Flow cytometric testing of green fluorescent protein-tagged Lactobacillus rhamnosus GG for response to defensins

Genetic tools for the electrotroph Sporomusa ovata and autotrophic biosynthesis

Sporomusa ovata is a Gram-negative acetogen of the Sporomusaceae family with a unique physiology. This anerobic bacterium is a core microbial catalyst for advanced CO2-based biotechnologies including gas fermentation, microbial electrosynthesis, and hybrid photosystem. Until now, no genetic tools exist for S. ovata, which is a critical obstacle to its optimization as an autotrophic chassis and the acquisition of knowledge about its metabolic capacities. Here, we developed an electroporation protocol for S. ovata. With this procedure, it became possible to introduce replicative plasmids such as pJIR751 and its derivatives into the acetogen. This system was then employed to demonstrate the feasibility of heterologous expression by introducing a functional β-glucuronidase enzyme under the promoters of different strengths in S. ovata. Next, a recombinant S. ovata strain producing the non-native product acetone both from an organic carbon substrate and from CO2 was constructed. Finally, a replicative plasmid capable of integrating itself on the chromosome of the acetogen was developed as a tool for genome editing, and gene deletion was demonstrated. These results indicate that S. ovata can be engineered and provides a first-generation genetic toolbox for the optimization of this biotechnological workhorse.IMPORTANCES. ovata harbors unique features that make it outperform most microbes for autotrophic biotechnologies such as a capacity to acquire electrons from different solid donors, a low H2 threshold, and efficient energy conservation mechanisms. The development of the first-generation genetic instruments described in this study is a key step toward understanding the molecular mechanisms involved in these outstanding metabolic and physiological characteristics. In addition, these tools enable the construction of recombinant S. ovata strains that can synthesize a wider range of products in an efficient manner.

Expression of heterologous heparan sulphate binding protein of Helicobacter pylori on the surface of Lactobacillus rhamnosus GG

Helicobacter pylori (H. pylori) is one of most commonly found pathogen in the stomach. In spite of emergence of different treatment strategies, H. pylori infection remains difficult to treat. The bioengineered probiotic lactobacilli that could displace H. pylori and simultaneously present immunogenic peptides such as heparan sulphate binding protein (Hsbp) to elicit immune response could emerge as a potential therapeutic agent. The aim of this study was to discover the anti-H. pylori activities and faster exclusion of H. pylori from host cells by the recombinant strain of Lactobacillus expressing the immunogenic Hsbp protein. The results were promising and showed a 65% reduction in H. pylori adhesion after two hours of pre-incubation with recombinant-LGG and HeLa S3 cells, followed by the adhesion of H. pylori pathogen (P < 0.002). Additionally, 36% and 39% reduction were examined in co-incubation and post-incubation with recombinant-LGG, respectively. When challenged with H. pylori, the proinflammatory cytokine expression was also down regulated in recombinant-LGG treated HeLa S3 cells. This promising result provides a new insight of bioengineered probiotic lactobacilli which could displace H. pylori and simultaneously has immunogenic properties thereby may be useful to prevent H. pylori infection.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03428-4.

Lactobacilli as a Vector for Delivery of Nanobodies against Norovirus Infection

Passive administration of neutralizing antibodies (Abs) is an attractive strategy for the control of gastrointestinal infections. However, an unanswered practical concern is the need to assure the stability of sufficient amounts of orally administered neutralizing Abs against intestinal pathogens (e.g., norovirus) in the harsh environment of the gastrointestinal tract. To this end, we expressed a single-domain Ab (VHH, nanobody) against norovirus on the cell surface of Lactobacillus, a natural and beneficial commensal component of the gut microbiome. First, we used intestinal epithelial cells generated from human induced pluripotent stem cells to confirm that VHH 1E4 showed neutralizing activity against GII.17 norovirus. We then expressed VHH 1E4 as a cell-wall-anchored form in Lactobacillus paracasei BL23. Flow cytometry confirmed the expression of VHH 1E4 on the surface of lactobacilli, and L. paracasei that expressed VHH 1E4 inhibited the replication of GII.17 norovirus in vitro. We then orally administered VHH 1E4-expressing L. paracasei BL23 to germ-free BALB/c mice and confirmed the presence of lactobacilli with neutralizing activity in the intestine for at least 10 days after administration. Thus, cell-wall-anchored VHH-displaying lactobacilli are attractive oral nanobody deliver vectors for passive immunization against norovirus infection.

Engineered lactobacilli display anti-biofilm and growth suppressing activities against Pseudomonas aeruginosa

Biofilms are an emerging target for new therapeutics in the effort to address the continued increase in resistance and tolerance to traditional antimicrobials. In particular, the distinct nature of the biofilm growth state often means that traditional antimcirobials, developed to combat planktonic cells, are ineffective. Biofilm treatments are designed to both reduce pathogen load at an infection site and decrease the development of resistance by rendering the embedded organisms more susceptible to treatment at lower antimicrobial concentrations. In this work, we developed a new antimicrobial treatment modality using engineered lactic acid bacteria (LAB). We first characterized the natural capacity of two lactobacilli, L. plantarum and L. rhamnosus, to inhibit P. aeruginosa growth, biofilm formation, and biofilm viability, which we found to be dependent upon the low pH generated during culture of the LAB. We further engineered these LAB to secrete enzymes known to degrade P. aeruginosa biofilms and show that our best performing engineered LAB, secreting a pathogen-derived enzyme (PelAh), degrades up to 85% of P. aeruginosa biofilm.

Lipoteichoic Acid Is Involved in the Ability of the Immunobiotic Strain Lactobacillus plantarum CRL1506 to Modulate the Intestinal Antiviral Innate Immunity Triggered by TLR3 Activation

Studies have demonstrated that lipoteichoic acid (LTA) is involved in the immunomodulatory properties of some immunobiotic lactobacilli. The aim of this work was to evaluate whether LTA contributes to the capacity of Lactobacillus plantarum CRL1506 in modulating the intestinal innate antiviral immune response. A D-alanyl-lipoteichoic acid biosynthesis protein (dltD) knockout CRL1506 strain (L. plantarumΔdltD) was obtained, and its ability to modulate Toll-like receptor (TLR)-3-mediated immune response was evaluated in vitro in porcine intestinal epithelial (PIE) cells and in vivo in Balb/c mice. Wild-type (WT) CRL1506 (L. plantarum WT) was used as positive control. The challenge of PIE cells with the TLR3 agonist poly(I:C) significantly increased interferon (IFN)-β, interleukin (IL)-6, and monocyte chemoattractant protein (MCP)-1 expressions. PIE cells pretreated with L. plantarumΔdltD or L. plantarum WT showed higher levels of IFN-β while only L. plantarum WT significantly reduced the expression of IL-6 and MCP-1 when compared with poly(I:C)-treated control cells. The oral administration of L. plantarum WT to mice prior the intraperitoneal injection of poly(I:C) significantly increased IFN-β and IL-10 and reduced intraepithelial lymphocytes (CD3⁺NK1.1⁺CD8αα⁺) and pro-inflammatory mediators (TNF-α, IL-6, and IL-15) in the intestinal mucosa. Similar to the WT strain, L. plantarumΔdltD-treated mice showed enhanced levels of IFN-β after poly(I:C) challenge. However, treatment of mice with L. plantarumΔdltD was not able to increase IL-10 or reduce CD3⁺NK1.1⁺CD8αα⁺ cells, TNF-α, IL-6, or IL-15 in the intestine. These results indicate that LTA would be a key molecule in the anti-inflammatory effect induced by the CRL1506 strain in the context of TLR3-mediated inflammation.

Study of the cwaRS-ldcA Operon Coding a Two-Component System and a Putative L,D-Carboxypeptidase in Lactobacillus paracasei

The cell surface is the primary recognition site between the bacterium and the host. An operon of three genes, LSEI_0219 (cwaR), LSEI_0220 (cwaS), and LSEI_0221 (ldcA), has been previously identified as required for the establishment of Lactobacillus paracasei in the gut. The genes cwaR and cwaS encode a predicted two-component system (TCS) and ldcA a predicted D-alanyl-D-alanine carboxypeptidase which is a peptidoglycan (PG) biosynthesis enzyme. We explored the functionality and the physiological role of these three genes, particularly their impact on the bacterial cell wall architecture and on the bacterial adaptation to environmental perturbations in the gut. The functionality of CwaS/R proteins as a TCS has been demonstrated by biochemical analysis. It is involved in the transcriptional regulation of several genes of the PG biosynthesis. Analysis of the muropeptides of PG in mutants allowed us to re-annotate LSEI_0221 as a putative L,D-carboxypeptidase (LdcA). The absence of this protein coincided with a decrease of two surface antigens: LSEI_0020, corresponding to p40 or msp2 whose implication in the host epithelial homeostasis has been recently studied, and LSEI_2029 which has never been functionally characterized. The inactivation of each of these three genes induces susceptibility to antimicrobial peptides (hBD1, hBD2, and CCL20), which could be the main cause of the gut establishment deficiency. Thus, this operon is necessary for the presence of two surface antigens and for a suitable cell wall architecture.

The Effect of Donor Human Milk Fortification on The Adhesion of Probiotics In Vitro

Preterm delivery complications are the primary cause of death among children under the age of five. Preventive strategies include the use of pasteurized donor human milk (DHM), its fortification with human milk fortifiers (protein supplements), and supplementation with probiotics. Our aim was to examine the impact of DHM and fortified DHM (FDHM) on the mucus adhesion properties of two widely used probiotics. The study covered two forms of human milk fortifier, liquid and powdered, with or without probiotics and storage at 4 °C for 24 h. To test the adhesion properties of the probiotic strains, DHM+probiotics and FDHM+probiotics were prepared and added to immobilized mucus isolated from the stool of healthy Finnish infants. The probiotic adhesion was then measured by liquid scintillation. Our results suggest that addition of liquid or powdered human milk fortifier in donor human milk had no impact on probiotic adhesion. In addition, given the increased adhesion of probiotics suspended in buffer, other matrices should be further studied. These factors need to be considered when designing future intervention strategies using probiotics in preterm infants.

Role of Sortase A in Lactobacillus gasseri Kx110A1 Adhesion to Gastric Epithelial Cells and Competitive Exclusion of Helicobacter pylori

We have previously shown that Lactobacillus gasseri Kx110A1, a human stomach isolate, can colonize mouse stomach and reduce the initial colonization of Helicobacter pylori. Here, we investigated the role of sortase-dependent proteins (SDPs) involved in these functions by the construction of a mutant for srtA, the gene encoding the housekeeping sortase that covalently anchors SDPs to the cell surface. The srtA mutant showed a decrease in hydrophobicity and autoaggregation under acidic conditions, indicating the effect of SDPs on cell surface properties. Correspondingly, the srtA mutant lost the capacity to adhere to gastric epithelial cells, thus resulting in an inability to provide a physical barrier to prevent H. pylori adherence. These results indicate that sortase A is a key determinant of the cell surface properties of L. gasseri Kx110A1 and contributes to Lactobacillus-mediated exclusion of H. pylori. Understanding the molecular mechanisms by which lactobacilli antagonize H. pylori might contribute to the development of novel therapeutic strategies that take advantage of health-promoting bacteria and reduce the burden of antibiotic resistance.

Putative Adhesion Factors in Vaginal Lactobacillus gasseri DSM 14869: Functional Characterization

Lactobacilli play an important role in the maintenance of a healthy vaginal microbiota, and some select species are widely used as probiotics. Vaginal isolates of Lactobacillus gasseri DSM 14869 and Lactobacillus rhamnosus DSM 14870 were previously selected to develop the probiotic EcoVag capsules and showed therapeutic effects in women with bacterial vaginosis (BV). However, the molecular mechanisms involved in their probiotic activity are largely unknown. In this study, we identified three cell surface molecules in L. gasseri DSM 14869 that promote adhesion to vaginal epithelial cells (VEC) by constructing dedicated knockout mutants, including exopolysaccharides (EPSs), a protein containing MucBP-like domains (N506_1778), and a putative novel adhesin (N506_1709) with rib/alpha-like domain repeats. EPS knockout mutants revealed 20-fold and 14-fold increases in adhesion to Caco-2 and HeLa cells, respectively, compared with wild type, while the adhesion to VEC was reduced 30% by the mutation, suggesting that EPSs might mediate tissue tropism for vaginal cells. A significant decrease in adhesion to Caco-2 cells, HeLa cells, and VEC was observed in the N506_1778 knockout mutant. The N506_1709 mutant showed no significant difference for the adhesion to Caco-2 and HeLa cells compared with wild type (WT); in contrast, the adhesion to VEC revealed a significant decrease (42%), suggesting that N506_1709 might mediate specific binding to stratified squamous epithelial cells, and this putative novel adhesin was annotated as Lactobacillus vaginal epithelium adhesin (LVEA). Thus, we have discovered an important role for EPSs and a novel adhesin, LVEA, in the adhesive capacity of a vaginal probiotic Lactobacillus strain.IMPORTANCE Lactobacilli are known to contribute to the maintenance of a healthy vaginal microbiota and some are selected as probiotics for the prevention or treatment of urogenital diseases, such as bacterial vaginosis. However, the molecular mechanisms for these health-promoting effects are not fully understood. Here, we functionally identified three cell surface factors of a Lactobacillus gasseri strain potentially involved in its adhesion to vaginal epithelial cells, including exopolysaccharides (EPSs) and two sortase-dependent proteins (N506_1778 and N506_1709). We could demonstrate the tissue-specific adhesion of EPSs to vaginal cells and that N506_1709 might be a novel adhesin specifically mediating bacterial binding to stratified squamous epithelial cells. The results provide important new information on the molecular mechanisms of vaginal Lactobacillus spp. adhesion.

Inducible Plasmid Self-Destruction (IPSD) Assisted Genome Engineering in Lactobacilli and Bifidobacteria

Genome engineering is essential for application of synthetic biology in probiotics including lactobacilli and bifidobacteria. Several homologous recombination system-based mutagenesis tools have been developed for these bacteria, but still have many limitations in different species or strains. Here we developed a genome engineering method based on an inducible self-destruction plasmid delivering homologous DNA into bacteria. Excision of the replicon by induced recombinase facilitates selection of homologous recombination events. This new genome editing tool called inducible plasmid self-destruction (IPSD) was successfully used to perform gene knockout and knock-in in lactobacilli and bifidobacteria. Due to its simplicity and universality, the IPSD strategy may provide a general approach for genetic engineering of various bacterial species.

Adhesion and Colonization of the Probiotic Lactobacillus rhamnosus Labeled by Dsred2 in Mouse Gut

The health-promoting effects of the probiotic strain Lactobacillus rhamnosus are based on its adherence and colonization ability. However, little is known about its adhesion and colonization rates. Lactobacillus rhamnosus in mouse intestinal mucosa a mutant of the red fluorescence protein (RFP) DSred2 was used to tag L. rhamnosus to observe the adhesion and distribution of L. rhamnosus in mouse intestinal mucosa. A mutant of the red fluorescence protein (RFP) Dsred2 was used to tag L. rhamnosus to allow us to observe and distinguish it in the mouse intestine. Seven-week-old female BALB/c mice were fed once (at day 0) with an oral administration of the labeled L. rhamnosus, and the number of labeled bacteria was detected in different regions of the intestinal tract at 3 h and at day 1, 2, 3, 4, 5, 6, 7, and 15 after administration. The labeling process changed the morphology of L. rhamnosus, as it appeared after observation under the microscope, but did not change its basic probiotic properties in vitro. In vivo, labeled L. rhamnosus reached the colonization peak at the fourth day after gavage. From the distribution point of view, the number of colonization strains increased from the proximal to the distal small intestine (duodenum < jejunum < ileum) and the number of strains in the colon was less than the distal small intestine (ileum). The labeling protocol actually allowed the detection of the distribution and adhesion of this bacterium to the intestine, thus demonstrating that the health-promoting effects of this probiotic are satisfied. This study provides a scientific basis in the use of probiotics such as L. rhamnosus in functional foods.

Thirty Years of Lactobacillus rhamnosus GG: A Review

Lactobacillus rhamnosus GG (LGG) was the first strain belonging to the genus Lactobacillus to be patented in 1989 thanks to its ability to survive and to proliferate at gastric acid pH and in medium containing bile, and to adhere to enterocytes. Furthermore LGG is able to produces both a biofilm that can mechanically protect the mucosa, and different soluble factors beneficial to the gut by enhancing intestinal crypt survival, diminishing apoptosis of the intestinal epithelium, and preserving cytoskeletal integrity. Moreover LGG thanks to its lectin-like protein 1 and 2 inhibits some pathogens such as Salmonella species. Finally LGG is able to promote type 1 immune-responsiveness by reducing the expression of several activation and inflammation markers on monocytes and by increasing the production of interleukin-10, interleukin-12 and tumor necrosis factor-α in macrophages. A large number of research data on Lactobacillus GG is the basis for the use of this probiotic for human health. In this review we have considered predominantly randomized controlled trials, meta-analysis, Cochrane Review, guide lines of Scientific Societies and anyway studies whose results were evaluated by means of relative risk, odds ratio, weighted mean difference 95% confidence interval. The effectiveness of LGG in gastrointestinal infections and diarrhea, antibiotic and Clostridium difficile associated diarrhea, irritable bowel syndrome, inflammatory bowel disease, respiratory tract infections, allergy, cardiovascular diseases, nonalcoholic fatty liver disease, nonalcoholic steatohepatitis, cystic fibrosis, cancer, elderly end sport were analyzed.

Expression of fluorescent proteins in Lactobacillus rhamnosus to study host-microbe and microbe-microbe interactions

Probiotic Lactobacillus strains are widely used to benefit human and animal health, although the exact mechanisms behind their interactions with the host and the microbiota are largely unknown. Fluorescent tagging of live probiotic cells is an important tool to unravel their modes of action. In this study, the implementation of different heterologously expressed fluorescent proteins for the labelling of the model probiotic strains Lactobacillus rhamnosusGG (gastrointestinal) and Lactobacillus rhamnosusGR-1 (vaginal) was explored. Heterologous expression of mTagBFP2 and mCherry resulted in long-lasting fluorescence of L. rhamnosusGG and GR-1 cells, using the nisin-controlled expression (NICE) system. These novel fluorescent strains were then used to study in vitro aspects of their microbe-microbe and microbe-host interactions. Lactobacillus rhamnosusGG and L. rhamnosusGR-1 expressing mTagBFP2 and mCherry could be visualized in mixed-species biofilms, where they inhibited biofilm formation by Salmonella Typhimurium-gfpmut3 expressing the green fluorescent protein. Likewise, fluorescent L. rhamnosusGG and L. rhamnosusGR-1 were implemented for the visualization of their adhesion patterns to intestinal epithelial cell cultures. The fluorescent L. rhamnosus strains developed in this study can therefore serve as novel tools for the study of probiotic interactions with their environment.

The lectin-like protein 1 in Lactobacillus rhamnosus GR-1 mediates tissue-specific adherence to vaginal epithelium and inhibits urogenital pathogens

The probiotic Lactobacillus rhamnosus GR-1 has been documented to survive implantation onto the vaginal epithelium and interfere with urogenital pathogens. However, the molecular mechanisms involved are largely unknown. Here, we report for the first time the construction of dedicated knock-out mutants in L. rhamnosus GR-1 to enable the study of gene functions. In a search for genes responsible for the adherence capacity of L. rhamnosus GR-1, a genomic region encoding a protein with homology to lectin-like proteins was identified. Phenotypic analyses of the knock-out mutant of L. rhamnosus GR-1 revealed a two-fold decreased adhesion to the vaginal and ectocervical epithelial cell lines compared to wild-type. In contrast, the adhesion to gastro-intestinal epithelial (Caco2) and endocervical cell lines (Hela and End1/E6E7) was not drastically affected by the mutation, suggesting that the LGR-1_Llp1 lectins mediates tissue tropism. The purified LGR-1_Llp1 protein also inhibited biofilm formation and adhesion of uropathogenic Escherichia coli. For the first time, an important role for a novel lectin-like protein in the adhesion capacity and host cell-specific interaction of a vaginal probiotic Lactobacillus strain has been discovered, with an additional role in pathogen inhibition.

Polymorphisms, Chromosomal Rearrangements, and Mutator Phenotype Development during Experimental Evolution of Lactobacillus rhamnosus GG

Lactobacillus rhamnosus GG is a lactic acid bacterium widely marketed by the food industry. Its genomic analysis led to the identification of a gene cluster encoding mucus-binding SpaCBA pili, which is located in a genomic island enriched in insertion sequence (IS) elements. In the present study, we analyzed by genome-wide resequencing the genomic integrity of L. rhamnosus GG in four distinct evolutionary experiments conducted for approximately 1,000 generations under conditions of no stress or salt, bile, and repetitive-shearing stress. Under both stress-free and salt-induced stress conditions, the GG population (excluding the mutator lineage in the stress-free series [see below]) accumulated only a few single nucleotide polymorphisms (SNPs) and no frequent chromosomal rearrangements. In contrast, in the presence of bile salts or repetitive shearing stress, some IS elements were found to be activated, resulting in the deletion of large chromosomal segments that include the spaCBA-srtC1 pilus gene cluster. Remarkably, a high number of SNPs were found in three strains obtained after 900 generations of stress-free growth. Detailed analysis showed that these three strains derived from a founder mutant with an altered DNA polymerase subunit that resulted in a mutator phenotype. The present work confirms the stability of the pilus production phenotype in L. rhamnosus GG under stress-free conditions, highlights the possible evolutionary scenarios that may occur when this probiotic strain is extensively cultured, and identifies external factors that affect the chromosomal integrity of GG. The results provide mechanistic insights into the stability of GG in regard to its extensive use in probiotic and other functional food products.

IMPORTANCE

Lactobacillus rhamnosus GG is a widely marketed probiotic strain that has been used in numerous clinical studies to assess its health-promoting properties. Hence, the stability of the probiotic functions of L. rhamnosus GG is of importance, and here we studied the impact of external stresses on the genomic integrity of L. rhamnosus GG. We studied three different stresses that are relevant for understanding its robustness and integrity under both ex vivo conditions, i.e., industrial manufacturing conditions, and in vivo conditions, i.e., intestinal tract-associated stress. Overall, our findings contribute to predicting the genomic stability of L. rhamnosus GG and its ecological performance.

The N-Terminal GYPSY Motif Is Required for Pilin-Specific Sortase SrtC1 Functionality in Lactobacillus rhamnosus Strain GG

Predominantly identified in pathogenic Gram-positive bacteria, sortase-dependent pili are also found in commensal species, such as the probiotic-marketed strain Lactobacillus rhamnosus strain GG. Pili are typically associated with host colonization, immune signalling and biofilm formation. Comparative analysis of the N-terminal domains of pilin-specific sortases from various piliated Gram-positive bacteria identified a conserved motif, called GYPSY, within the signal sequence. We investigated the function and role of the GYPSY residues by directed mutagenesis in homologous (rod-shaped) and heterologous (coccoid-shaped) expression systems for pilus formation. Substitutions of some of the GYPSY residues, and more specifically the proline residue, were found to have a direct impact on the degree of piliation of Lb. rhamnosus GG. The present findings uncover a new signalling element involved in the functionality of pilin-specific sortases controlling the pilus biogenesis of Lb. rhamnosus GG and related piliated Gram-positive species.

An Exopolysaccharide-Deficient Mutant of Lactobacillus rhamnosus GG Efficiently Displays a Protective Llama Antibody Fragment against Rotavirus on Its Surface

Rotavirus is the leading cause of infantile diarrhea in developing countries, where it causes a high number of deaths among infants. Two vaccines are available, being highly effective in developed countries although markedly less efficient in developing countries. As a complementary treatment to the vaccines, a Lactobacillus strain producing an anti-rotavirus antibody fragment in the gastrointestinal tract could potentially be used. In order to develop such an alternative therapy, the effectiveness of Lactobacillus rhamnosus GG to produce and display a VHH antibody fragment (referred to as anti-rotavirus protein 1 [ARP1]) on the surface was investigated. L. rhamnosus GG is one of the best-characterized probiotic bacteria and has intrinsic antirotavirus activity. Among four L. rhamnosus GG strains [GG (CMC), GG (ATCC 53103), GG (NCC 3003), and GG (UT)] originating from different sources, only GG (UT) was able to display ARP1 on the bacterial surface. The genomic analysis of strain GG (UT) showed that the genes welE and welF of the EPS cluster are inactivated, which causes a defect in exopolysaccharide (EPS) production, allowing efficient display of ARP1 on its surface. Finally, GG (UT) seemed to confer a level of protection against rotavirus-induced diarrhea similar to that of wild-type GG (NCC 3003) in a mouse pup model, indicating that the EPS may not be involved in the intrinsic antirotavirus activity. Most important, GG (EM233), a derivative of GG (UT) producing ARP1, was significantly more protective than the control strain L. casei BL23.

An L-Fucose Operon in the Probiotic Lactobacillus rhamnosus GG Is Involved in Adaptation to Gastrointestinal Conditions

L-Fucose is a sugar present in human secretions as part of human milk oligosaccharides, mucins, and other glycoconjugates in the intestinal epithelium. The genome of the probiotic Lactobacillus rhamnosus GG (LGG) carries a gene cluster encoding a putative L-fucose permease (fucP), L-fucose catabolic pathway (fucI, fucK, fucU, and fucA), and a transcriptional regulator (fucR). The metabolism of L-fucose in LGG results in 1,2-propanediol production, and their fucI and fucP mutants displayed a severe and mild growth defect on L-fucose, respectively. Transcriptional analysis revealed that the fuc genes are induced by L-fucose and subject to a strong carbon catabolite repression effect. This induction was triggered by FucR, which acted as a transcriptional activator necessary for growth on L-fucose. LGG utilized fucosyl-α1,3-N-acetylglucosamine and contrarily to other lactobacilli, the presence of fuc genes allowed this strain to use the L-fucose moiety. In fucI and fucR mutants, but not in fucP mutant, L-fucose was not metabolized and it was excreted to the medium during growth on fucosyl-α1,3-N-acetylglucosamine. The fuc genes were induced by this fucosyl-disaccharide in the wild type and the fucP mutant but not in a fucI mutant, showing that FucP does not participate in the regulation of fuc genes and that L-fucose metabolism is needed for FucR activation. The l-fucose operon characterized here constitutes a new example of the many factors found in LGG that allow this strain to adapt to the gastrointestinal conditions.

Piliation of Lactobacillus rhamnosus GG promotes adhesion, phagocytosis, and cytokine modulation in macrophages

Recently, spaCBA-encoded pili on the cell surface of Lactobacillus rhamnosus GG were identified to be key molecules for binding to human intestinal mucus and Caco-2 intestinal epithelial cells. Here, we investigated the role of the SpaCBA pilus of L. rhamnosus GG in the interaction with macrophages in vitro by comparing the wild type with surface mutants. Our results show that SpaCBA pili play a significant role in the capacity for adhesion to macrophages and also promote bacterial uptake by these phagocytic cells. Interestingly, our data suggest that SpaCBA pili also mediate anti-inflammatory effects by induction of interleukin-10 (IL-10) mRNA and reduction of interleukin-6 (IL-6) mRNA in a murine RAW 264.7 macrophage cell line. These pili appear to mediate these effects indirectly by promoting close contact with the macrophages, facilitating the exertion of anti-inflammatory effects by other surface molecules via yet unknown mechanisms. Blockage of complement receptor 3 (CR3), previously identified to be a receptor for streptococcal pili, significantly decreased the uptake of pilus-expressing strains in RAW 264.7 cells, while the expression of IL-10 and IL-6 mRNA by these macrophages was not affected by this blocking. On the other hand, blockage of Toll-like receptor 2 (TLR2) significantly reduced the expression of IL-6 mRNA irrespective of the presence of pili.

Adhesion properties of Lactobacillus rhamnosus mucus-binding factor to mucin and extracellular matrix proteins

We previously described potential probiotic Lactobacillus rhamnosus strains, isolated from fermented mare milk produced in Sumbawa Island, Indonesia, which showed high adhesion to porcine colonic mucin (PCM) and extracellular matrix (ECM) proteins. Recently, mucus-binding factor (MBF) was found in the GG strain of L. rhamnosus as a mucin-binding protein. In this study, we assessed the ability of recombinant MBF protein from the FSMM22 strain, one of the isolates of L. rhamnosus from fermented Sumbawa mare milk, to adhere to PCM and ECM proteins by overlay dot blot and Biacore assays. MBF bound to PCM, laminin, collagen IV, and fibronectin with submicromolar dissociation constants. Adhesion of the FSMM22 mbf mutant strain to PCM and ECM proteins was significantly less than that of the wild-type strain. Collectively, these results suggested that MBF contribute to L. rhamnosus host colonization via mucin and ECM protein binding.

The highly autoaggregative and adhesive phenotype of the vaginal Lactobacillus plantarum strain CMPG5300 is sortase dependent

Lactobacilli are important for the maintenance of a healthy ecosystem in the human vagina. Various mechanisms are postulated but so far are poorly substantiated by molecular studies, such as mutant analysis. Bacterial autoaggregation is an interesting phenomenon that can promote adhesion to host cells and displacement of pathogens. In this study, we report on the identification of a human vaginal isolate, Lactobacillus plantarum strain CMPG5300, which shows high autoaggregative and adhesive capacity. To investigate the importance of sortase-dependent proteins (SDPs) in these phenotypes, a gene deletion mutant was constructed for srtA, the gene encoding the housekeeping sortase that covalently anchors these SDPs to the cell surface. This mutant lost the capacity to autoaggregate, showed a decrease in adhesion to vaginal epithelial cells, and lost biofilm-forming capacity under the conditions tested. These results indicate that the housekeeping sortase SrtA of CMPG5300 is a key determinant of the peculiar surface properties of this vaginal Lactobacillus strain.

Optimization and shelf life of a low-lactose yogurt with Lactobacillus rhamnosus HN001

Lactose intolerance results in gastrointestinal discomfort and the malabsorption of certain nutrients, such as calcium. The replacement of milk with low-lactose and probiotic-enriched dairy products is an effective strategy of mitigating the symptoms of lactose intolerance. Lactobacillus rhamnosus HN001 (HN001) is a safe, immunity-stimulating probiotic. We have developed a process to increase the hydrolysis of lactose and HN001 growth in yogurt versus β-galactosidase (βG) concentration and enzymatic hydrolysis time (EHT) before bacterial fermentation. The objective of this study was to optimize the conditions by which yogurt is processed as a function of βG and EHT using a multifactorial design, with lactose content, HN001 growth, process time, and sensory quality as dependent variables. Further, the shelf life of the optimized yogurt was evaluated. In the optimization study, polynomials explained the dependent variables. Based on Pearson correlation coefficients, HN001 growth correlated positively with the hydrolysis of lactose. However, low lactose content and high HN001 count increased the fermentation time and lowered the sensory quality. The optimized conditions-using polynomials to obtain yogurt with >1 × 10(7) cfu of HN001/mL, <10 g of lactose/L, and a minimum overall sensory quality of 7 on the Karlsruhe scale-yielded a theoretical value of 910 neutral lactose units/kg for βG and 2.3h for EHT, which were validated in an industrial-scale assay. Based on a shelf-life study at 3 temperatures, the hydrolysis of lactose and the growth of HN001 continue during storage. Arrhenius equations were developed for the variables in the shelf-life study. Our results demonstrate that it is feasible to develop a low-lactose yogurt to which HN001 has been added for lactose-intolerant persons who wish to strengthen their immune system.

Genetic and biochemical characterization of the cell wall hydrolase activity of the major secreted protein of Lactobacillus rhamnosus GG

Lactobacillus rhamnosus GG (LGG) produces two major secreted proteins, designated here Msp1 (LGG_00324 or p75) and Msp2 (LGG_00031 or p40), which have been reported to promote the survival and growth of intestinal epithelial cells. Intriguingly, although each of these proteins shares homology with cell wall hydrolases, a physiological function that correlates with such an enzymatic activity remained to be substantiated in LGG. To investigate the bacterial function, we constructed knock-out mutants in the corresponding genes aiming to establish a genotype to phenotype relation. Microscopic examination of the msp1 mutant showed the presence of rather long and overly extended cell chains, which suggests that normal daughter cell separation is hampered. Subsequent observation of the LGG wild-type cells by immunofluorescence microscopy revealed that the Msp1 protein accumulates at the septum of exponential-phase cells. The cell wall hydrolyzing activity of the Msp1 protein was confirmed by zymogram analysis. Subsequent analysis by RP-HPLC and mass spectrometry of the digestion products of LGG peptidoglycan (PG) by Msp1 indicated that the Msp1 protein has D-glutamyl-L-lysyl endopeptidase activity. Immunofluorescence microscopy and the failure to construct a knock-out mutant suggest an indispensable role for Msp2 in priming septum formation in LGG.

Lactobacillus rhamnosus GG secreting an antigen and Interleukin-2 translocates across the gastrointestinal tract and induces an antigen specific immune response

Lactobacillus rhamnosus strain GG (LGG) is a probiotic organism. In this present study, LGG that express the green fluorescence protein (LGG-GFP) and IL-2 and GFP as a fusion protein (LGG-IL-2-GFP) were used to examine bacterial uptake and the immune response induced by oral immunization. Using TEM to examine the intestinal tissue, the Lactobacilli were localized in M cells and in venules. After oral immunization, most of the bacteria were excreted in feces only a small fraction (0.15%) was retained in the intestine at 48 hr. However, more LGG-IL-2-GFP was found in the MLN and spleen than LGG-GFP. The loop ligation method was used to evaluate LGG uptake and both LGG-GFP and LGG-IL-2-GFP were found to translocate at the same rate. Analysis of LGG internalization in J774 macrophage cells indicated that IL-2 increased survival of LGG and this may explain the increased presence of these bacteria in the MLN for a longer period. After oral immunization, specific mucosal antibody production as well as GFP specific CTL activity was demonstrated. IL-2 co-expression with GFP further enhanced antibody production and CTL activity. In conclusion, Lactobacillus rhamnosus GG expressing an antigen could generate an effective immune response to the antigen and IL-2 improved the response generated probably by increasing LGG expressing antigen survival in immune cells.

Functional analysis of Lactobacillus rhamnosus GG pili in relation to adhesion and immunomodulatory interactions with intestinal epithelial cells

Lactobacillus rhamnosus GG, a probiotic with good survival capacity in the human gut, has well-documented adhesion properties and health effects. Recently, spaCBA-encoded pili that bind to human intestinal mucus were identified on its cell surface. Here, we report on the phenotypic analysis of a spaCBA pilus knockout mutant in comparison with the wild type and other adhesin mutants. The SpaCBA pilus of L. rhamnosus GG showed to be key for efficient adherence to the Caco-2 intestinal epithelial cell (IEC) line and biofilm formation. Moreover, the spaCBA mutant induces an elevated level of interleukin-8 (IL-8) mRNA in Caco-2 cells compared to the wild type, possibly involving an interaction of lipoteichoic acid with Toll-like receptor 2. In contrast, an L. rhamnosus GG mutant without exopolysaccharides but with an increased exposure of pili leads to the reduced expression of IL-8. Using Transwells to partition bacteria from Caco-2 cells, IL-8 induction is blocked completely regardless of whether wild-type or mutant L. rhamnosus GG cells are used. Taken together, our data suggest that L. rhamnosus GG SpaCBA pili, while promoting strong adhesive interactions with IECs, have a functional role in balancing IL-8 mRNA expression induced by surface molecules such as lipoteichoic acid.

Lactobacilli secreting a tumor antigen and IL15 activates neutrophils and dendritic cells and generates cytotoxic T lymphocytes against cancer cells

Lactobacillus rhamnosus GG (LGG) has been used to successfully induce tumor regression in an orthotopic model of bladder cancer. Increased infiltration of neutrophils and macrophages into the tumor mass was observed after therapy. This study evaluates the potential of LGG to induce a directed anti-tumor response. Lactobacilli were modified to secrete the prostate specific antigen (PSA) or IL15 and PSA (IL-15-PSA). Neutrophils and DC were exposed to LGG for 2 h as in clinical therapy for bladder cancer. Recombinant LGG activated neutrophils (elevated MHC class I expression) induced DC maturation (increased expression of CD86, CD80, CD40, MHC II and CD83), T cell proliferation and PSA specific cytotoxic T lymphocytes (CTL) activity. IL15 enhanced direct DC activation of CTL. Thus LGG secreting tumor antigens may activate antigen specific immune responses when instilled intravesically and IL15 could enhance this response.

Transformation of, and heterologous protein expression in, Lactobacillus agilis and Lactobacillus vaginalis isolates from the chicken gastrointestinal tract

Lactobacilli are naturally found in the gastrointestinal tract of chickens, and there is interest in utilizing autochthonous strains for the delivery of therapeutic proteins. Previously we identified three chicken-derived Lactobacillus strains, Lactobacillus agilis La3, Lactobacillus vaginalis Lv5, and Lactobacillus crispatus Lc9, which persist in the gastrointestinal tract of chickens fed either a commercial or high-protein diet. In the current study, we investigated the ability to electrotransform these strains, determined plasmid vector stability, and compared reporter gene expression directed by several different promoters. The La3 and Lv5 strains were reproducibly transformed with efficiencies of 10(8) and 10(6) transformants per microgram of plasmid DNA, respectively. The third strain tested, L. crispatus Lc9, was recalcitrant to all transformation protocols examined. The plasmid vectors pTRK563 and pTRKH2 were maintained over 100 generations in La3 and Lv5, respectively. The ability of La3 and Lv5 to express the heterologous reporter gene gfp was analyzed using heterologous and homologous promoters. Transformants of both La3 and Lv5 containing the La3 ldhL promoter were the most fluorescent. To our knowledge, this is the first report of successful transformation and heterologous protein expression in L. agilis and L. vaginalis. The ability of these strains to express heterologous proteins in vitro indicates their potential utility as in vivo delivery vectors for therapeutic peptides to the chicken gastrointestinal tract.

Exopolysaccharides of Lactobacillus rhamnosus GG form a protective shield against innate immune factors in the intestine

Probiotic bacteria are administered as live microorganisms to provide a health benefit to the host. Insight into the adaptation factors that promote the survival and persistence of probiotics in the gastrointestinal tract (GIT) is important to understand their performance. In this study, the role of the long galactose‐rich exopolysaccharides (EPS) of the prototypical probiotic strain Lactobacillus rhamnosus GG (LGG) was investigated. In a competition experiment with wild type, the isogenic EPS mutant CMPG5351 exhibited a reduced persistence in the murine GIT, especially in the lower parts of the intestine. This was surprising as our previous in vitro studies had shown an increased adhesion capacity for this EPS mutant. Follow‐up assays indicated that this mutant is more sensitive towards host innate defence molecules, such as the LL‐37 antimicrobial peptide and complement factors. This suggests that EPS forms a protective shield for LGG against these molecules in the GIT. Moreover, culturing LGG wild‐type in subinhibitory concentrations of host defence factors such as LL‐37 resulted in increased production of EPS, indicating that bacterial EPS production is modulated in the host to fine‐tune the balance between adhesion and immune evasion. These observations are of interest in understanding the dynamics of adaptation of probiotics to the host environments.

Characterization of MabA, a modulator of Lactobacillus rhamnosus GG adhesion and biofilm formation

Abstract The probiotic Lactobacillus rhamnosus GG, first isolated from healthy human gut microbiota, has been reported to adhere very well to components of the intestinal mucosa, thereby enabling transient colonization of the gastrointestinal tract (GIT). In a search for the genes responsible for the good adherence capacity of this strain, a genomic region encoding a protein with homology to putative adhesion proteins (LGG_01865) and its putative regulator (LGG_01866) was identified. The sequence of the L. rhamnosus GG LGG_01865 encodes a polypeptide of 2419 amino acid residues containing 26 repetitive DUF1542 domains and a C-terminal LPxTG cell wall-anchoring motif. Phenotypic analyses of a dedicated LGG_01865 knockout mutant revealed a reduced biofilm formation capacity on abiotic surfaces and decreased adhesion to intestinal epithelial cells and tissues of the murine GIT. This suggests a modulating role for LGG_01865 in L. rhamnosus GG-host interactions. Therefore, we propose a new name for LGG_01865, i.e. MabA, modulator of adhesion and biofilm. Expression analysis indicated that LGG_01866 plays a conditional role in the regulation of LGG_01865 expression, i.e. when cells are grown under conditions of sugar starvation.

Host interactions of probiotic bacterial surface molecules: comparison with commensals and pathogens

How can probiotic bacteria transduce their health benefits to the host? Bacterial cell surface macromolecules are key factors in this beneficial microorganism-host crosstalk, as they can interact with host pattern recognition receptors (PRRs) of the gastrointestinal mucosa. In this Review, we highlight the documented signalling interactions of the surface molecules of probiotic bacteria (such as long surface appendages, polysaccharides and lipoteichoic acids) with PRRs. Research on host-probiotic interactions can benefit from well-documented host-microorganism studies that span the spectrum from pathogenicity to mutualism. Distinctions and parallels are therefore drawn with the interactions of similar molecules that are presented by gastrointestinal commensals and pathogens.

Identification of a Gene Cluster for the Biosynthesis of a Long, Galactose-Rich Exopolysaccharide in Lactobacillus rhamnosus GG and Functional Analysis of the Priming Glycosyltransferase

Cell surface polysaccharides have an established role as virulence factors in human bacterial pathogens. Less documented are the biosynthesis and biological functions of surface polysaccharides in beneficial bacteria. We identified a gene cluster that encodes the enzymes and regulatory and transporter proteins for the different steps in the biosynthesis of extracellular polysaccharides (EPS) of the well-documented probiotic strain Lactobacillus rhamnosus GG. Subsequent mutation of the welE gene, encoding the priming glycosyltransferase within this cluster, and comparative phenotypic analyses of wild-type versus mutant strains confirmed the specific function of this gene cluster in the biosynthesis of high-molecular-weight, galactose-rich heteropolymeric EPS molecules. The phenotypic analyses included monomer composition determination, estimation of the polymer length of the isolated EPS molecules, and single-molecule force spectroscopy of the surface polysaccharides. Further characterization of the welE mutant also showed that deprivation of these long, galactose-rich EPS molecules results in an increased adherence and biofilm formation capacity of L. rhamnosus GG, possibly because of less shielding of adhesins such as fimbria-like structures.

Genes and molecules of lactobacilli supporting probiotic action

Lactobacilli have been crucial for the production of fermented products for centuries. They are also members of the mutualistic microbiota present in the human gastrointestinal and urogenital tract. Recently, increasing attention has been given to their probiotic, health-promoting capacities. Many human intervention studies demonstrating health effects have been published. However, as not all studies resulted in positive outcomes, scientific interest arose regarding the precise mechanisms of action of probiotics. Many reported mechanistic studies have addressed mainly the host responses, with less attention being focused on the specificities of the bacterial partners, notwithstanding the completion of Lactobacillus genome sequencing projects, and increasing possibilities of genomics-based and dedicated mutant analyses. In this emerging and highly interdisciplinary field, microbiologists are facing the challenge of molecular characterization of probiotic traits. This review addresses the advances in the understanding of the probiotic-host interaction with a focus on the molecular microbiology of lactobacilli. Insight into the molecules and genes involved should contribute to a more judicious application of probiotic lactobacilli and to improved screening of novel potential probiotics.

Impact of luxS and suppressor mutations on the gastrointestinal transit of Lactobacillus rhamnosus GG

It is generally believed that probiotic bacteria need to survive gastrointestinal transit to exert a health-promoting effect. In this study, a genuine luxS mutant and a luxS mutant containing unknown suppressor mutations of the probiotic strain Lactobacillus rhamnosus GG were compared to the wild type for survival and persistence in the murine gastrointestinal tract. The LuxS enzyme, catalyzing the production of the autoinducer-2 signaling molecule, also forms an integral part of the activated methyl cycle and the metabolism of methionine and cysteine. The genuine luxS mutant CMPG5412 showed drastically reduced persistence in mice, which was related to less survival in simulated gastric juice, indicating that LuxS metabolism is crucial for the gastric stress resistance of L. rhamnosus GG. The suppressor mutations in the other luxS mutant, CMPG5413, appear to compensate for the metabolic defects of the luxS mutation and to restore the resistance to gastric juice but cause a defect in adherence, biofilm formation, and exopolysaccharide production. The shorter residence time of this suppressor mutant in the murine gastrointestinal tract indicates a role for biofilm formation and exopolysaccharides in the persistence capacity of L. rhamnosus GG.

Impact of environmental and genetic factors on biofilm formation by the probiotic strain Lactobacillus rhamnosus GG

Lactobacillus rhamnosus GG (ATCC 53103) is one of the clinically best-studied probiotic organisms. Moreover, L. rhamnosus GG displays very good in vitro adherence to epithelial cells and mucus. Here, we report that L. rhamnosus GG is able to form biofilms on abiotic surfaces, in contrast to other strains of the Lactobacillus casei group tested under the same conditions. Microtiter plate biofilm assays indicated that in vitro biofilm formation by L. rhamnosus GG is strongly modulated by culture medium factors and conditions related to the gastrointestinal environment, including low pH; high osmolarity; and the presence of bile, mucins, and nondigestible polysaccharides. Additionally, phenotypic analysis of mutants affected in exopolysaccharides (wzb), lipoteichoic acid (dltD), and central metabolism (luxS) showed their relative importance in biofilm formation by L. rhamnosus GG.

Identification and characterization of starter lactic acid bacteria and probiotics from Columbian dairy products

AIMS

Considering the significant rise in the probiotic market in Columbia, and given the lack of reports concerning the microbial population and strain performance in products from different producers, this study aims at determining the number of viable starter bacteria and probiotics in bio-yoghurts available at the Columbian market, identifying the species and analysing the performance of the isolated strains in bile acid resistance, antagonistic activity against pathogens, and adherence capacity to human intestinal epithelial cells.

METHODS AND RESULTS

Seven bio-yoghurts were analysed for the bacterial species present. Species identification was carried out using 16S rRNA gene targeted PCR. The cultured bacteria were tested for bile acid resistance, adherence to a human intestinal epithelial cell line, and antagonism against the pathogen Salmonella enterica serovar Typhimurium. A total of 17 different strains were identified. Based on plate counting, all bio-yoghurts have at least total viable cells of approximately 10(7) CFU ml(-1). Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus were the most frequently isolated bacteria. Viable Bifidobacterium was only recovered from one product. However, after PCR analysis, DNA of this genus was confirmed in five out of seven products. Major differences were found for S. typhimurium antagonism. The adherence capacity to Caco-2 cells was observed in 10 of the isolated strains. In general, low survival to simulated gastric juice was observed.

CONCLUSIONS

Some of the isolated strains have probiotic potential, although not all of them were present in the advised amount to exert beneficial health effects. However, the full correct scientific name of the isolated bacteria and their viable counts were not included on the product label.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report describing the identification and functionality of starter bacteria and probiotics present in dairy products on the Columbian market.

Functional analysis of D-alanylation of lipoteichoic acid in the probiotic strain Lactobacillus rhamnosus GG

Lipoteichoic acid (LTA) is a macroamphiphile molecule which performs several functions in gram-positive bacteria, such as maintenance of cell wall homeostasis. D-alanylation of LTA requires the proteins encoded by the dlt operon, and this process is directly related to the charge properties of this polymer strongly contributing to its function. The insertional inactivation of dltD of the probiotic strain Lactobacillus rhamnosus GG (ATCC 53103) resulted in the complete absence of D-alanyl esters in the LTA as confirmed by nuclear magnetic resonance analysis. This was reflected in modifications of the bacterial cell surface properties. The dltD strain showed 2.4-fold-increased cell length, a low survival capacity in response to gastric juice challenge, an increased sensitivity to human beta-defensin-2, an increased rate of autolysis, an increased capacity to initiate growth in the presence of an anionic detergent, and a decreased capacity to initiate growth in the presence of cationic peptides compared to wild-type results. However, in vitro experiments revealed no major differences for adhesion to human intestinal epithelial cells, biofilm formation, and immunomodulation. These properties are considered to be important for probiotics. The role of the dlt operon in lactobacilli is discussed in view of these results.

Functional analysis of luxS in the probiotic strain Lactobacillus rhamnosus GG reveals a central metabolic role important for growth and biofilm formation

Quorum sensing is involved in the regulation of multicellular behavior through communication via small molecules. Given the high number and diversity of the gastrointestinal microbiota, it is postulated that members of this community communicate to coordinate a variety of adaptive processes. AI-2 is suggested to be a universal bacterial signaling molecule synthesized by the LuxS enzyme, which forms an integral part of the activated methyl cycle. We have previously reported that the well-documented probiotic strain Lactobacillus rhamnosus GG, a human isolate, produces AI-2-like molecules. In this study, we identified the luxS homologue of L. rhamnosus GG. luxS seems to be located in an operon with a yxjH gene encoding a putative cobalamin-independent methionine synthase. In silico analysis revealed a methionine-specific T box in the leader sequence of the putative yxjH-luxS operon. However, transcriptional analysis showed that luxS is expressed mainly as a monocistronic transcript. Construction of a luxS knockout mutant confirmed that the luxS gene is responsible for AI-2 production in L. rhamnosus GG. However, this mutation also resulted in pleiotropic effects on the growth of this fastidious strain. Cysteine, pantothenate, folic acid, and biotin could partially complement growth, suggesting a central metabolic role for luxS in L. rhamnosus GG. Interestingly, the luxS mutant also showed a defect in monospecies biofilm formation. Experiments with chemically synthesized (S)-4,5-dihydroxy-2,3-pentanedione, coculture with the wild type, and nutritional complementation suggested that the main cause of this defect has a metabolic nature. Moreover, our data indicate that suppressor mutations are likely to occur in luxS mutants of L. rhamnosus GG. Therefore, results of luxS-related studies should be carefully interpreted.