Probiotics are micro-organisms that have been proven to exert health-promoting influences in humans and animals. Probiotic bacterial cultures are intended to assist the body’s naturally occurring gut flora. The gut flora can be broadly divided into three main groups:
• Bifidobacteria (probiotic beneficial ‘good’ bacteria)
• Bacteroides species (pathogenic micro-organisms)
• Prebiotics (non-digestible food ingredients that encourage the growth of bifidobacteria).
Most probiotics fall into the group of organisms known as lactic acid-producing bacteria, and are normally consumed in the form of yoghurt and other fermented foods.
The most commonly used species of lactic acid bacteria in probiotic preparations include Lactobacillus, Bifidobacterium, Enterococcus and Streptococcus. Other organisms currently being used in probiotic preparations, include Bacillus yeasts (e.g. Saccharomyces boulardii) and filamentous fungi (e.g. Aspergillus oryzae).
Lactobacillus
The Lactobacillus genus has over 125 species. It is a major part of the lactic acid bacteria group – named as such because most of the strains convert lactose into lactic acid. Lactobacillus lowers gut pH, thereby limiting the growth of pathogenic bacteria, competing for pathogen binding and receptor sites, and modulating the local and systemic inflammatory immune response.
The gastrointestinal (GI) tract
The gastrointestinal (GI) tract serves as an interface between the gut and immune system. The intestinal lining functions as a barrier that decreases the passage of bacteria from the gut into the bloodstream. Harmless micro-organisms, such as species of Lactobacilli and Bifidobacteria, can occupy a space or a biofilm that otherwise would be colonised by a pathogen. When inflamed, the GI tract becomes permeable and can serve as a link between inflammatory diseases of the tract itself and extra-inflammatory disorders such as arthritis. Consuming probiotics can modulate or ‘down regulate’ the immune system and subsequently reduce the permeability of the GI tract.
Oral health benefits
Probiotics, literally meaning ‘for life’, have been extensively studied for their health-promoting effects. While the main field of research has been the GI tract, the past decade has seen the oral health perspective of probiotics investigated. Probiotic treatment of oral diseases such as dental caries and periodontal disease is being researched due to their mechanisms of bacterial adhesion, competitive colonisation of bioflim and possible effects on immunomodulation.
Lactobacilli have attracted particular interest in dental research over the last decade. This is due to their ability to inhibit a wide range of bacterial species including oral Streptococci, Aggregatibacter actinomycetemcomitans (AA), and Porphyromonas gingivalis (PG).
Caries
Dairy micro-organisms have been tested in vitro for their ability to become a part of the supragingival dental biofilm, as well as for their ability to compete with cariogenic micro-organisms. Promising results have been obtained for several strains. Milk fermented with Lactobacillus GG was also recently shown to reduce the adherence of Streptococcus mutans to saliva-coated hydroxyapatite beads.
The preventive effect of Lactobacillus GG in milk has been evaluated in a clinical study of caries involving nearly 600 pre-school children over a period of seven months. Statistically significant differences were found in the development of caries in children treated with the probiotic strain compared with those given a placebo milk product. A further, separate study also demonstrated decreased counts of yeast and Streptococcus mutans in saliva.
Periodontal disease
More recently, a number of probiotics have been studied with reference to the prevention and treatment of periodontal disease. Koll et al (2008) studied 10 species of oral lactobacilli and reported suppression in the growth of Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis, and Streptococcus mutans.
A further study also identified Lactobacillus brevis as having anti-inflammatory effects on periodontal disease. After exposure to L. brevis-containing lozenges, salivary concentration of pro-inflammatory markers was significantly reduced.
This research suggests a new and innovative approach to the treatment of chronic periodontal disease.
Halitosis
Other areas under investigation include the use of probiotics in the treatment of halitosis and reduction of volatile sulphur compounds (VSC).
Oral malodour is primarily treated by reducing bacterial populations (especially those present on the tongue) using a variety of anti-microbial agents or mechanical devices. However, problematic bacteria can quickly repopulate the tongue shortly after treatment and the malodour returns.
Encouraging results have been shown using Streptococcus salivarius and Weissella cibaria respectively. These preliminary studies indicate that the replacement of the colonising bacteria implicated in halitosis with competitive bacteria such as S. salivarius may provide an effective strategy to reduce the severity of oral malodour.
Probiotic bacterial strains sourced from the indigenous oral microbiotas of healthy humans may have potential application as adjuncts for the prevention and treatment of halitosis.
Oral colonisation
Colonisation of the oral cavity is mixed, with some strains colonising more effectively than others.
Lactobacillus GG was found to colonise the oral cavity in one week, with lactobacilli present for up to two weeks after discontinuation of probiotic application. No installation was found after one week with L. acidophilus, L. casei or B. bifidum. The most effective colonisation appeared to be from L. reuteri, with 95% colonisation after 14 days.
For a probiotic to be effective it should adhere to dental tissues as a part of the biofilm (or plaque) and compete with the growth of cariogenic bacteria or periodontal pathogens. None of the studies were able to demonstrate permanent colonisation. This could be due to the relatively short exposure time. Daily intake of a probiotic may therefore be required for continued effect.
Lacobacillus reuteri
Gerhard Reuter first isolated L. reuteri from human faecal and intestinal samples in the 1960s. This work was later repeated by other researchers. L. reuteri is now well-established as one of the most ubiquitous members of the lactic acid-producing bacteria. In 1988, Dobrogosz and Casas et al discovered that L. reuteri produced a broad-spectrum antibiotic substance via the organism’s fermentation of glycerol. They named this substance ‘reuterin’, after Gerhard Reuter.
Research has found that reuterin inhibits the growth of some harmful Gram-negative and Gram-positive bacteria, along with yeasts, fungi and protozoa. L. reuteri can secrete sufficient amounts of reuterin to cause the desired anti-microbial effects. Furthermore, about four to five times the amount of reuterin is needed to kill bifido gut bacteria (i.e. L. reuteri and other Lactobacillus species) as bacteroides bacteria. This allows L. reuteri to remove pathogenic species while keeping normal gut flora intact.
Oral intake of L. reuteri has been shown to colonise the intestine of healthy people effectively; colonisation begins rapidly within days of ingestion, although the amount of the bacterium present in the body returns to low levels within several months once intake ceases. Once in the body, L. reuteri benefits its host in a variety of ways, in general health, immunomodulation and disease protection.
L. reuteri and oral health
Increasing attention is being given to L. reuteri with reference to oral colonisation and prevention of both caries and periodontal disease.
In a number of studies, L. reuteri has been shown to colonise the oral cavity, significantly reduce salivary concentrations of Streptococcus mutans and inhibit a number of pathogens associated with periodontal disease, most notably AA and PG.
Nikawa et al (2004) found that the consumption of yoghurt containing L. reuteri significantly inhibited the growth of Streptococcus mutans, which contrasted with other probiotic lactobacilli strains. In the study, dairy products such as milk, yoghurt and cheese were selected as delivery vehicles for the selected bacteria.
In 2006, Caglare et al investigated the effect of the probiotic bacterium L. reuteri on the levels of salivary mutans streptococci and lactobacilli when ingested via two different non-dairy delivery systems (straws and tablets). Levels of mutans streptococci were significantly reduced after ingesting probiotic bacteria via both straw and tablets, contrasting the placebo controls.
Recently, new research has evaluated the effect of probiotic chewing gums containing two strains of L. reuteri on the levels of salivary mutans streptococci and lactobacilli. Daily chewing of the gum containing probiotic bacteria significantly reduced the levels of salivary mutans streptococci.
Probiotics can also be delivered as lozenges, powder, gelatine, tablets or through straws. It has been suggested that slowly melting tablets would allow a more thorough contact between the probiotic and the oral environment. However, the best vehicle for probiotic delivery has yet to be identified.
L. reuteri and periodontal disease
More recently, studies have shown the ability of L. reuteri to inhibit the proliferation of the main periodontal pathogens, including AA and PG. Krasse et al (2006) demonstrated the ability of L. reuteri to colonise the oral cavity within two weeks of exposure. A reduction in both gingivitis and plaque levels was also observed. Two strains of L. reuteri were trialled in this study, (L. reuteri ATCC 55730/L. reuteri ATCC PTA 5289), both of which were shown to have successfully colonised the oral cavity. A 59% reduction in gingivitis levels and a 42% reduction in plaque levels were recorded after four weeks of treatment.
Probiotics and immune function
Probiotics can enhance both specific and non-specific immune responses. It is thought that these effects are achieved through activating macrophages, increasing levels of cytokines, increasing natural killer cell activity and/or increasing levels of immunoglobulins. Probiotics have also been found to upregulate anti-inflammatory cytokines. This is seen both as an immunostimulatory effect in healthy subjects and as a down regulation effect of immunoinflammatory responses in hypersensitive patients. Moreover, probiotics may prevent infection because they compete with pathogenic viruses or bacteria for binding sites on epithelial cells. Probiotics may also inhibit the growth of pathogenic bacteria by producing bacteriocins, such as reuterin.
The ability of a probiotic to influence immune function and colonise the gut, oral cavity and plaque biofilm is only as good as its adhesion factor. The ability of these micro-organisms to adhere to mucosal surfaces is potentially a major distinguishing feature when selecting bacteria as probiotic strains. Close interaction with host tissues provides probiotics with a distinct advantage when establishing residence in the GI tract or interacting with cells of the mucosa. However, not all probiotics have the same adhesion factors. When compared to other lactobacilli, L. reuteri has been shown to have superior adhesion and ability to colonise the entire digestive tract, including the oral cavity.
Emerging evidence suggests that L. reuteri may be able to modulate the immune system in the GI tract. Valeur et al (2004) observed a significant increase in CD4+ T cells and B lymphocytes. Furthermore, Mao et al (1996) found that L. reuteri could increase sIgA levels in addition to CD4+ T cells. The stimulation of T-helper cells by L. reuteri may be a central symbiotic mechanism for improving the health of the gut and one of the probiotics main actions. Valeur showed colonisation of the human GI tract by L. reuteri ATCC 55730 when delivered in a tablet formulation and observed consequent modulation of local immune cell populations. It therefore seems likely that this response to exogenous L. reuteri may be involved in maintaining gastrointestinal well-being and acting as a defence against bacterial pathogens.
L. reuteri prodentis
Two strains of L. reuteri were trialled in a number of studies (L. reuteri ATCC 55730/L. reuteri ATCC PTA 5289); both strains were shown to have successfully colonised the oral cavity.
A multi-strain probiotic, L. reuteri prodentis, has been developed as a combination of two complementary strains of L. reuteri (L.reuteri ATCC 55730 and L. reuteri ATCC PTA 5289). While a monostrain has to overcome barriers presented by the host and its endogenous microflora, multi-strain probiotics have a greater divergency and there is an increased chance of at least one strain surviving.
The safety of consuming L. reuteri on a daily basis has been well-established through a number of studies on diverse populations, including adults, children, newborn infants, premature infants and immunocompromised adults.
Conclusion
The oral administration of probiotic therapies may be beneficial in a multitude of disorders, both inside and outside the GI tract. Their application in the treatment and prevention of oral disease appears to be both local (colonisation of plaque biofilm) and systemic (modulation of the immune system).
The direct effects of probiotics in the GI tract are well-documented and include upregulation of immunoglobulins such as IgA, down regulation of inflammatory cytokines, and enhancement of gut barrier function. New research supports indirect, systemic effects of probiotics for a widely divergent set of disorders, including atopic disease and immune compromise. It also appears to affect caries and chronic inflammatory periodontal disease.
Unfortunately, the concentration of probiotics in food products varies significantly and there are currently no national standards of identity for levels of bacteria required in yoghurt or other fermented products.
There is considerable potential for the benefits of probiotics over a wide range of clinical conditions. As oral health professionals we are in an ideal position to guide patients towards appropriate uses of probiotics to deliver the desired beneficial oral health effects.
