| Lactobacillus plantarum, a predominent Lactobacillus species in the human gasrointes-tinal tract, could also be isolated with high frequency from its natural habitat, i.e. fermented plant products such as pickles and silage. As a candidate probiotics, Lactobacillus plantarum could adjust host intestinal microbial balance by inhibiting the growth of the pathogen via nu-trition competition and production of antagonistic metabolites such as organic acid, bacterio-cins, hydrogen peroxide and diacetyl etc. Lactobacilli had been proven to exert critical physiological impacts on host health including resisting pathogen colonization, adjusting mi-croecology balance, anti-tumor forming, improving host defense and immune system and re-ducing serum cholesterol levels as well. Lb. plantarum ST-Ⅲhad demonstrated the ability to remove cholesterol in vitro and in vivo. Our previous study results showed that Lb. plantarum ST-Ⅲcould hardly grow in skim milk. As a result, it showed poor ability to clot milk protein via lactic acid production. In this paper, the impacts of a variety of nutrient factors which might restrict the growth of Lb. plantarum ST-Ⅲin skim milk were investigated and it was prospeculated that the poor ability of Lb. plantarum ST-Ⅲto grow and clot the skim milk could be attributed to its lack of cell wall-bound extrocellar proteinase activity.First of all, the influences of microelement, vitamin, carbon sources and nitrogen sources on growth of Lb. plantarum ST-Ⅲwere studied respectively. Results showed that no significant growth-promoting effects were observed by addition of vitamins, carbon sources and amino acids. On the contrary, supplement of 1% (w/w) peptone prepared from fish tissues (PF) could significantly stimulate the growth of Lb. plantarum ST-Ⅲin skim milk and facili-tate milk curd by acidification. And after cultivation the number of viable bacterium in the fermented milk could reach 1.4×109 cfu/mL and the titration acidity was up to 60°T. There-fore, it was inferred fish peptone might contain some nutrients factors which could promote the growth of Lb. plantarum ST-Ⅲin skim milk.Then, it was found the ultrafiltration permeate of fish peptone by membrane of Molecu-lar weight Cut-off of 5,000 Dalton could also promote the growth of Lb.plantarum ST-Ⅲin skim milk. The permeatement was further isolated step by step by chromatographing the per-meatate on Sephadex G-25 and Sephadex G-15 column, with the elution detected by measur-ing the adsorbance at the wavelenth of 254nm. A fraction (B33) with single peak at 220nm, which could strongly stimulate the growth of Lb. plantarum ST-Ⅲin skim milk, was col-lected and lyophilized.The components of the lyophilized B33 were further characterized by IR spectra. It was shown that B33 demonstrated strong absorbance of C-O in the range of 1000 to 1200cm-1. Therefore, the posibility of polysacchride was excluded for B33. In addition, IR spectra of B33 displayed the characteristic peak of NH3+, the flex libration of asymmetry and symmetry of COO-, the flex libration of N-H asymmetry curl libration and symmetry curl libration. Be-sides, the flex libration of O-H and N-H, and the C=O and C-O were also very strong. There- fore, it could infer that the predominent components of B33 were peptides. The composition of B33 was further assayed by LC/MS/MS which gets eight peptides.To illustrate the impacts of the proteolytic activity of Lb.plantarum ST-Ⅲon the growth of itself, the extracellular protease and intracellular peptidase activity of Lb. plantarum ST-Ⅲwere compared with Lb. plantarum CZ 2112, a strain growing well in skim milk supple-mented with glucose and clotting milk proteins rapidly. Compared with Lb. plantarum CZ 2112, Lb. plantarum ST-Ⅲhad lower protease activity, whereas there was no obvious dif-ference in the activity of aminopeptidase and carboxypeptidase activities between the two Lactobacillus strains. Therefore, it was presumed that deficiency in protease activity of Lb. plantarum ST-Ⅲ, which was responsible for hydrolyzing large milk proteins into small pep-tides when the bacteria were cultivated in skim milk, be correlated with the poor ability of Lb. plantarum ST-Ⅲto grow in and clot milk.
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