The effect of immobilization on some probiotic properties ofStreptococcus thermophilus strains

Isolation of 60 strains from fermented milk of mares and donkeys in Algeria and identification by 16S rRNA sequencing of lactobacilli: Assessment of probiotic skills of important strains and aromatic productivity power

Background and Aim

Donkey and mare milk have high nutritional and functional values, but their lactic acid bacteria (LAB) content remains poorly studied and undervalued in the Algerian dairy industry. This study aimed to isolate and select LAB strains that produce antimicrobial substances during fermentation and to characterize the probiotic profiles of each extracted strain to indicate their potential for antioxidant and proteolytic activity.

Materials and Methods

This study focuses on isolating and identifying lactic acid bacterial strains from 10 Equid-fermented milk samples collected in two regions of El Bayed Wilaya (Algeria). Identification of LAB strains was obtained by 16S rRNA sequencing. The probiotic properties of important strains and their aromatic productivity power are assessed. To evaluate their antibacterial activity against Listeria monocytogenes, Staphylococcus aureus, Chryseobacterium joostei, Pseudomonas aeruginosa, and Escherichia coli, we selected 21 strains. Different induction methods have been used to amplify the antibacterial effects against these pathogenic strains.

Results

Among a total of 60 identified strains, 31 had a probiotic profile, and most were catalase-negative. Aromatic productivity power was observed in eight strains: Lactiplantibacillus plantarum, Lactobacillus casei, Lactobacillus paracasei, Weissella confusa, Weissella cibaria, Leuconostoc mesenteroides, Leuconostoc lactis, and Lactobacillus sp1.

Conclusion

Our results provide insight into the considerable diversity of LAB present in fermented donkey and mare milk. To meet the expectations of the Algerian dairy industry, it is important that the probiotic skills of the nine selected strains are met. In addition, a significant number of these strains may have important probiotic activity and biotechnological potential.

Control of tyramine and histamine accumulation by lactic acid bacteria using bacteriocin forming lactococci

The aim of this study was to evaluate the competitive effects of three bacteriocin producing strains of Lactococcus lactis subsp. lactis against two aminobiogenic lactic acid bacteria, i.e. the tyramine producing strain Enterococcus faecalis EF37 and the histamine producing strain Streptococcus thermophilus PRI60, inoculated at different initial concentrations (from 2 to 6 log cfu/ml). The results showed that the three L. lactis subsp. lactis strains were able to produce bacteriocins: in particular, L. lactis subsp. lactis VR84 and EG46 produced, respectively, nisin Z and lacticin 481, while for the strains CG27 the bacteriocin has not been yet identified, even if its peptidic nature has been demonstrated. The co-culture of E. faecalis EF37 in combination with lactococci significantly reduced the growth potential of this aminobiogenic strain, both in terms of growth rate and maximum cell concentration, depending on the initial inoculum level of E. faecalis. Tyramine accumulation was strongly reduced when E. faecalis EF37 was inoculated at 2 log cfu/ml and, to a lesser extent, at 3 log cfu/ml, as a result of a lower cell load of the aminobiogenic strain. All the lactococci were more efficient in inhibiting streptococci in comparison with E. faecalis EF37; in particular, L. lactis subsp. lactis VR84 induced the death of S. thermophilus PRI60 and allowed the detection of histamine traces only at higher streptococci inoculum levels (5-6 log cfu/ml). The other two lactococcal strains did not show a lethal action against S. thermophilus PRI60, but were able to reduce its growth extent and histamine accumulation, even if L. lactis subsp. lactis EG46 was less effective when the initial streptococci concentration was 5 and 6 log cfu/ml. This preliminary study has clarified some aspects regarding the ratio between bacteriocinogenic strains and aminobiogenic strains with respect to the possibility to accumulate BA and has also showed that different bacteriocins can have different effects on BA production on the same strain. This knowledge is essentially aimed to use bacteriocinogenic lactococci as a predictable strategy against aminobiogenic bacteria present in cheese or other fermented foods.

The effect of cell immobilization on the antibacterial activity of Lactobacillus reuteri DPC16 cells during passage through a simulated gastrointestinal tract system

Cell immobilization has the ability to influence the survival and functional characteristics of probiotic bacterial strains in harsh environments. This study investigated the effect of cell immobilization and passage through a simulated gastrointestinal tract (GI) on the antibacterial activity of Lactobacillus reuteri DPC16. Antibacterial activity, reuterin production and diol dehydratase activity were assayed in recovered isolates of L. reuteri that had been immobilized in Ca alginate-skim milk, and incubated in simulated GI fluids. Among all the recovered isolates tested, any that had undergone immobilization followed by immediate recovery of the cells without subsequent incubation in any fluids demonstrated the highest reuterin production, antimicrobial activity and diol dehydratase enzyme activity. L. reuteri DPC16 cells that had been immobilized, incubated in simulated GI fluids, and subsequently recovered from the beads often showed some loss of antimicrobial activity compared to the immobilized cells. The data confirm that the process of immobilization of L. reuteri in Ca alginate-skim milk, rather than the passage through simulated GI fluids, resulted in enhanced antibacterial activity. This is attributed to increased diol dehydratase activity, resulting in increased reuterin production.