Kinetics of cell growth and functional characterization of probiotic strains lactobacillus delbrueckii and lactobacillus paracasei isolated from breast milk

Probiotics are living microbes that benefit the host by influencing the composition and or metabolic activity of the gastrointestinal tract (GIT) flora. In the present work, the cell growth kinetics and the potential use of two lactobacilli strains: Lactobacillus delbrueckii and L. paracasei, isolated from healthy breast milk, as probiotics were investigated. Data showed that both strains grew successfully in MRS medium with almost the same growth rate of about 0.22 [h -1 ]. When cells grew in mixed culture, the growth rate was 0.29 [h -1 ], reaching maximal cell dry weight of 1.27 g l -1 after 8 h cultivation. Its also worthy to note that, the amount of lactic acid produced in mixed culture was higher than those obtained in pure cultures of each individual strains. To evaluate the possible use of these two lactobacilli strains as probiotics, studies on the effect of gastric juice, pH, bile salts and antibiotic resistance were conducted. In general, L. paracasei showed higher resistance to SGJ and bile salt compared to L. delbrueckii. It has been shown also that both strains can withstand in simulated gastric juice (SGJ) in acidic pH down to 3 with different extent. Further decrease in pH value was inhibitory to L. delbrueckii viability whereas, L. paracasei tolerate the pH down to 1 with certain extent. However, both strains were also tolerant to the bile salt lethal effect when applied in low concentration 2―3 %. Higher concentration of bile salt resulted in complete cell death (more than 99.99 %) for both strains. On the other hand, L. paracasei was susceptible to antibiotics like vancomycin, streptomycin, chloramphenicol, tetracyclines and gentamycin, whereas L. delbrueckii showed the similar result except that its susceptibility to gentamycin was intermediate. In conclusion, both of breast milk isolated strains of Lactobacillus paracasei and Lactobacillus delbrueckii have the potential to be used as probiotic in food industries.