγ and GLUT4 in adipose tissue induced by a high fructose diet in rats was suppressed with coingestion of 2x10⁹ CFU of L. reuteri over 14 weeks, which may be related to restoring Lactobacilli levels while decreasing levels of Clostridia.^[27]

L. reuteri may confer a degree of protection against insulin resistance by preventing the suppression of PPARγ and GLUT4 in response to a high fructose diet.

Feeding of L. reuteri to rats also fed a high fructose diet for 14 weeks at the daily dose of 2x10⁹ CFU suppressed the increase of inflammatory cytokines TNF-α and IL-6 in adipocytes that was noted in high fructose only-controls.^[27] This has been noted in the intestines as well,^[72] which is thought to occur via restoration of the balance between two subpopulations of T cells known as Treg cells and Th17 cells. L. reuteri appears to increase Treg cells, which suppresses the aberrant elevation of Th17 populations associated with inflammatory disorders.^[73]

Lactobacillus supplementation has been shown to influence weight in animals in a strain-specific manner, with certain strains promoting weight gain and others promoting weight loss.^[74] Although this effect does not appear to extend to humans, possible positive correlations between L. acidophilus intake and weight gain have been noted.^[74] The relevance of this association has recently been contested however,^[75] and there is insufficient evidence to draw any correlations with L. reuteri intake.^[74]

In obese humans, there appeared to be a trend for increased fecal Lactobacillus (species not specified) independent of supplementation relative to lean controls,^[76] and self-reported consumption of probiotic yogurts (primarily Lactobacillus) is associated with weight loss.^[77]

While oral intake of probiotic yogurts tends to be associated with weight loss (likely confounded with other lifestyle factors), the effect of specific Lactobacillus strains on human body weight is not well established.

A few studies not directly assessing weight have noted that rats given L. reuteri (ATCC PTA 6475) exhibited reduced fat mass relative to control groups. This was noted with 10⁹ CFU L. reuteri thrice weekly in male mice over four weeks, which was associated with a 50% reduction in visceral fat pads (interestingly, this was ineffective in females).^[78] Another study noted that L. reuteri ATCC PTA 6475 given to rats of mixed gender at a dose of 3.5x10⁵ CFU daily 20 weeks inhibited most of the fat gain caused by high fat 'western' diets.^[73]

This effect may be explained by an increase IL-17 signaling (from Th17 cells) with a concomitant reduction in IL-10 signaling (from Treg cells) in the obese state, as L. reuteri ATCC 6475 has been shown to increase Treg cells and thereby reduce IL-17 levels through an increase in IL-10 signaling.^[73] Importantly, these effects are likely strain dependent. While L. reuteri ATCC 4659 has also been shown to prevent diet induced obesity and increases in serum IL-10, L reuteri DSM 17938 failed in the same study to affect either of these parameters.^[79]

In an overectomized mouse model of menopausal osteoporosis, L. reuteri could suppress the osteoclastogenesis induced by RANKL and M-CSF (factors important in osteoclast activation and migration), suggesting a possible water-soluble factor which could inhibit bone loss.^[80] Preventing abnormal pro-osteoclastic increases in CD4⁺ T cell activity has also been implicated as a possible mechanism for anti-osteoclastic effects of L.reuteri.^[80]

A rat study using L. reuteri ATCC PTA 6475 at 10⁹ CFU thrice weekly, which noted increases in bone mass in male rats only, found that there was an increase in osteoblastic activity relative to control.^[78] This differs from the results seen in ovarectomized rats, where the same thrice weekly dose and additional L. reuteri in the drinking water failed to increase osteoblast activity,instead increasing via reduced osteoclast function).^[80]

L. reuteri ATCC PTA 6475 has been implicated in protecting rats from bone loss during ovarectomy,^[80] which has been noted with other Lactobacillus probiotics.^[81] When dosed at 10⁹ CFU thrice weekly (and 1.5x10⁸ CFU/mL in drinking water) for four weeks, L. reuteri inhibited all vertebral and femoral bone loss relative to ovarectomized control.^[80] This same dose given by gavage (10⁹ CFU thrice weekly) in aged male mice was able to increase bone mass and intestinal inflammation; this effect was attributed to changes in osteoblast function.^[78]

Mechanistically, it seems that the proliferation of osteoclasts (negative regulators of bone mass) is attenuated without affecting osteoblast activity in overectomized rats.^[80] This effect that may be mediated by bone marrow CD4⁺ T cell activity, which enhances osteoclast activity.^[82] Although increased upon ovarectomy, this phenomenon was prevented with L. reuteri ATCC PTA 6475.^[80] 

Rat evidence suggests that Lactobacillus reuteri ATCC PTA 6475 has bone sparing effects,which may occur via modulation of gut immunology and T cell populations that could result in less secretion of cytokines that contribute to bone loss.

Hypothesizing that probiotics may help oral health by stimulating antibody production against bacteria that cause cavities, researchers have searched for a possible immunostimulatory effect of L. reuteri supplmenetation. Usage of a chewing gum containing L. reuteri (equal parts ATCC 55730 and ATCC PTA 5289 collectively at 2x10⁸ CFU) twice daily for 10 minutes each time over the course of 12 weeks increased total IgA concentrations in saliva despite the percentage of antibodies specific to both L. reuteri and pathogenic Streptococcus species decreasing during the course of the study. The researchers speculated that this unexpected decrease could be caused by cross-tolerance of the immune system to Streptococcus species after exposure to L. reuteri.^[11]

Oral ingestion of L. reuteri ATCC 6475 appears to increase IL-10 in serum, ^[73] which mediates its effects on the skin of mice. ^[13] The effects of IL-10 on bowels and skin are thought to be primarily mediated through the inhibitory effects of IL-10 on IL-17A noted in these tissues,^[83] since inhibition of IL-17A mimicks the effects of L. reuteri on skin in mice.^[13] Inhibition of IL-17A is also implicated in the improvement in female fertility^[13],testicular function, and serum testosterone levels seen in mice with this particular strain of L. reuteri (ATCC 6475).^[47]

Other studies measuring IL-10 after ingestion of L. reuteri strains have noted a failure for DSM 17938 and L6798 to either increase IL-10 in serum or to exert antiobese actions^[79] which are secondary to IL-10^[13] However, ATCC PTA 4659 had some antiobesity effects following oral ingestion of 109 CFU daily in mice even though a statistically significant change in IL-10 was not seen either.^[79] A similar dose of ATCC 23272 after nine days in mice has been shown to increase IL-10 production, however, by inducing a two-fold increase in IL-10-producing CD4+ T cells in the spleen.^[16]

In humans with cystic fibrosis, DSM 17938 at 10⁸ CFU daily for six months was not associated with any alterations in serum IL-10 relative to placebo.^[84]

At least in mice, Lactobacillus reuteri appears to hinder IL-17 signaling, which is thought to occur via increased IL-10 signaling (IL-10 suppresses IL-17 in some tissues). This alteration in cytokine function appears to underlie the effects of Lactobacillus on testosterone, fertility, and aesthetics, which appear to be strain-specific.

T cells are an essential part of the adaptive immmune system; they recognize and mount an immune response to specific antigens presented to them by antigen-presenting cells. ^[85] T cells are generally divided into different classes depending on the kind of coreceptors they express on their surface. There are roughly two types: Helper T cells produce cytokines, and express both CD3 and CD4 on their surfaces (this is denoted as CD3⁺CD4⁺ since they are positive for both of these molecules), while cytotoxic T cells destroy cells expressing a specific antigen and are CD3⁺CD8⁺.^[85] However, a heterogenous group of T cells known as regulatory T cells (Treg) also exist, and are not classified just by the coreceptors expressed on their surface but by their expression of the transcription factor Foxp3 and their function to downregulate immune responses.^[86]

L. reuteri DSM 17938 given at a dose of 10⁶ CFU/g body weight per day to rat pups with necrotizing enterocolitis (NEC) increased CD4⁺CD8⁺Treg⁺ and CD4⁺Treg⁺ cells in the first few days of life (via standard feeding from their mothers) in the intestines and lymph relative to control.^[87] This increase was associated with an increase in survival of the rat pups^[72][87] as CD4⁺CD8⁺Treg cells are lower in pups with NEC relative to controls and these abnormalities are ablated with L. reuteri (without appearing to influence other helper or cytotoxic T cell populations).^[87] Increases in Treg cell populations have been noted in other populations, including otherwise normal mice given L. reuteri (ATCC 23272) at 10⁹ CFU which increased Treg cells in splenic tissue (60% within nine days).^[16]

Ultimately, it may be that preserving the populations of regulatory T cells through feeding of L. reuteri exerts anti-inflammatory effects as it is noted to be associated with less proinflammatory (IL-6, TNF-α) and more antinflammatory (IL-10) cytokines.^[72]

In rat models, feeding of L. reuteri seems to restore abnormalities in T-cell populations, specifically by increasing regulatory T cell levels.

One study in mice feeding 10⁹ CFU of ATCC 23272 for nine days noted that the expansion of CD4⁺CD25⁺Foxp3⁺ T cells in splenic tissue and increase in secreted IL-10 was associated with immunosuppressive actions in vitro, which reduced airway inflammation and hyperresponsiveness upon exposure to antigens.^[16]

In mice, ATCC 23272 has been noted to desensitize the lungs to antigens and is thus thought to have antiallergic properties.

When tested in infants, supplementaton of ATCC 55730 (10⁸ CFU) in families known to have allergic diseases (where supplementation started the last month of gestation, and continued in the offspring for one year) who were then followed up after seven years failed to find any differences in the occurrence rates for asthma, rhinoconjunctivitis, eczema, and skin prick test reactivity.^[88] These null results are similar to other probiotics that also exert acute antiallergic actions such as L. paracasei LF19.^[89]

Supplementation of the mother and offspring with ATCC 55730 when the family is known to be at risk for allergic diseases failed to modify the incidence of disease in the offspring after seven years.

In mice, ingestion of L. reuteri ATCC 6475 at 3.5x10⁵ CFU daily over one year increased testicular weight from five months onwards when placed on a control diet. When these mice were placed on a high fat diet, L. reuteri negated decreases in testicular weight.^[47] The mice given L. reuteri also showed improved testicular histology (increased Leydig cell number and seminiferous tuble cross-sectional profile) with an increase in testosterone after five months in both the control and high fat diet groups (more pronouned in the latter).^[47]

The effects of L. reuteri on testicular function were mimicked with an IL-17 antibody, suggesting a pathological role for this cytokine similar to its implications in skin/hair quality and fertility.^[47]

Ingestion of L. reuteri ATCC 6475 in rats has been noted to increase social grooming as well as serum oxytocin,^[13] both of which are linked in many species.^[90] Due to the ability of IL-10 to influence hypothalamic function where oxytocin is produced,^[91][92] it has been hypothesized that oxytocin mediates the interaction between the gut and skin.^[93]

Chewing a gum containing L. reuteri (mixture of strains ATCC 55730 and ATCC PTA 5289) at 2x10⁸ twice daily for twelve weeks appears to be able to increase salivary concentrations of the latter strain.^[12] A desired reduction in S. mutans, a bacterium which contributes to tooth decay,^[94] was seen in the oral cavity, even though there was not a statistically significant increase oral colonization of Lactobacillus.^[95]

Antibody count specific for S. mutans has been noted to be decrease with L. reuteri chewing gum,^[11] and L. reuteri (ATCC 55730 and ATCC PTA 5289) as well as L. rhamnosus (LC 705 and GG ATCC 53103) given over the course of 1-3 weeks via food products or lozenges can reduce colony counts of S. mutans in the oral cavity.^[96][97][98] This reduction in S. mutans is thought to mediate the benefits seen with L. rhamnosus on dental caries in children,^[99] although no studies have been conducted specifically with L. reuteri on this endpoint beyond seven months.

One study using L. reuteri ATCC 55730 tablets as well as through a straw (to avoid direct interaction with the oral cavity) noted that both methods were capable to reducing oral S. mutans colonies when dosed at 10⁸ CFU over three weeks.^[95]

Helicobacter pylori is a bacteria known to exist endogenously in the stomach of many people at concentrations of 10⁴-10⁷ CFU/g in gastric mucus. Although people have no symptoms, ^[100]Helicobacter pylori has a causative role in gastric cancers and ulcerative diseases.^[101] It is thought that probiotics could play a beneficial role here by competing with H. pylori for colonization of the stomach.^[102]

In subjects who tested positive for H. pylori yet did not reach the criteria for standard eradication therapy (Maastricht Guidelines^[103]), four tablets of 5x10⁹ CFU (daily dose of 2x10¹⁰ CFU) of the DSMZ 17648 strain of L. reuteri for two weeks significantly decreased H. pylori load. This decrease persisted over the course of the 24-week study, beyond the cessation of supplementation.^[26] Benefits have also been noted with the live strain ATCC 55730 in chewable tablets at 10⁸ colonies once daily. After a month of supplementation, a 13% reduction in breath urea (a measure of H. pylori levels) was observed in subjects given L. reuteri versus a 3% increase in the placebo group.^[19] A reduction in gastrointestinal symptoms upon treatment with L. reuteri was also noted.^[19]

There is some evidence to suggest that L. reuteri may decrease the concentration of H. pylori in the gut.

In otherwise healthy adult humans given supplementation of L. reuteri DSM 17938 for two months, (5x10⁸ CFU) there was a significant elevation in fecal calprotectin, a marker of intestinal inflammation. Although still within the normal range,increased calprotectin levels persisted to a small degree six months after supplement cessation.^[104] This same dose, in people with cystic fibrosis (whose symptoms include intestinal inflammation^[105]), was capable of reducing calprotectin by 40%.^[84]

In hospitalized adults at-risk for antibiotic-associated diarrhea (due to detecting Clostridium difficile, known to cause diarrhea^[106]), supplementation of L. reuteri ATCC 55730 at 10⁸ CFU for one month reduced the frequency of diarrhea from 50% to 7.7%.^[107]

L. reuteri may reduce diarrhea caused by some antibiotics.

Lactobacillus bacteria are of interest in women's health as their numbers are reduced during instances of vaginal bacteriosis^[108] and it is thought that the reduced production of lactic acid by these bacteria are part of what creates a supportive environment for pathogenic bacteria or other microorganisms.^[109]

The RC-14 strain of L. reuteri and the GR-1 strain of L. rhamnosus both appear to be able to increase vaginal Lactobacillus microflora when orally ingested^[25] (although one study was not able to detect L. reuteri in vaginal swabs^[3]). Studies testing the this mixture's use for vaginal health have been dosed at 10⁹ CFU daily.^[25][3]

In postmenopausal female urinary tract infections (UTIs), supplementation of mixed Lactobacillus (10⁹ CFU of L. rhamnosus GR-1 and L. reuteri RC-14 twice daily) was compared against the pharmaceutical trimethoprim-sulfamethoxazole (480mg) for one year. Both groups performed equally,as the probiotic reduced the mean number of symptomatic UTIs from 6.8 to 3.3 while the pharmaceutical reduced them from 7.0 to 2.9.^[3] While performing equally to the pharmaceutical, Lactobacillus did not increase antibiotic resistance against the causative pathogen (E. coli) as trimethoprim-sulfamethoxazole did. On the other hand, there was a statistically non-significant increase in withdrawals from the trial in the Lactobacillus group relative to the antibiotic group due to side-effects.^[3] While this study found that Lactobacillus treatment was clearly not inferior to antibiotic treatment, this interpretation has been contested on both technical grounds and due to concern over antibiotic resistance generated by antibiotic treatment.^[110][111] 

Mixed Lactobacillus probiotics appear to be able to reduce urinary tract infections in postmenopausal women, with one study suggesting comparable potency to a reference antibiotic for this purpose.

One human study using L. reuteri RC-14 alongside L. rhamnosus GR-1 (collectively 10⁹ CFU, taken twice daily) in women with bacterial vaginosis noted that oral supplementation for 12 weeks resulted in greater restoration of vaginal microflora (61.5%) compared to the placebo group (26.9%).^[2] This study also noted that approximately half of overall subjects recieving the probiotic had normal microflora, and that supplementation resulted in over 80% of subjects showing increased Lactobacillus counts in vaginal secretions.^[2]

Lactobacillus genus probiotics have been noted to be beneficial for vaginal health at times, specifically both L. reuteri ATCC 6475 and l. reuteri RC-14 have been implicated.

L. reuteri ATCC 6475 supernatant has been noted to increase the apoptosis of leukemia cells (KBM-5) caused by TNF-α, an effect which may be occur via inhibition of NF-kB.^[56] L. reuteri did not appear to inhibit DNA binding of the NF-kB complex (p50 and p65 subunits) but appeared to inhibit p65 subunits from translocating to the nucleus by reducing ubiquination and degradation of IκBα .^[56] These effects were noted alongside an increase in JNK and p38 phosphorylation from TNF-α, while another MAPK protein known as ERK1/2 was suppressed.^[56]

The NF-kB inhibitory effects of L. reuteri (likely through one of its metabolites) have been shown to sensitize some cancer cells to endogenous cytotoxic agents, although the practical relevance of this information is not yet known.

Supplementation of Lactobacillus reuteri ATCC 6475 (3.5×10⁵ CFU) for 20-24 weeks increased fur quality within seven days in female mice, with no significant effect in males.^[13] This was thought to be related to the increase in proliferation and activity of follicular sebocytes noted with ATCC 6475 relative to control, and did not occur in mice lacking IL-10.^[13]

Skin was noted to be thickened in both sexes and is known to thicken during the anagen phase of hair growth normally. Ingesting the probiotic increased hair follicle count and amount of follicles in the anagen phase (70% relative to 36% in control) with substantially less in the telogen phase (16% relative to 64%). This was absent in mice which did not express IL-10.^[13] To futher support this argument, since IL-10 works via downregulating IL-17^[83] the investigatos used an IL-17 antibody to mimick the effects of Lactobacillus reuteri.^[13]

Probiotic supplements of the Lactobacillus genus including both casei^[112] and johnsonii^[113] have been previously noted to influence skin quality following oral ingestion. This has been hypothesized to be related to a gut-skin axis, also referred to as a gut-brain-skin axis due to similarities with the gut-brain axis.^[114][115])

Ingestion of Lactobacillus reuteri ATCC 6475 (3.5×10⁵ organisms) for 20-24 weeks has been noted to increase dermal thickness in mice relative to control.^[13] This was not seen in mice lacking IL-10, suggesting a important role for this antiinflammatory cytokine,^[13] which is known to mediate antiinflammatory effects in both intestines and skin.^[83]

L. reuteri is known to colonize the vaginal tract amongst other Lactobacillus bacteria,^[116] although it is one of the less common species, (most common being jensenii, gasseri, iners and crispatus^{[117][118][119]}) quantified at 0.3% total bacteria.^[117] 

In mice, oral ingestion of supplemental L. reuteri (ATCC PTA 6475) at 3.5x10⁵ CFU per mouse was noted to decrease vaginal pH which occurred alongside female-exclusive improvements in fur lustre.^[13] This was hypothesized to exert a pro-fertility effect, since vaginal acidity (due to Lactobacillus colonization) correlates with ages of peak fertility^[120][117] and the phenotype induced by supplementation (increased hair lustre and skin quality) was one which displays health and reproductive fitness.^[13]

Infantile colic is a temporary condition affecting between 10-25% of infants,^[121] where crying of a fussy nature exceeds three hours daily for at least three times a week.^[122] It has been suggested that colic may be related to the intestines,^[123] with alterations in microflora count including less Lactobacillus species having been noted in colicky infants relative to those not suffering from colic.^[124][125]

In infantile colic, L. reuteri DSM 17938 at 1x10⁸ CFU daily (liquid suspension given as 5 drops before feeding) appeared to be better than placebo at reducing crying over the course of three weeks supplementation, with more responders (a reduction of crying by 50% or more) occurring with weekly supplementation.^[126] Benefit has also been noted to occur with the parent L. reuteri strain ATCC 55730, where the same dose of 10⁸ CFU via liquid suspension daily for one month was associated with a 95% response rate (active control of simethicone reaching 7%) and decreased crying time at all weeks, ultimately reaching a 75% reduction in crying time.^[127] These benefits to crying have been replicated in numerous trials all showing substantial benefits over placebo at a similar 10⁸ CFU dosage in upwards of one week.^{[128][129][130]} Moreover, supplementation of L. reuteri DSM 17938 for the first three months of life reduced the overall risk of crying at all measured time points.^[131] 

However, in a meta-analysis of trials treating cholic, it was noted that all trials had potential bias warranting caution when drawing positive conclusions.^[132] Notably, a subsequent trial on DSM 17938 at the 10⁸ CFU dosage in infants under three months with colic failed to find a beneficial influence of L. reuteri relative to placebo.^[133]

In regards to infantile colic (excessive crying, which is thought to be gastrointestinal in nature), supplementation of L. reuteri DSM 17938 via liquid suspension has been noted in numerous trials to greatly reduce crying. A recent meta-analysis, however, found potential bias with these studies and was followed up by an additional study which failed to note any benefit. Although promising, more independent research needs to be conducted to determine if L. reuteri is an effective treatment for colic.

L. reuteri DSM 17938 appears to confer other gastrointestinal benefits to children, and prophylactic treatment for the first three months of life has been noted to reduce overall regurgitation (37%) and improve bowel frequency (16%) relative to placebo.^[131] the latter This is thought to be related to a reduction in risk of constipation seen with L. reuteri DSM 17938 treatment.^[134]

Supplementation of L. reuteri DSM 17938 appears to support gastrointestinal motility in infants, resulting in less symptoms of both regurgitation and constipation.

Pseudomonas aeruginosa is a pathogenic bacteria with clinical relevance to cystic fibrosis, as the biofilms it can produces can cause lung infection and aggravate pulmonary functions over time.^[135] Probiotics that compete with P. aeruginosa are thought to be preventative in people with cystic fibrosis.

L. reuteri ATCC 55730 given to youth with cystic fibrosis at 10¹⁰ CFU daily (liquid suspension) over the course of six months was associated with a significantly reduced risk of pulmonary exacerbations (OR 0.06; reducing eleven exacerbations down to one) with a reduced risk of upper respiratory tract infections.^[136] These benefits were without changes in sputum or FEV₁ (lung power) between groups,with no changes in serum or sputum IL-8 or TNF-α.^[136] Elsewhere, supplementation of 10⁸ CFU of DSM 17938 via once daily tablets for six months also failed to influence IL-8 or TNF-α or to improve pulmonary function, but did appear to be associated with increased gastrointestinal comfort associated with beneficial alterations in intestinal microflora (a significant decrease in Proteobacteria and increase in both Firmicutes and Bacteroidetes).^[84]

Preliminary evidence indicates that L. reuteri may greatly reduce the risk of pulmonary exacerbations in cystic fibrosis and may have a beneficial influence on the gut microbiome, which is altered from the norm in cystic fibrosis.

Serum Vitamin D has been noted to increase when 2.9x10⁹ CFU of the strain of L. reuteri known as NCIMB 30242 was consumed twice daily over the course of 13 weeks by hypercholesterolemic adults.^[24] Over the course of this trial, there was a slight decrease in vitamin D levels noted in the placebo arm, while there was a 25.5% increase noted with NCIMB 30242, increasing serum 25-dihydroxyvitamin D from 67.91nM to 82.64nM with no influence on other measured vitamins in serum (β-carotene or Vitamin E).^[24]

Cobalamin (Vitamin B12) is known to be synthesized by a few bacteria which include L. reuteri CRL 1098,^[137][138] which the bacterium uses in the production of the antimicrobial reuterin since the first enzyme in reuterin synthesis (glycerol dehydratase) is dependent on B12.^[139] The genes required for B12 synthesis have also been detected in the strains JCM 1112^[140][141] and ATCC PTA 6475.^[140] It is thought that most strains of L. reuteri produce B12 since most strains produce reuterin.^[4] 

The form of B12 that is produced from L. reuteri (CRL 1098) has been identified as pseudovitamin B12.^[142] While one study found that oral ingestion of L. reuteri CRL 1098 by mice improved symptoms of B12 deficiency (1x10⁷ CFU daily),^[143] it was noted that the medium for cultivating CRL 1098 may have been contaminated with B12 of other origins.^[144]

Lactobacillus reuteri is capable of synthesizing vitamin B12 for its own usage in order to produce antimicrobial compounds, but the variant synthesized is pseudovitamin B12. There is currently no good evidence to support that B12 synthesized in the human gut from this bacteria contributes to overall B12 status.

Similar to Vitamin B12, Lactobacillus reuteri JCM 1112 can synthesize folate^[145] although in lesser amounts; it is uncertain if the synthesis of folate confers any systemic benefit after human consumption of L. reuteri.

The safety of various L. reuteri strains has been tested in healthy adult populations. Daily treatment in healthy adults with 5x10⁸ CFU L. reuteri DSM 17938 over the course of two months showed no significant differences in adverse events versus placebo in a double-blinded trial.^[104] A shorter double-blind, placebo-controlled study in healthy adults with a higher dose (1x10¹¹ CFU) given daily for 21 days (and a total study length of 28 days) also found no difference between treatment and placebo groups.^[57] The NCIMB 30242 strain has also been found to be well-tolerated compared to placebo in a direct study on its possible toxicity at 2.9x10⁹ CFU twice daily in otherwise healthy hypercholesterolemic men and women over 9 weeks in capsule form.^[146] similar Results in the same population were found when the same strain was administered in yogurt form at a dose of 5x10¹⁰ CFU twice daily for 6 weeks.^[147]

Safety in diseased adult populations has also been assessed. Oral ingestion of L. reuteri SD 2122 freeze-dried powder (5x10⁹ CFU added to a cool beverage) twice daily for three weeks in adults with HIV appears to be well-tolerated with no clinical or biochemical signs of toxicity.^[58] In patients with rheumatoid arthritis, a combination of L. reuteri RC-14 and L. rhamnosus GR-1 at a total dose of 2x10⁹ CFU for both species combined twice daily for 90 days (with a 120 day follow-up) found no adverse events reported among the treatment group and no significant differences between serum creatinine or liver enzyme levels compared to placebo.^[148]

Studies on the safety of L. reuteri have also been performed in infants. Otherwise healthy infants aged 3-65 days fed a formula at a concentration of 2.2x10⁸ CFU/180mL L. reuteri ATCC 55730 (also called SD 2112) for 4 weeks showed no adverse effects and no differences in growth, stooling behaviors, instances of crying, or irritability compared to placebo formula.^[149] No adverse events were seen in children 6-36 months old with either or acute diarrhea^[150] or chronic constipation^[134] were given the DSM 17938 strain of L. reuteri.

Various strains of L. reuteri appear to be safe in both healthy adults as well as adults with certain disease states. L. reuteri also seems to be well-tolerated in healthy infants as well as those with bowel problems.