Inhibitory effect of Lactobacillus salivarius on Streptococcus mutans biofilm formation

Opposing effect of Lactobacillus on in vitro Klebsiella pneumoniae in biofilm and in an in vivo intestinal colonisation model

Beneficial bacteria represent potential sources of therapy, particularly in the battle against antibiotic-resistant pathogens. The Gram-negative bacillus Klebsiella pneumoniae is not only a paradigm of multi-resistant opportunistic pathogen, but it is also able to colonise the human intestine and displays a high capacity to form biofilm. In this study, the anti-biofilm activity of 140 neutralised Lactobacillus supernatants was assessed against K. pneumoniae. Among the 13 strains whose supernatant significantly impaired biofilm formation, Lactobacillus plantarum CIRM653 was selected because it was also able to impair K. pneumoniae preformed biofilm, independently of a bactericidal effect. Mixed K. pneumoniae/L. plantarum CIRM653 biofilms had reduced tridimensional structures associated with a significant decrease in K. pneumoniae biomass. Further investigation showed that L. plantarum CIRM653 supernatant induced transcriptional modifications of K. pneumoniae biofilm-related genes, including down-regulation of the quorum sensing-related lsr operons and over-expression of type 3 pili structure genes. Increased production of type 3 pili was validated by Western-blot, hemagglutination and adhesion assays. L. plantarum CIRM653 activity against K. pneumoniae was also assessed in a murine intestinal colonisation model: a constant faecal pathogen burden was observed, as against a gradual decrease in the control group. These results reveal that an in vitro a priori attracting anti-biofilm activity of Lactobacillus might be counterbalanced by an in vivo behaviour in a complex microbiota environment with potential deleterious dispersal of highly adherent K. pneumoniae cells, raising the question of the accuracy of in vitro assays in screening of beneficial microbes.

Lactobacillus rhamnosus GG in Experimental Oral Biofilms Exposed to Different Carbohydrate Sources

Probiotic administration may favour caries prevention, as recent research has shown. This in vitro study aimed to investigate the growth of Lactobacillus rhamnosus GG (LGG) in experimental biofilms exposed to various carbohydrates, and also to assess its cariogenic potential. Multispecies experimental oral biofilms with or without LGG were grown with a sole-carbohydrate source (fructose/glucose/lactose/sorbitol/sucrose). The viable cells of LGG and structure of the biofilms were examined after 64.5 h of incubation, and pH values of spent media were measured at 16.5, 40.5, and 64.5 h. Fermentation profiles of LGG in biofilm media were assessed with study carbohydrate as the sole energy source. Our results showed that LGG reached higher viable cell numbers with glucose and sucrose in 64.5-h multispecies experimental oral biofilms compared to other carbohydrates. When LGG was incorporated in biofilms, no distinct pH changes at any time points were observed under any of the carbohydrates used; the pH values of spent media at each time point were lower when lactose was used, compared to other carbohydrates. The fermentation profiles of LGG in biofilm media were similar to its growth in MRS (no obvious growth with lactose or sucrose). In conclusion, LGG in our in vitro multispecies experimental oral biofilms was capable of surviving and growing well in each carbohydrate source. LGG might not have harmful effects on dental hard tissues. Another finding from our study was that the lowest pH values were observed in the presence of lactose, and the thickest biofilms were in sucrose.

Effects of Sub-lethal Concentrations of Silver Nanoparticles on a Simulated Intestinal Prokaryotic-Eukaryotic Interface

Nanotechnology applications are expected to bring a range of benefits to the food sector, aiming to provide better quality and conservation. In this research, the physiological response of both an Escherichia coli mono-species biofilm and Caco-2 intestinal cells to sub-lethal concentrations of silver nanoparticles (AgNPs) has been investigated. In order to simulate the anaerobic and aerobic compartments required for bacteria and intestinal cells growth, a simplified semi-batch model based on a transwell permeable support was developed. Interaction between the two compartments was obtained by exposing Caco-2 intestinal cells to the metabolites secreted by E. coli biofilm after its exposure to AgNPs. To the best of the authors’ knowledge, this study is the first to investigate the effect of AgNPs on Caco-2 cells that takes into consideration previous AgNP-intestinal biofilm interactions, and at concentrations mimicking real human exposure. Our data show that 1 μg/mL AgNPs in anaerobic conditions (i) promote biofilm formation up to 2.3 ± 0.3 fold in the first 72 h of treatment; (ii) increase reactive oxygen species (ROS) production to 84 ± 21% and change the physiological status of microbial cells after 96 h of treatment; (iii) seriously affect a 72-h old established biofilm, increasing the level of oxidative stress to 86 ± 21%. Moreover, the results indicate that oxygen renders the biofilm more adequate to counteract AgNP effects. Comet assays on Caco-2 cells demonstrated a protective role of biofilm against the genotoxic effect of 1 μg/mL AgNPs on intestinal epithelial cells.

Streptococcus mutans Displays Altered Stress Responses While Enhancing Biofilm Formation by Lactobacillus casei in Mixed-Species Consortium

Like Streptococcus mutans, lactobacilli are commonly isolated from carious sites, although their exact role in caries development remains unclear. This study used mixed-species models to analyze biofilm formation by major groups of oral lactobacilli, including L. casei, L. fermentum, L. rhamnosus, L. salivarius ssp. salivarius, and L. gasseri. The results showed that lactobacilli did not form good biofilms when grown alone, although differences existed between different species. When grown together with S. mutans, biofilm formation by L. gasseri and L. rhamnosus was increased by 2-log (P < 0.001), while biofilms by L. fermentum reduced by >1-log (P < 0.001). L. casei enhanced biofilm formation by ~2-log when grown with S. mutans wild-type, but no such effects were observed with S. mutans deficient of glucosyltransferase GtfB and adhesin P1. Both S. mutans and L. casei in dual-species enhanced resistance to acid killing with increases of survival rate by >1-log (P < 0.001), but drastically reduced the survival rates following exposure to hydrogen peroxide (P < 0.001), as compared to the respective mono-species cultures. When analyzed by RNA-seq, more than 134 genes were identified in S. mutans in dual-species with L. casei as either up- or down-regulated when compared to those grown alone. The up-regulated genes include those for superoxide dismutase, NADH oxidase, and members of the mutanobactin biosynthesis cluster. Among the down-regulated genes were those for GtfB and alternative sigma factor SigX. These results further suggest that interactions between S. mutans and oral lactobacilli are species-specific and may have significant impact on cariogenic potential of the community.

Protective Microbiota: From Localized to Long-Reaching Co-Immunity

Resident microbiota do not just shape host immunity, they can also contribute to host protection against pathogens and infectious diseases. Previous reviews of the protective roles of the microbiota have focused exclusively on colonization resistance localized within a microenvironment. This review shows that the protection against pathogens also involves the mitigation of pathogenic impact without eliminating the pathogens (i.e., “disease tolerance”) and the containment of microorganisms to prevent pathogenic spread. Protective microorganisms can have an impact beyond their niche, interfering with the entry, establishment, growth, and spread of pathogenic microorganisms. More fundamentally, we propose a series of conceptual clarifications in support of the idea of a “co-immunity,” where an organism is protected by both its own immune system and components of its microbiota.

Effect of a Lactobacillus Salivarius Probiotic on a Double-Species Streptococcus Mutans and Candida Albicans Caries Biofilm

The aim of the study was to evaluate the anti-cariogenic effects of Lactobacillus salivarius by reducing pathogenic species and biofilm mass in a double-species biofilm model. Coexistence of S. mutans with C. albicans can cause dental caries progression or recurrence of the disease in the future. Fifty-nine children with diagnosed early childhood caries (ECC) were recruited onto the study. The condition of the children's dentition was defined according to the World Health Organization guidelines. The participants were divided into children with initial enamel demineralization and children showing dentin damage. The study was performed on the S. mutans and C. albicans clinical strains, isolated from dental plaque of patients with ECC. The effect of a probiotic containing Lactobacillus salivarius on the ability of S. mutans and C. albicans to produce a double-species biofilm was investigated in an in vitro model. The biomass of the formed/non-degraded biofilm was analyzed on the basis of its crystal violet staining. The number of colonies of S. mutans and C. albicans (CFU/mL, colony forming units/mL) forming the biofilm was determined. Microorganism morphology in the biofilm was evaluated using a scanning electron microscope (SEM). In vitro analysis demonstrated that the presence of S. mutans increased the number of C. albicans colonies (CFU/mL); the double-species biofilm mass and hyphal forms produced in it by the yeast. L. salivarius inhibited the cariogenic biofilm formation of C. albicans and S. mutans. Under the influence of the probiotic; the biofilm mass and the number of S. mutans; C. albicans and S. mutans with C. albicans colonies in the biofilm was decreased. Moreover; it can be noted that after the addition of the probiotic; fungi did not form hyphae or germ tubes of pathogenic potential. These results suggest that L. salivarius can secrete intermediates capable of inhibiting the formation of cariogenic S. mutans and C. albicans biofilm; and may inhibit fungal morphological transformation and thereby reduce the pathogenicity of C. albicans; weakening its pathogenic potential. Further research is required to prove or disprove the long-term effects of the preparation and to achieve preventive methods.

Health-Associated Niche Inhabitants as Oral Probiotics: The Case of Streptococcus dentisani

Oral diseases, including dental caries and periodontitis, are among the most prevalent diseases worldwide and develop as a consequence of a microbial dysbiosis. Several bacterial strains are being tested as potential oral health-promoting organisms, but usually they are species isolated from niches other than the site where they must exert its probiotic action, typically from fecal samples. We hypothesize that oral inhabitants associated to health conditions will be more effective than traditional, gut-associated probiotic species in key aspects such as colonization of the oral site where disease takes place or the possession of oral health promoting functions, as well as more practical issues like safety and toxicity, and establishing proper doses for administration. As an example of these active colonizers, we describe the case of Streptococcus dentisani, a new streptococcal species isolated from dental plaque of caries-free individuals. We have detected it in 98% of dental plaque samples from healthy individuals and, as expected, it does not produce any toxic secondary metabolite and does not survive a simulated stomach digestion, preventing potential secondary effects. Besides, this species has a double probiotic action, as it inhibits the growth of major oral pathogens through the production of bacteriocins, and also buffers acidic pH (the primary cause of dental caries) through an arginolytic pathway. We propose the use of S. dentisani as a promising probiotic against tooth decay.

Demineralization-remineralization dynamics in teeth and bone

Biomineralization is a dynamic, complex, lifelong process by which living organisms control precipitations of inorganic nanocrystals within organic matrices to form unique hybrid biological tissues, for example, enamel, dentin, cementum, and bone. Understanding the process of mineral deposition is important for the development of treatments for mineralization-related diseases and also for the innovation and development of scaffolds. This review provides a thorough overview of the up-to-date information on the theories describing the possible mechanisms and the factors implicated as agonists and antagonists of mineralization. Then, the role of calcium and phosphate ions in the maintenance of teeth and bone health is described. Throughout the life, teeth and bone are at risk of demineralization, with particular emphasis on teeth, due to their anatomical arrangement and location. Teeth are exposed to food, drink, and the microbiota of the mouth; therefore, they have developed a high resistance to localized demineralization that is unmatched by bone. The mechanisms by which demineralization-remineralization process occurs in both teeth and bone and the new therapies/technologies that reverse demineralization or boost remineralization are also scrupulously discussed. Technologies discussed include composites with nano- and micron-sized inorganic minerals that can mimic mechanical properties of the tooth and bone in addition to promoting more natural repair of surrounding tissues. Turning these new technologies to products and practices would improve health care worldwide.

Anti-biofilm Activity as a Health Issue

The formation and persistence of surface-attached microbial communities, known as biofilms, are responsible for 75% of human microbial infections (National Institutes of Health). Biofilm lifestyle confers several advantages to the pathogens, notably during the colonization process of medical devices and/or patients’ organs. In addition, sessile bacteria have a high tolerance to exogenous stress including anti-infectious agents. Biofilms are highly competitive communities and some microorganisms exhibit anti-biofilm capacities such as bacterial growth inhibition, exclusion or competition, which enable them to acquire advantages and become dominant. The deciphering and control of anti-biofilm properties represent future challenges in human infection control. The aim of this review is to compare and discuss the mechanisms of natural bacterial anti-biofilm strategies/mechanisms recently identified in pathogenic, commensal and probiotic bacteria and the main synthetic strategies used in clinical practice, particularly for catheter-related infections.

Effect of the Probiotic Lactobacillus rhamnosus LB21 on the Cariogenicity of Streptococcus mutans UA159 in a Dual-Species Biofilm Model

Despite promising results using probiotics, evidence of the preventive effect on enamel demineralization is insufficient and the cariogenic potential of probiotics is still controversial. Probiotics could affect biofilm formation and interfere with adherence, growth or coaggregation with Streptococcus mutans in biofilms. However, most of the studies have been conducted using planktonic bacteria. Hence, the aim of the study was to assess the effect of probiotic bacteria on the cariogenicity of S. mutans using an in vitro biofilm caries model on enamel. Single-species biofilms (S. mutans UA159, SM or Lactobacillus rhamnosus LB21, LB) or dual-species biofilms simultaneously inoculated (SM + LB) or LB inoculated 8 h after SM (SM x2192; LB) were grown for 96 h. Biofilms were formed on bovine enamel saliva-coated slabs of known surface hardness (SH) and immersed in culture media. Biofilms were exposed 8 times per day to 10% sucrose. Medium pH was monitored twice daily as a biofilm acidogenicity indicator. After 96 h, biofilms were collected to determine biomass and bacteria viability. Slab demineralization was calculated as percentage of SH loss (%SHL). Additionally, the model was tested with different concentrations of the initial inoculum (103, 106, 108 cells/ml) and different adhesion times (2 or 8 h). The dual-species biofilm revealed no LB effects on SM cariogenicity, without changes in acidogenicity or %SHL among groups (p > 0.05, n = 12). Lack of activity of LB on SM cariogenicity persisted even when 105 times higher concentration of the probiotic was tested. Coaggregation was not observed. In conclusion, findings suggest that LB does not reduce cariogenicity of SM in a validated experimental caries model.

Isolation of a Novel Phage with Activity against Streptococcus mutans Biofilms

Streptococcus mutans is one of the principal agents of caries formation mainly, because of its ability to form biofilms at the tooth surface. Bacteriophages (phages) are promising antimicrobial agents that could be used to prevent or treat caries formation by S. mutans. The aim of this study was to isolate new S. mutans phages and to characterize their antimicrobial properties. A new phage, ɸAPCM01, was isolated from a human saliva sample. Its genome was closely related to the only two other available S. mutans phage genomes, M102 and M102AD. ɸAPCM01 inhibited the growth of S. mutans strain DPC6143 within hours in broth and in artificial saliva at multiplicity of infections as low as 2.5x10-5. In the presence of phage ɸAPCM01 the metabolic activity of a S. mutans biofilm was reduced after 24 h of contact and did not increased again after 48 h, and the live cells in the biofilm decreased by at least 5 log cfu/ml. Despite its narrow host range, this newly isolated S. mutans phage exhibits promising antimicrobial properties.

L-arginine destabilizes oral multi-species biofilm communities developed in human saliva

The amino acid L-arginine inhibits bacterial coaggregation, is involved in cell-cell signaling, and alters bacterial metabolism in a broad range of species present in the human oral cavity. Given the range of effects of L-arginine on bacteria, we hypothesized that L-arginine might alter multi-species oral biofilm development and cause developed multi-species biofilms to disassemble. Because of these potential biofilm-destabilizing effects, we also hypothesized that L-arginine might enhance the efficacy of antimicrobials that normally cannot rapidly penetrate biofilms. A static microplate biofilm system and a controlled-flow microfluidic system were used to develop multi-species oral biofilms derived from pooled unfiltered cell-containing saliva (CCS) in pooled filter-sterilized cell-free saliva (CFS) at 37^oC. The addition of pH neutral L-arginine monohydrochloride (LAHCl) to CFS was found to exert negligible antimicrobial effects but significantly altered biofilm architecture in a concentration-dependent manner. Under controlled flow, the biovolume of biofilms (μm³/μm²) developed in saliva containing 100-500 mM LAHCl were up to two orders of magnitude less than when developed without LAHCI. Culture-independent community analysis demonstrated that 500 mM LAHCl substantially altered biofilm species composition: the proportion of Streptococcus and Veillonella species increased and the proportion of Gram-negative bacteria such as Neisseria and Aggregatibacter species was reduced. Adding LAHCl to pre-formed biofilms also reduced biovolume, presumably by altering cell-cell interactions and causing cell detachment. Furthermore, supplementing 0.01% cetylpyridinium chloride (CPC), an antimicrobial commonly used for the treatment of dental plaque, with 500 mM LAHCl resulted in greater penetration of CPC into the biofilms and significantly greater killing compared to a non-supplemented 0.01% CPC solution. Collectively, this work demonstrates that LAHCl moderates multi-species oral biofilm development and community composition and enhances the activity of CPC. The incorporation of LAHCl into oral healthcare products may be useful for enhanced biofilm control.