Overview
Lactobacillus salivarius is a Gram-positive, rod-shaped, facultatively anaerobic bacterium belonging to the phylum Firmicutes, family Lactobacillaceae. As its name suggests, it was originally isolated from human saliva and is a natural inhabitant of the oral cavity, but it is also found throughout the gastrointestinal tract and in breast milk. L. salivarius has attracted research interest for its antimicrobial properties, its role in oral health, and its potential to modulate immune responses in conditions such as atopic dermatitis and inflammatory bowel disease.
The strain L. salivarius UCC118, isolated from the human ileal-caecal region, has been one of the most extensively studied and was among the first Lactobacillus strains to have its complete genome sequenced, providing foundational insights into probiotic mechanisms of action.
Characteristics
L. salivarius is a versatile lactic acid bacterium that can thrive in multiple body sites, reflecting its adaptability to different environmental conditions.
Key features include:
- Morphology: Gram-positive, non-motile, non-spore-forming rods, occurring singly, in pairs, or in short chains
- Metabolism: Facultatively heterofermentative; produces both L-lactic acid and D-lactic acid, along with acetic acid and ethanol from certain carbohydrate substrates
- Habitat: Human saliva, oral mucosa (tongue, buccal surfaces), gastrointestinal tract (particularly ileum), breast milk, and vaginal tract
- Genome: The UCC118 strain has a chromosome of approximately 1.83 Mbp and harbors megaplasmids (pMP118, ~242 kbp) that carry genes for bacteriocin production and other survival functions
- Bacteriocin production: Produces Abp118, a potent Class IIb two-peptide bacteriocin with broad-spectrum antimicrobial activity
- pH tolerance: Survives at low pH (2.0-3.0) and tolerates bile salts, enabling survival through the upper gastrointestinal tract
Bacteriocin-Mediated Antimicrobial Activity
One of the defining characteristics of L. salivarius UCC118 is its production of the bacteriocin Abp118, a two-component antimicrobial peptide. In a landmark study by Corr et al. (2007), researchers demonstrated that Abp118 production was directly responsible for protecting mice against lethal infection with the foodborne pathogen Listeria monocytogenes.[1]
Key findings from this research include:
- Mice fed L. salivarius UCC118 showed significantly improved survival following L. monocytogenes challenge
- A mutant strain of UCC118 engineered to be deficient in Abp118 production lost this protective effect entirely
- This provided direct genetic evidence that bacteriocin production, rather than other probiotic mechanisms such as competitive exclusion or immune stimulation, was the primary protective mechanism
This study was significant because it was one of the first to use molecular Koch's postulates to demonstrate a specific mechanism by which a probiotic exerts its anti-infective activity in vivo, moving the field beyond correlative observations to mechanistic understanding.
Oral Health Benefits
Given its natural presence in the oral cavity, L. salivarius has been investigated for several oral health applications:
Halitosis Management
Iwamoto et al. (2010) conducted a clinical trial evaluating L. salivarius WB21 for the management of halitosis (bad breath). Participants who consumed tablets containing L. salivarius WB21 daily for 4 weeks showed significant reductions in volatile sulfur compound (VSC) levels, the primary chemical mediators of oral malodor.[2] Additional improvements were observed in:
- Organoleptic scores (clinician-assessed breath odor)
- Plaque index scores
- Gingival pocket depth in subjects with deeper baseline pockets
Periodontal Health
L. salivarius may contribute to periodontal health through several mechanisms:
- Competitive exclusion: By colonizing oral mucosal surfaces, L. salivarius may limit the adhesion and proliferation of periodontal pathogens such as Porphyromonas gingivalis, Tannerella forsythia, and Treponema denticola
- Bacteriocin activity: Abp118 and related antimicrobial peptides have demonstrated activity against oral pathogens in vitro
- Anti-inflammatory effects: Modulation of pro-inflammatory cytokine production in gingival tissues may reduce the tissue destruction characteristic of periodontitis
Research into probiotic applications for periodontal disease is still emerging, and while early results are encouraging, larger randomized controlled trials are needed to establish clinical guidelines for probiotic use in periodontal therapy.
Immune Modulation and Atopic Dermatitis
L. salivarius has shown promise in modulating immune responses, with particular relevance to allergic and atopic conditions. The bacterium can influence the balance between Th1 and Th2 immune responses, which is central to the pathogenesis of atopic diseases.
Key immunological effects observed in preclinical and clinical studies include:
- Dendritic cell maturation: L. salivarius can stimulate dendritic cell maturation and promote the production of IL-10 and IL-12, cytokines that may help redirect immune responses away from the Th2 polarization seen in allergic disease
- Regulatory T cell induction: Enhancement of Treg cell populations that suppress excessive inflammatory and allergic responses
- IgE modulation: Some studies have reported reductions in allergen-specific IgE levels with L. salivarius supplementation
In infant populations, supplementation with L. salivarius has been explored as a strategy for the prevention and management of atopic dermatitis. Breast milk naturally contains L. salivarius, and this vertical transfer from mother to infant is believed to play a role in early immune education and the establishment of a balanced infant microbiome.
Breast Milk and Infant Colonization
L. salivarius is one of the most frequently identified Lactobacillus species in human breast milk. This presence is significant for several reasons:
- Vertical transmission: Transfer of L. salivarius from mother to infant through breastfeeding contributes to the establishment of the infant's gut and oral microbiome
- Mastitis management: Clinical trials have explored L. salivarius supplementation in lactating mothers for the prevention and treatment of mastitis, with some studies showing reduced bacterial counts of Staphylococcus species in breast milk and decreased pain scores
- Immune priming: Early colonization with L. salivarius may help educate the neonatal immune system, potentially reducing the risk of allergic and autoimmune conditions later in life
Gastrointestinal Applications
In the gut, L. salivarius UCC118 has been shown to modulate the intestinal microbiota and influence gut immune function.[3] Demonstrated gastrointestinal effects include:
- Pathogen inhibition: Reduction of pathogenic bacteria including Salmonella, Listeria, and Clostridium species through bacteriocin production and competitive exclusion
- Cytokine modulation: Alteration of the pro-inflammatory to anti-inflammatory cytokine balance in the gut-associated lymphoid tissue (GALT)
- Survival in the GI tract: UCC118 has demonstrated the ability to survive gastric acid and bile exposure and can be recovered from human fecal samples after oral administration, confirming its transit through the entire gastrointestinal tract[3]
Safety Profile
L. salivarius has a well-established safety profile and is considered safe for human consumption. It holds QPS status in Europe and has been used without significant adverse events in clinical trials involving infants, children, adults, and the elderly. As a natural component of the healthy human oral and gut microbiome, it is encountered daily through normal physiological processes.
Key Takeaways
- L. salivarius is a versatile probiotic naturally found in human saliva, the oral cavity, gut, and breast milk
- The UCC118 strain produces the bacteriocin Abp118, which provides direct antimicrobial protection against pathogens such as Listeria monocytogenes
- Clinical evidence supports oral health applications including halitosis reduction and potential periodontal benefits
- L. salivarius may help modulate immune responses relevant to atopic dermatitis and allergic conditions
- Its natural presence in breast milk suggests an important role in mother-to-infant microbial transmission and early immune development
- As with all probiotics, effects are strain-specific, and clinical benefits observed with one strain should not be automatically assumed for others