Prevalence and impact of single-strain starter cultures of lactic acid bacteria on metabolite formation in sourdough

Microbial ecology of sourdough fermentations: diverse or uniform?

Sourdough is a specific and stressful ecosystem inhabited by yeasts and lactic acid bacteria (LAB), mainly heterofermentative lactobacilli. On the basis of their inocula, three types of sourdough fermentation processes can be distinguished, namely backslopped ones, those initiated with starter cultures, and those initiated with a starter culture followed by backslopping. Typical sourdough LAB species are Lactobacillus fermentum, Lactobacillus paralimentarius, Lactobacillus plantarum, and Lactobacillus sanfranciscensis. Typical sourdough yeast species are Candida humilis, Kazachstania exigua, and Saccharomyces cerevisiae. Whereas region specificity is claimed in the case of artisan backslopped sourdoughs, no clear-cut relationship between a typical sourdough and its associated microbiota can be found, as this is dependent on the sampling, isolation, and identification procedures. Both simple and very complex consortia may occur. Moreover, a series of intrinsic and extrinsic factors may influence the composition of the sourdough microbiota. For instance, an influence of the flour (type, quality status, etc.) and the process parameters (temperature, pH, dough yield, backslopping practices, etc.) occurs. In this way, the presence of Lb. sanfranciscensis during sourdough fermentation depends on specific environmental and technological factors. Also, Triticum durum seems to select for obligately heterofermentative LAB species. Finally, there are indications that the sourdough LAB are of intestinal origin.

Role of lactic acid bacteria as a biosanitizer to prevent attachment of Listeria monocytogenes F6900 on deli slicer contact surfaces

The study was conducted to evaluate the attachment of three lactic acid bacteria (LAB) strains and their combination in a cocktail, to stainless steel coupons from a deli slicer, and their ability to inhibit the attachment of Listeria monocytogenes. In a previous study, three LAB strains, Pediococcus acidilactici, Lactobacillus amylovorus, and Lactobacillus animalis, were isolated from ready-to-eat meat and exhibited antilisterial effect. In the study reported here, hydrophobicity tests were determined according to the method of microbial adhesion to solvent. The attachment of the cells was evaluated on stainless steel coupons from deli slicers. Extracellular carbohydrates were determined with a colorimetric method. Based on these tests, L. animalis exhibited the greatest hydrophobicity (26.3%), and its adherence increased sharply from 24 to 72 h, whereas L. amylovorus yielded the lowest hydrophobicity (3.86%) and was weakly adherent. Although P. acidilactici had moderate hydrophobicity (10.1%), it adhered strongly. The attached LAB strains produced significantly (P < 0.05) higher total carbohydrates than their planktonic counterparts did, which is an important characteristic for attachment. Three conditions were simulated to evaluate the ability of the LAB cocktail (10(8) CFU/ml) to competitively exclude L. monocytogenes (10(3) CFU/ml) on the surface of the coupons. The coupons were pretreated with the LAB cocktail for 24 h prior to the addition of L. monocytogenes, simultaneously treated with the LAB cocktail and L. monocytogenes, or pretreated with L. monocytogenes 24 h prior to the addition of the LAB cocktail. The LAB cocktail was able to reduce the attachment L. monocytogenes significantly (P < 0.05). The LAB cocktail indicated potential attachment on stainless steel and bacteriostatic activity toward L. monocytogenes attached on stainless steel, which indicates a possible role for LAB as a biosanitizer in the food industry.

Expression of the arginine deiminase pathway genes in Lactobacillus sakei is strain dependent and is affected by the environmental pH

The adaptation of Lactobacillus sakei to a meat environment is reflected in its metabolic potential. For instance, the ability to utilize arginine through the arginine deiminase (ADI) pathway, resulting in additional ATP, represents a competitive benefit. In L. sakei CTC 494, the arc operon (arcABCTDR) shows the same gene order and organization as that in L. sakei 23K, the genome sequence of which is known. However, differences in relative gene expression were found, and these seemed to be optimal in different growth phases, namely, the highest relative gene expression level was in the end exponential growth phase in the case of L. sakei CTC 494 and in the mid-exponential growth phase of L. sakei 23K. Also, the environmental pH influenced the relative expression level of the arc operon, as shown for L. sakei CTC 494, with the highest relative expression level occurring at the optimal pH for growth (pH 6.0). Deviations from this optimal pH (pH 5.0 and pH 7.0) resulted in an overall decline of the relative expression level of all genes of the arc operon. Furthermore, a differential relative expression of the individual genes of the arc operon was found, with the highest relative gene expression occurring for the first two genes of the arc operon (arcA and arcB). Finally, it was shown that some L. sakei strains were able to convert agmatine into putrescine, suggesting an operational agmatine deiminase pathway in these strains, a metabolic trait that is undesirable in meat fermentations. This study shows that this metabolic trait is most probably encoded by a previously erroneously annotated second putative arc operon.

Typing of Lactobacillus sanfranciscensis isolates from traditional sourdoughs by combining conventional and multiplex RAPD-PCR profiles

In the present work, a rapid and reproducible molecular method, based on the combination of conventional and multiplex RAPD-PCR reactions, was developed for typing Lactobacillus sanfranciscensis isolates from traditional sourdoughs. At first, four random primers, two used singly and two combined with the primer RD1, were chosen on the basis of their differentiating capability and reproducibility. The four resulting profiles for each isolate were integrated into a unique profile to be statistically treated by cluster analysis. The method was validated on 58 L. sanfranciscensis isolates coming from three traditional Italian sourdoughs. This new RAPD method was useful for determining the genomic diversity within the L. sanfranciscensis species. In particular, the intraspecific diversity of this species seemed to be related to the sourdough origin.

Biotechnology and pasta-making: lactic Acid bacteria as a new driver of innovation

Cereals-derived foods represent a key constituent in the diet of many populations. In particular, pasta is consumed in large quantities throughout the world in reason of its nutritive importance, containing significant amounts of complex carbohydrates, proteins, B-vitamins, and iron. Lactic acid bacteria (LAB) are a heterogeneous group of bacteria that play a key role in the production of fermented foods and beverages with high relevance for human and animal health. A wide literature testifies the multifaceted importance of LAB biotechnological applications in cereal-based products. Several studies focused on LAB isolation and characterization in durum wheat environment, in some cases with preliminary experimental applications of LAB in pasta-making. In this paper, using sourdough as a model, we focus on the relevant state-of-art to introduce a LAB-based biotechnological step in industrial pasta-making, a potential world driver of innovation that might represent a cutting-edge advancement in pasta production.

The pentose moiety of adenosine and inosine is an important energy source for the fermented-meat starter culture Lactobacillus sakei CTC 494

The genome sequence of Lactobacillus sakei 23K has revealed that the species L. sakei harbors several genes involved in the catabolism of energy sources other than glucose in meat, such as glycerol, arginine, and nucleosides. In this study, a screening of 15 L. sakei strains revealed that arginine, inosine, and adenosine could be used as energy sources by all strains. However, no glycerol catabolism occurred in any of the L. sakei strains tested. A detailed kinetic analysis of inosine and adenosine catabolism in the presence of arginine by L. sakei CTC 494, a fermented-meat starter culture, was performed. It showed that nucleoside catabolism occurred as a mixed-acid fermentation in a pH range (pH 5.0 to 6.5) relevant for sausage fermentation. This resulted in the production of a mixture of acetic acid, formic acid, and ethanol from ribose, while the nucleobase (hypoxanthine and adenine in the case of fermentations with inosine and adenosine, respectively) was excreted into the medium stoichiometrically. This indicates that adenosine deaminase activity did not take place. The ratios of the different fermentation end products did not vary with environmental pH, except for the fermentation with inosine at pH 5.0, where lactic acid was produced too. In all cases, no other carbon-containing metabolites were found; carbon dioxide was derived only from arginine catabolism. Arginine was cometabolized in all cases and resulted in the production of both citrulline and ornithine. Based on these results, a pathway for inosine and adenosine catabolism in L. sakei CTC 494 was presented, whereby both nucleosides are directly converted into their nucleobase and ribose, the latter entering the heterolactate pathway. The present study revealed that the pentose moiety (ribose) of the nucleosides inosine and adenosine is an effective fermentable substrate for L. sakei. Thus, the ability to use these energy sources offers a competitive advantage for this species in a meat environment.

Species diversity and metabolic impact of the microbiota are low in spontaneously acidified Belgian sausages with an added starter culture of Staphylococcus carnosus

Quality of fermented sausages is affected by acidifying lactic acid bacteria (LAB) and colour- and flavour-promoting coagulase-negative staphylococci (CNS), whether or not used as starter culture. Artisan fermented sausages are often perceived as superior to industrial variants, partially because of the specific microbiota due to spontaneous acidification, which may be considered as an artisan characteristic. Therefore, two kinds of spontaneously acidified Belgian sausages were prepared (Belgian-type salami and Boulogne sausage), but with addition of a Staphylococcus carnosus culture. The Belgian-type salami was made from pork and beef, whereas the Boulogne sausage contained pork and horse meat. In all cases, Lactobacillus sakei was the dominant LAB species present on the raw materials and during fermentation, whereas enterococci remained present in the background. Enterobacteriaceae vanished after fermentation. The CNS species diversity on the raw materials was large and differed between the pork, beef, and horse meat. Nevertheless, this species diversity was annihilated during fermentation by the added S. carnosus culture. The volatiles fraction was mainly composed of aldehydes that originated from lipid oxidation and spices-derived compounds. Aromatic compounds that are typically associated to CNS activity, such as end-products from the metabolism of branched-chain amino acids, were not present in the Belgian-type salami and only marginally present in the Boulogne sausage. In conclusion, spontaneous acidification of Belgian-type fermented sausages leads to dominance of L. sakei and is no guarantee for bacterial contribution to the aroma profile when S. carnosus is added as a starter culture.