Selection of yoghurt starter culture from indigenous isolates ofStreptococcus thermophilus andLactobacillus delbrueckii subsp.bulgaricus on the basis of technological properties

Study on Streptococcus thermophilus isolated from Qula and associated characteristic of acetaldehyde and diacetyl in their fermented milk

In this study, the lactic acid bacterial population of Qula cheese from the Gansu and Sichuan provinces of China were isolated and identified. Eight strains of Streptococcus thermophilus were isolated, of which five strains were selected for further characterization based on their fermentation properties. The changes in a number of parameters, including titration acidity, pH, viable counts, PrtS protease activity and the production of acetaldehyde, diacetyl and organic acid, were monitored during fermentation and the storage of fermented milks produced by the respective strain. All of the strains displaying acidifying capacity and all five fermented milks maintained high viable counts of S. thermophilus from fermentation to storage. Our study found that the changes in the monitored parameters were strain-specific and varied considerably among the five tested strains. Fermented milks produced by strain IMAU80809 had the highest concentration of acetaldehyde and were most favorable in the sensory evaluation. This study confirms that Qula cheese is a good source for isolating novel lactic acid bacterial strains with different fermentation properties, which will be very useful for further development and industrialization of traditionally fermented dairy products.

Identification of Coccoidal Bacteria in Traditional Fermented Milk Products from Mongolia, and the Fermentation Properties of the Predominant Species, Streptococcus thermophilus

The objective of this study was to identify the coccoidal bacteria present in 188 samples of fermented yaks', mares' and cows' milk products collected from 12 different regions in Mongolia. Furthermore, we evaluated the fermentation properties of ten selected isolates of the predominant species, Streptococcus (S.) thermophiles, during the process of milk fermentation and subsequent storage of the resulting yoghurt at 4℃. Overall, 159 isolates were obtained from 188 samples using M17 agar. These isolates were presumed to be lactic acid bacteria based on their gram-positive and catalase-negative properties, and were identified to species level using 16S rRNA gene sequence analysis. These coccoid isolates were distributed in four genera and six species: Enterococcus (E.) durans, Enterococcus (E.) faecalis, Lactococcus (Lac.) subsp. lactis, Leuconostoc (Leuc.) lactis, Leuconostoc (Leuc.) mesenteroides. subsp. mesenteroides and S. thermophilus. Among these S. thermophilus was the most common species in most samples. From evaluation of the fermentation characteristics (viable counts, pH, titratable acidity [TA]) of ten selected S. thermophilus isolates we could identify four isolates (IMAU 20246, IMAU20764, IMAU20729 and IMAU20738) that were fast acid producers. IMAU20246 produced the highest concentrations of lactic acid and formic acid. These isolates have potential as starter cultures for yoghurt production.

Fluorescent-BOX-PCR for resolving bacterial genetic diversity, endemism and biogeography

Background

BOX-A1R-based repetitive extragenic palindromic-PCR (BOX-PCR) is one of the most used techniques in biogeography studies of microbial isolates. However the traditional separation of BOX-PCR patterns by agarose gel electrophoresis suffers many limitations. The aim of this research was to set up a fluorescent BOX-PCR (F-BOX-PCR) assay in which separation of PCR products is automated in a capillary electrophoresis system. F-BOX-PCR was compared with the traditional BOX-PCR using bacterial strains with different G+C content (Bacillus cereus; Escherichia coli; isolates of the family Geodermatophilaceae). Resolution, discriminatory power and reproducibility were evaluated by assaying different electrophoretic runs, PCR reactions and independent DNA extractions. BOX-PCR and F-BOX-PCR were compared for the analysis of 29 strains of Modestobacter multiseptatus isolated from three different microsites in an altered carbonatic wall from Cagliari, Italy, and 45 strains of Streptococcus thermophilus isolated from 34 samples of the hand-made, yogurt-like product Matsoni, collected in different locations in Georgia.

Results

Fluorophore 6-FAM proved more informative than HEX and BOX-PCR both in agarose gel electrophoresis (p < 0.004 and p < 0.00003) and in capillary electrophoresis (compared only with HEX, p < 2 × 10^-7). 6-FAM- and HEX-based F-BOX-PCR respectively detected up to 12.0 and 11.3 times more fragments than BOX-PCR. Replicate separations of F-BOX-PCR showed an accuracy of the size calling of ± 0.5 bp until 500 bp, constantly decreasing to ± 10 bp at 2000 bp. Cluster analysis of F-BOX-PCR profiles grouped M. multiseptatus strains according to the microsite of isolation and S. thermophilus strains according to the geographical origin of Matsoni, but resulted intermixed when a BOX-PCR dataset was used.

Conclusion

F-BOX-PCR represents an improved method for addressing bacterial biogeography studies both in term of sensitivity, reproducibility and data analysis.