M. Vescovo

Inhibitory effect of selected lactic acid bacteria on microflora associated with ready-to-use vegetables

The addition of selected lactic acid bacteria strains had a remarkable inhibitory effect on the growth dynamics of microflora associated with ready‐to‐use vegetables, during refrigerated storage. In particular, coliforms and enterococci were strongly reduced or eliminated from the products from the third day of storage. Lactobacillus casei strains proved more effective than pediococci. The use of lactic cultures able to produce bacteriocins and to grow at low temperatures could be a useful tool to preserve fresh vegetables and to ensure their microbiological safety.

Plasmid-encoded ropiness production inLactobacillus casei SSP.casei

SummaryGenetic determinants of the Muc+ character were investigated in two ropy strains,Lactobacillus delbrueckii ssp.bulgaricus 201 andL. casei ssp.casei NCIB 4114, which secrete a large amount of slime in culture media. Plasmid DNA analysis revealed the presence of two plasmids (4.5 and 2.3 Mdal) inL. casei ssp.casei, whileL. delbrueckii ssp.bulgaricus was plasmid free, suggesting a chromosomal location of Muc+ character in this strain. Curing experiments carried out onL. casei ssp.casei NCIB 4114 indicated a correlation between the Muc+ phenotype and the 4.5 Mdal plasmid.

Potential of Lactobacillus casei, Culture Permeate, and Lacti Acid To Control Microorganisms in Ready-To-Use Vegetables

The effects of various treatments (i.e., the addition of a strain of Lactobacillus that produces antimicrobial agents, Lactobacillus casei IMPC LC34, its sterile permeate, and 0.5 or 1% lactic acid) on the growth of microorganisms associated with ready-to-use mixed salad vegetables were compared during refrigerated (8°C) storage. The addition of 3% culture permeate to mixed salads reduced the total mesophilic bacteria counts from 6 to 1 log CFU/g, and suppressed coliforms, enterococci, and Aeromonas hydrophila after 6 days of storage at 8°C. A similar effect was shown when the L. casei culture was inoculated in the vegetables. One percent lactic acid had a bacteriostatic effect on the bacterial groups examined, except for total and fecal coliforms, which were reduced by about 2 and 1 log unit, respectively, while 0.5% lactic acid did not affect the indigenous microflora of the vegetables. The potential of these new hurdles to prevent the growth of spoilage and pathogenic bacteria in ready-to-use salad vegetables is suggested.

Lactic Acid Bacteria in Ensiled High-Moisture Corn Grain: Physiological and Genetic Characterization

Summary Of 335 lactic acid bacteria, isolated from high-moisture corn grain, 212 were checked for their physiological characteristics and 59 representative strains were chosen for DNA-DNA hybridization. All the homofermentative lactobacilli tested were highly related to the type strain of Lactobacillus plantarum . The pediococci isolated were allotted to Pediococcus pentosaceus . Of the heterofermentative lactic acid bacteria the cocci were classified as Leuconostoc paramesenteroides , the lactobacilli as Lactobacillus brevis . The presence of plasmids was demonstrated in all the species studied.

Instability of lactose and citrate metabolism of Leuconostoc strains

SummaryThe instability of Lac+ and Cit+ phenotypes was investigated inLeuconostocmesenteroides subsp.cremoris ATCC 19245 and in four strains ofLeuconostocmesenteroides subsp.dextranicum. The two phenotypes were linked respectively to a 14 Mdal and a 34 Mdal plasmid in Leuconostoc mesenteroides subsp.cremoris ATCC 19245. InLeuconostocmesenteroides subsp.dextranicum the character Lac+ was linked to a 28 Mdal plasmid, while the Cit+ phenotype was stable.

Partial characterization and plasmid linkage of a non-proteinaceous antimicrobial compound in a Lactobacillus casei strain of vegetable origin

Lactobacillus casei IMPC LC34 of vegetable origin produces a non‐proteinaceous inhibitory compound with a broad spectrum of activity towards Gram‐positive and Gram‐negative bacteria, including pathogens. The active substance, mainly produced in the stationary phase of growth, is insensitive to proteolytic enzymes, lipase and catalase, and is stable at 121 °C for 30 min. The inhibitory activity was detected either at 8 °C or at 37 °C. The active compound does not contain glucidic groups, is inactivated by Na‐metaperiodate, and its molecular mass is between 2000 and 5000 Da. Plasmid curing experiments showed that both antimicrobial compound immunity and production determinants were encoded by an 8·8 kbp plasmid. The effectiveness of the active agent was verified on ready‐to‐use vegetables, using either the Lact. casei strain or its culture supernatant fluid as inoculant, compared with cured clone. The application potential of the Lact. casei strain or its culture supernatant fluid for assuring the microbiological safety of ready‐to‐use vegetables is discussed.

Plasmid dna transformation of Lactobacillus strains by electropermeabilization

Two thermophilic strains of Lactobacillus were transformed by electroporation; L.fermentum with a maximum of frequency of 1×105/ug of plasmid vector pPSC20DNA and 1.4×103/ug pSA3DNA. L.helveticus showed a very low frequency of transformation, from 9 to 26 transformants/ug DNA in all the experiments carried out with both the vectors. While L.fermentum transformants were very stable, in L.helveticus the acquired plasmid was lost after 30–50 generations.

Peptidase localization in Lactobacillus casei 5 Mn373 by stable protoplasts formation

Pure subcellular fractions were obtained from a Lactobacillus casei strain, selected among numerous isolates from Grana cheese on the basis of their peptidolytic activity. Several lytic conditions for the attainement of stable protoplasts were checked and three different cellular markers were employed to assess subcellular fraction purity. The maximum yeld and the best stability of protoplasts, as followed by Scanning Electron Microscopy, were obtained using 25mM Hepes pH 7, 15 mM MgCl2 and 20 U mutanolysin/mL, without the addition of osmotic stabilizers. Peptidolytic activities determined on subcellular fractions against Leu, Arg, Pro and Arg-Pro p-nitroanilides, had a cytoplasmic location. Comparative tests performed on either intact or broken cells by polyclonal antibodies against a general aminopeptidase and a X-prolyl dipeptidyl aminopeptidase confirmed this result. On the basis of such enzymatic activities, determined either on broken or intact cells, it is suggested that chromogenic substrates must be transported inside the cells.

Immuno-based strategy for Clostridium tyrobutyricum specific recognition

Polyclonal antibodies of C.tyrobutyricum were prepared and coupled to an immunoadsorbent resin specific for this microorganism contaminant of cheese milk. The immunoadsorbent selectivity was evaluated also by scanning electron microscope an aqueous suspension of C.tyrobutyricum vegetative cells. The immunoadsorbent showed valuable efficiency in cell retaining process and could be completely recycled.

Peptidase profiling of Lactobacillus casei

Fifty four Lactobacillus casei strains were investigated and compared for their peptidase profiling by statistical analysis of aminoacids released from milk proteins. Forty one strains formed a homogeneous group; only two strains, not included in the above group, resulted the most suitable for grana cheese production either for their aminoacidic pattern or total aminoacid amount.