Abstract:

With the prohibition of antibiotics, sub-health issues in livestock and poultry production have attracted increasing attention, and the screening of high-performance probiotics has evolved into a key approach to tackle this prominent industrial challenge. In this study, cow milk was used as the sample source to screen Lactococcus lactis strains with excellent gastrointestinal tolerance and strong adhesion ability. Lactic acid bacteria were isolated and purified using MRS-CaCO₃ selective medium, and the isolated strains were identified as Lactococcus lactis through morphological observation, biochemical tests and 16S rDNA molecular identification. Subsequently, the gastrointestinal tolerance of Lactococcus lactis was evaluated via tests of resistance to artificial gastrointestinal fluid, acid resistance and bile salt resistance, while the surface adhesion ability of the strains was assessed by determining the auto-aggregation rate, hydrophobicity rate, co-aggregation rate with pathogenic bacteria and biofilm formation ability. The results showed that a total of 3 strains of Lactococcus lactis (H1-H3) were isolated in this study. Their survival rates in artificial gastric juice and intestinal juice for 3 hours were as high as 104.9%-171.0% and 99.3%-234.2%, respectively, and the survival rates gradually increased with the increase of pH (3.0-5.0) (P<0.05). In addition, the three strains exhibited certain bile salt tolerance, with survival rates ranging from 18.6% to 57.5% at bile salt concentrations of 0.1% to 0.3%. Meanwhile, all three strains were sensitive to 5 antibiotics, including enrofloxacin, erythromycin, penicillin, doxycycline and streptomycin, indicating their high biological safety. In terms of surface adhesion characteristics, the auto-aggregation rates of strains H1, H2 and H3 were 17.52%, 52.93% and 48.47%, respectively, and their hydrophobicity rates were 10.40%, 56.55% and 54.39%, respectively. Moreover, the co-aggregation rates of the three strains with Staphylococcus aureus and Escherichia coli were all above 78%, and all strains showed strong biofilm formation ability. In conclusion, the three isolated Lactococcus lactis strains possess strong gastrointestinal tolerance, among which strain H2 presents more prominent adhesion ability. This strain can provide excellent strain resources for the development and application of feed probiotics for livestock.

Key words: probiotics, Lactococcus lactis, stress tolerance, adhesion properties