The intestinal microbiota are pivotal in determining the developmental, metabolic and immunological status of the mammalian host. However, the intestinal tract may also accommodate pathogenic organisms, including helminth parasites which are highly prevalent in most tropical countries. Both microbes and helminths must evade or manipulate the host immune system to reside in the intestinal environment, yet whether they influence each other’s persistence in the host remains unknown. We now show that abundance of Lactobacillus bacteria correlates positively with infection with the mouse intestinal nematode, Heligmosomoides polygyrus, as well as with heightened regulatory T cell (Treg) and Th17 responses. Moreover, H. polygyrus raises Lactobacillus species abundance in the duodenum of C57BL/6 mice, which are highly susceptible to H. polygyrus infection, but not in BALB/c mice, which are relatively resistant. Sequencing of samples at the bacterial gyrB locus identified the principal Lactobacillus species as L. taiwanensis, a previously characterized rodent commensal. Experimental administration of L. taiwanensis to BALB/c mice elevates regulatory T cell frequencies and results in greater helminth establishment, demonstrating a causal relationship in which commensal bacteria promote infection with an intestinal parasite and implicating a bacterially-induced expansion of Tregs as a mechanism of greater helminth susceptibility. The discovery of this tripartite interaction between host, bacteria and parasite has important implications for both antibiotic and anthelmintic use in endemic human populations.

SummaryThe probiotic Lactobacillus casei catabolizes galacto-N-biose (GNB) and lacto-N-biose (LNB) by using a transport system and metabolic routes different from those of Bifidobacterium. L. casei contains a gene cluster, gnbREFGBCDA, involved in the metabolism of GNB, LNB and also N-acetylgalactosamine. Inactivation of gnbC (EIIC) or ptsI (Enzyme I) of the phosphoenolpyruvate : sugar phosphotransferase system (PTS) prevented the growth on those three carbohydrates, indicating that they are transported and phosphorylated by the same PTS Gnb . Enzyme activities and growth analysis with knockout mutants showed that GnbG (phospho-β-galactosidase) hydrolyses both disaccharides. However, GnbF (N-acetylgalactosamine-6P deacetylase) and GnbE (galactosamine-6P isomerase/deaminase) are involved in GNB but not in LNB fermentation. The utilization of LNB depends on nagA (Nacetylglucosamine-6P deacetylase), showing that the N-acetylhexosamine moieties of GNB and LNB follow different catabolic routes. A lacAB mutant (galactose-6P isomerase) was impaired in GNB and LNB utilization, indicating that their galactose moiety is channelled through the tagatose-6P pathway. Transcriptional analysis showed that the gnb operon is regulated by substrate-specific induction mediated by the transcriptional repressor GnbR, which binds to a 26 bp DNA region containing inverted repeats exhibiting a 2T/2A conserved core. The data represent the first characterization of novel metabolic pathways for human milk oligosaccharides and glycoconjugate structures in Firmicutes.

Three putative ␣-L-fucosidases encoded in the Lactobacillus casei BL23 genome were cloned and purified. The proteins displayed different abilities to hydrolyze natural fucosyloligosaccharides like 2-fucosyllactose, H antigen disaccharide, H antigen type II trisaccharide, and 3-, 4-, and 6-fucosyl-GlcNAc. This indicated a possible role in the utilization of oligosaccharides present in human milk and intestinal mucosa.L-Fucose is one of the most common monosaccharides occurring at the nonreducing end of many glycans on mammalian cell surfaces, intestinal mucin, blood group antigens, and human milk oligosaccharides (HMO) (2). ␣-L-Fucosidases (EC 3.2.1.51), which are exoglycosidases capable of cleaving ␣-linked L-fucose residues from fucosyloligosaccharides, play important roles in the adaptation of bacteria to particular niches. Therefore, infant intestinal bacteria such as bifidobacteria are able to use HMO (4, 9).The genome of Bifidobacterium longum strains carries gene clusters related to the utilization of these substrates (8), which contain the necessary activities to degrade all of their glycosidic linkages, including ␣-L-fucosidases. In Bifidobacterium bifidum, two ␣-L-fucosidases, AfcA and AfcB, have been characterized as belonging to glycoside hydrolase (GH) families 95 and 29, respectively, and degrade ␣-(1,2)-and ␣-(1,3/4)-fucosylated HMO, respectively (1, 3).However, there are no reports of ␣-L-fucosidases in lactobacilli, another important group of probiotic bacteria which are common inhabitants of the human intestine. Genome analysis of 25 Lactobacillus species reveals that only the Lactobacillus casei-Lactobacillus rhamnosus group encodes putative ␣-L-fucosidases (6).Cloning and purification of three putative ␣-L-fucosidases from L. casei BL23. Analysis of the L. casei BL23 genome sequence (GenBank accession no. FM177140) (5) showed the presence of three genes coding for putative ␣-L-fucosidases of GH family 29: LCABL_20390 (which was already automatically annotated as ␣-L-fucosidase A [alfA]), LCABL_28270, and LCABL_29340 (hereafter alfB and alfC, respectively). Alignment of the deduced amino acid sequences of the three putative ␣-L-fucosidases from L. casei BL23 showed low homology (21% identity), suggesting that they can have different substrate specificities.alfA, alfB, and alfC were amplified by PCR using specific oligonucleotides and chromosomal DNA from BL23 as the template. They were cloned into the pQE80 vector (Qiagen) for expression as 6ϫHis-tagged proteins in Escherichia coli. E. coli DH5␣ containing pQE80 derivatives carrying alfA, alfB, and alfC was grown in 500 ml of LB with 100 g/ml ampicillin, and when the cultures reached an optical density at 550 nm of 0.6 to 0.8, expression of the proteins was induced with 0.1 mM isopropyl-␤-D-thiogalactopyranoside at 25°C for 5 h. Bacterial cells were lysed by sonication, the cleared extracts were directly loaded onto Ni-nitrilotriacetic acid agarose (1 ml) columns (Qiagen), and His-tagged proteins were purified according to the supplier's recomm...