Phenotypic, biochemical and molecular identification
The probiotic, technological and protective characterization of three bacterial isolates from fermented wheat was carried out through this experimental study. Strains were characterized as gram-positive, catalase-negative, non-motile and rod-shaped bacteria that could grow at temperatures between 15°C and 40°C and at 3% NaCl concentration. Other biochemical and phenotypic characteristics of the isolates were summarized in Table 1.
Isolate LAn 12a was identified as
Lactiplantibacillus pentosus based on the results of molecular identification 16S rDNA sequencing and BLAST analysis of its corresponding sequences on the NCBI website, whereas isolates LAn 91 and LAn110 were identified as
Levilactobacillus brevis. Table 1 lists the GenBank accession numbers corresponding to the 16S rRNA gene sequences of the identified isolates.
Probiotic properties
A variation in the survival rate of the isolated bacteria under different acidic conditions was observed (Table 2). All strains grew well on the control medium with a pH of 6.2 (>100%), resulting in an increase in the initial number of cells and a maximum survival rate in
L. brevis 91 (114.48%), followed by
L. pentosus 12a (114.15%) and
L. brevis 110 (104.15%). All of the isolates showed good growth at pH 4 with no significant differences from the control; however, their resistance to acidic conditions generally decreases significantly (p 0.0
5) in pH 2 by up to 98.16% (
L. pentosus 12a), 98.01% (
L. brevis 91) and 96.9% (
L. brevis 110). This study produced comparable results to previous research
(Angelescu et al., 2019) in which isolated bacteria showed a survival rate greater than 90%.
Regarding bile salt tolerance, a significant decrease (p 0.05) was observed in the viability rate of the strains in presence of 0.3% of bile salts compared to the control over the 3 h test period (Table 2), ranging from 114.82% (
L. brevis 110) to 122.03% (
L. pentosus 12a) in control and from 97.81% (
L. brevis 91) to 98.88% (
L. brevis 110) in bile salts. Our results were close to those found by
Adesulu-Dahunsi et al., (2018). Shokryazdan et al., (2017) provided an explanation for this resistance to bile salts, indicating that bile salts, which disrupt the membrane structure of viable cells, undergo hydrolysis through the action of an enzyme known as bile salt hydrolase (BSH), produced by certain LAB probiotic strains.
This study found that during the incubation period, the auto-aggregation values increased significantly (Table 2). Although in the first three hours, the strains’ behavior was difficult to distinguish, they exhibited an auto-aggregation rate that ranged from 17.61 to 38.05%; these characteristics support the findings reported by Ait
Chait et al., (2021). After 24 hours, all bacterial isolates tested showed more than 90% aggregation.
L. brevis 110 was found to be the best auto-aggregating strain among them, with 98.28% aggregation, followed by
L. pentosus 12a (97.67%) and
L. brevis 91 being the least auto-aggregative (94.25%), comparable finds are described by
Kowsalya et al., (2022).
Antibacterial activity
One of the main advantages of probiotics is their antimicrobial activity, which is ensured by the production of various substances, such as short-chain fatty acids, bacteriocins and organic acids
(Hojjati et al., 2020). With inhibition zones ranging from 20 to 48.33 mm, isolate
L. pentosus 12a showed the greatest inhibitory activity against all the indicator strains (Table 3). In contrast, isolate
L. brevis 91 had little inhibitory effect against the majority of the tested pathogens, with the exception of
E. coli and
B. subtilis, which were not susceptible. When the
L. brevis 110 had the least impact on the growth of pathogens, it was only able to inhibit
B. cereus and
K. pneumonia with inhibition zones larger than 13 mm in diameter.
Acid CFS from the tested isolate was significantly active against all pathogens (Fig 1). The CFS of
L. pentosus 12a had the highest antibacterial activity against all indicators evaluated, followed by that of
L. brevis 91, while the CFS from
L. brevis 110 had the minimal effect on the growth of pathogens. However, the neutralization of CFS to pH 6.5 eliminate the antibacterial activity of the three LAB strains which implies that the decrease in pH caused by organic acid production is the source of this significant inhibitory effect. A similar situation was described by
Obioha et al., (2021).
Technological characterization
Isolates of LAB were tested for protease, lipolytic activities and EPS production. The results anticipated that only
L. pentosus 12a was able to degrade casein in plate assays.
Moussa et al., (2008) find that
Lactiplantibacillus pentosus isolated from Tunisian raw milk has a high proteolytic character. As concerns lipolytic activity, bacterial strains did not show any extracellular lipolytic activity, a result that corroborated the data reported by
Araújo-Rodrigues et al., (2021).
Regarding EPS production, only the
L. pentosus 12a strain was able to produce shiny, mucoid and white colonies on sucrose agar. Several factors affect the variations and production of EPS by LAB strains, such as the culture conditions, medium composition, age of the cell, type of LAB strains, pH and temperature (
Ismail and Nampoothiri 2010).
Acidification capacity and viability during stockage period
L. pentosus 12a was able to decrease significantly the pH of milk after 24 h of incubation to pH 4.67 (Fig 2). Comparing with the inoculation level, an increase of 3 logs in the viability rate of this isolate during 21 days of post acidification period was found; during this time, the pH of the milk products remained relatively constant. These technological features are promising for future application in fermented products such as yogurt and acidified milk. The literature shows that some strains of
Lactiplantibacillus pentosus isolated from raw cereal material had a high acidification power
(Merabti et al., 2019). However,
L. brevis 91 and
L. brevis 110 were not able to acidify milk during the 24 h fermentation time (pH 5.35 and 5.07, respectively); according to
González et al., (2010), these findings may be due to the slow rate at which the lactose is metabolized by the Lactobacillus genus. Both strains survived the cold storage period, whereas the number of
L. brevis 91 cells decreased by 1 log during the third week of storage. These results are in agreement with those of
Rönkä et al., (2003) who found that
L. brevis strains are not appropriate for use alone in the manufacturing of fermented dairy products.
Lactose hydrolysis
The organoleptic and nutritional characteristics of milk and its derivatives are attributed mainly by lactose hydrolysis using the enzymes β-D-galactosidase (EC 3.2.1.23)
(Santos et al., 2015). In addition to their good technological performances, probiotics can reduce the symptoms of lactose intolerance through their B-galactosidase activity
(Sakoui et al., 2022). The concentration of lactose used by each strain is presented in Fig 3. A significant difference (p 0.05) was observed in the use of lactose by the LAB isolates compared to the control. In this study.
L. brevis 91 and
L. brevis 110 strains were the ones that used the least amount of lactose (0.92 and 0.97 g/100 g, respectively). In their study,
Martinez et al., (2013) demonstrated that
L. brevis strains have typically been characterized by a limited capacity for lactose utilization. While the amount of lactose consumed by the isolate
L. pentosus 12a was higher (1.79 g/ 100 g).
Maischberger et al., (2010) previously described very efficient growth of
L. pentosus on lactose, implying a strong β-galactosidase activity.