Summary -This review reports recent data regarding the metabolism and biochemistry of Streptococcus salivarius subsp thermophilus and Lactobacillus delbrueckii subsp bulgaricus related to yogurt manufacture. The taxonomy of these bacteria is presented. Different proposed pathways for carbohydrate metabolism are then discussed, as weil as recent molecular and genetic studies of the enzymes involved. Acetaldehyde is the major aromatic compound in yogurt, and so the different pathways of its formation are briefly described. Recent studies have concerned threonine aldolase which catalyzes acetaldehyde synthesis by yogurt bacteria. Exocellular polysaccharides produced by lactic acid bacteria improve the texture of stirred and liquid yogurts. Some of the polysaccharides of yogurt bacteria are currently known and particular aspects of their production are discussed. Some other properties, le proteolysis, Iipolysis, urease, oxygen metabolism, are also briefly presented. Interactions between streptococci and lactobacilli are weil established, but more data are required for the complete characterization and control of mixed populations. In particular, Iittle is known about antimicrobial compounds produced by these microorganisms. Bacteriophages of yogurt bacteria are now weIl characterized, but liltle is known about Iysogeny in thermophilic streptococci. Finally, progress in genetics (on both plasmid and chromosomal DNA) is briefly discussed.

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A cloned 2-kb EcoRI fragment (fragment f) from a 34-kb plasmid ofLactobacillus helveticus CNRZ 1094 was shown by dot blot to specifically hybridize to total DNAs of 75 L. helveticus strains. No hybridization was found with L. acidophilus, L. crispatus, L. delbrueckii subsp. bulgaricus, L. delbrueckii subsp. lactis, L. gasseri, or L. intestinalis strains. When Southern blots of EcoRI digests of L. helveticus strains were probed with fragment f, these strains displayed restriction fragment length polymorphisms on the basis of which they could be grouped into several clusters.

New species names, Brachybacterium alimentarium and Brachybacterium tyrofermentans, are proposed for two coryneform bacteria isolated from the surfaces of GruyGre and Beaufort cheeses. These two species are similar in their biochemical and chemotaxonomic characteristics but distinct from previously described bacteria. The most distinctive characteristics are the presence of meso-diaminopimelic acid-containing peptidoglycan with a D-G~u-D-As~ interpeptide bridge and the presence of erythritol teichoic acids that contain diaminoglucuronic acid (an uncommon substituent). The menaquinone pattern of these organisms is unique among coryneform bacteria. DNA-DNA hybridization experiments revealed that the level of hybridization between the two organisms is 15%, which indicates that they are distinct species. Despite the unique biochemical characteristics of these bacteria, a 16s rRNA sequence comparison revealed that they are unquestionably related to Brachybacterium faecium, Brachybacterium nesterenkovii, and Brachybacterium conglomeratum. DNA-DNA hybridization experiments performed with these three species, B. alimentarium, and B. tyrofermentans revealed that the levels of complementarity ranged from 11 to 38%, values that are similar to the values obtained for Brachybacterium strains described previously. With the inclusion of B. azimentarium and B. tyrofermentans the genus Brachybacterium becomes somewhat heterogeneous with respect to chemotaxonomic characteristics.Accolas et al. (2) described the isolation of a number of highly salt-tolerant (15 to 20% NaCl) coryneform bacteria from the surfaces of Gruyere and Beaufort cheeses. All of these organisms grew best at temperatures between 20 and 30°C. Most were able to hydrolyze gelatin but not starch and could not decompose uric acid. Deamination of arginine did not occur or occurred only after 1 month (1).The cell walls of two of the isolates were particularly interesting. These cell walls contained a peptidoglycan type not found in any previously described bacteria and also contained an uncommon erythritol teichoic acid (24). The novel features of these isolates, designated strains CNRZ 925T (T = type strain) and CNRZ 926T, made their taxonomic position interesting. The peptidoglycan type of strains CNRZ 925T and CNRZ 926T is the meso-diaminopimelic acid-D-Asp-D-Glu type and resembles the meso-diaminopimelic acid-( D-Glu), type found in members of the genus Brachybacterium (6, 11).We studied a number of taxonomic characteristics of strains CNRZ 925T and CNRZ 926T and compared these organisms with strains belonging to the genus Brachybacterium. MATERIALS AND METHODSBacterial strains and culture conditions. Strains CNRZ 925T and CNRZ 926T were obtained from the Centre National de Recherches Zootechniques (CNRZ), Jouy-en-Josas, France. Reference strain Brachybacterium conglomeratum NCIMB * Corresponding author. Mailing address: Institut fur Genetik und Mikrobiologie der Universitat Munchen, Maria-Ward-Strasse la, 80638 Munich, Germany. Phone: (089) 17 91 98 The strain...

After treatment with mitomycin C, 12 of the 120 strains of the French Collection of Lactic Acid Bacteria (CNRZ) belonging to the species Streptococcus salivarius subsp. thermophilus were found to be inducible and lysogenic. The corresponding temperate phages were multiplied on indicator strains and further characterized. It is noteworthy that some of the indicators were themselves lysogenic. The temperate phages belonged to Bradley's group B, or to the Siphoviridae family of the International Committee on Taxonomy of Viruses. Seven of them were shared among the two protein profiles previously established for virulent phages of Str. salivarius subsp. thermophilus. Seven different DNA restriction profiles were found for ten native temperate phages examined just after mitomycin C induction. Genome sizes varied from 40 to 45 kb and were classified into four related homology groups by DNA cross-hybridization, but there was no clear-cut relationship with the protein clusters previously shown. DNA homology with representatives of virulent phages was also found. The DNA restriction profiles of seven native temperate phages, examined just after mitomycin C induction of the lysogenic hosts, were noticeably different from those of the corresponding phages once they had been propagated on lysogenic indicators. Moreover, the host range of the latter phages was extended to a greater number of sensitive strains. The possible role of lysogenic strains of Str. salivarius subsp. thermophilus in the occurrence of phage outbreaks affecting this dairy streptococcal species is discussed.Streptococcus salivarius subsp. thermophilus is the streptococcal species characteristic of thermophilic dairy starters used in the manufacture of yogurt and several types of cheeses (Auclair & Accolas, 1983;Zourari et al. 1992). As in the case of other dairy lactic acid bacteria, these streptococci are often the prey of phage outbreaks which cause slow lactic acid fermentation and inferior products.The first phages of Str. salivarius subsp. thermophilus were described in the early 1950s (Deane et al. 1953; Kiuru & Tybeck, 1955) and numerous observations since then have shown that this streptococcus was the victim of specific phage infections relatively often. Nevertheless, work on Str. salivarius subsp. thermophilus phages remained scant and limited for some time, and it was only in the second half of the 1980s that several surveys were conducted. Their results filled in gaps in our * Present address: Schweizerische Milch-Gesellsohaft AG, CH-6280 Hoohdorf, Schweiz.10-2