The lactic acid bacterium Streptococcus thermophilus is widely used for the manufacture of yogurt and cheese. This dairy species of major economic importance is phylogenetically close to pathogenic streptococci, raising the possibility that it has a potential for virulence. Here we report the genome sequences of two yogurt strains of S. thermophilus . We found a striking level of gene decay (10% pseudogenes) in both microorganisms. Many genes involved in carbon utilization are nonfunctional, in line with the paucity of carbon sources in milk. Notably, most streptococcal virulence-related genes that are not involved in basic cellular processes are either inactivated or absent in the dairy streptococcus. Adaptation to the constant milk environment appears to have resulted in the stabilization of the genome structure. We conclude that S. thermophilus has evolved mainly through loss-of-function events that remarkably mirror the environment of the dairy niche resulting in a severely diminished pathogenic potential. Supplementary information The online version of this article (doi:10.1038/nbt1034) contains supplementary material, which is available to authorized users.
The eight IS231 variants characterized so far (IS231 A-F, V and W) display similar transposases with an overall 40% identity. Comparison with all the prokaryotic transposable elements sequenced so far revealed that the IS231 transposases share two conserved regions with those of 35 other insertion sequences of wide origins. These insertion sequences, defining the IS4 family, have a common bipartite organization of their ends and are divided into two similarity groups. Interestingly, the transposase domains conserved within this family display similarities with the well known integrase domain shared by transposases of the IS3 and IS15 families, and integrases of retroelements. This domain is also found in IS30-related elements and Tn7 TnsB protein. Amino acid residues conserved throughout all these prokaryotic and eukaryotic mobile genetic elements define a major transposase/integrase motif, likely to play an important role in the transposition process.
The potential of lactic acid bacteria as live vehicles for the production and delivery of therapeutic molecules is being actively investigated today. For future applications it is essential to be able to establish dose-response curves for the targeted biological effect and thus to control the production of a heterologous biopeptide by a live lactobacillus. We therefore implemented in Lactobacillus plantarum NCIMB8826 the powerful nisin-controlled expression (NICE) system based on the autoregulatory properties of the bacteriocin nisin, which is produced by Lactococcus lactis. The original two-plasmid NICE system turned out to be poorly suited to L. plantarum. In order to obtain a stable and reproducible nisin dose-dependent synthesis of a reporter protein (-glucuronidase) or a model antigen (the C subunit of the tetanus toxin, TTFC), the lactococcal nisRK regulatory genes were integrated into the chromosome of L. plantarum NCIMB8826. Moreover, recombinant L. plantarum producing increasing amounts of TTFC was used to establish a dose-response curve after subcutaneous administration to mice. The induced serum immunoglobulin G response was correlated with the dose of antigen delivered by the live lactobacilli.Lactic acid bacteria (LAB) are used worldwide in the preparation of fermented foods, including dairy products. They are also known for the potentially beneficial effects they may exert on the health of humans and animals (see, for example, reference 25). Their "generally recognized as safe" status (1), linked to their metabolic and technological properties, has recently led to their development as potential live-vaccine vehicles. Lactobacillus plantarum NCIMB8826 (17, 33) has been chosen for this purpose in our laboratory on the basis of its capability to persist in the mouse gastrointestinal and urogenital tracts (38). The ability to control the expression level of foreign proteins in LAB may offer certain advantages. However, while several controlled expression systems have been developed for Lactococcus lactis (9, 23), very few inducible promoters are available for lactobacilli: the xylR promoter from Lactobacillus pentosus (29), the ␣-amylase promoter from L. amylovorus (31), and the p-coumarate decarboxylase promoter from L. plantarum (4). One of the most promising lactococcal controlled expression systems is based on the autoregulatory properties of the L. lactis nisin gene cluster (7,23). Nisin is an antimicrobial peptide belonging to the family of lantibiotics (19) and is used as a natural preservative in the food industry (5). Nisin induces the transcription of the genes under control of the nisA and nisF promoters, via a two-component regulatory system (34, 37) consisting of the histidine protein kinase NisK and the response regulator NisR (14,21,22). A transferable nisin-controlled expression (NICE) system (24) based on the combination of the nisA promoter and the nisRK regulatory genes has recently been developed (7,20). It consists of two compatible replicons, a plasmid carrying the nisRK regulato...
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