| Lactic acid bacteria(LAB) which closely bound up to people’s life are widely used in food industry, medicine industry, feed industry and other industries. In recent years, the exposure of a series of food safety incidents focus more people’s attention on food safety. LAB used as food starters have a long history, and are generally recognized as safe(GRAS). However, negative reports about LAB occur in recent years, some studies show that some LAB carry the resistance genes that can thansfer to other bacteria, some bifidobacterium have mucin degradation activity in vitro, besides, the further studies find that some strains can produce harmful metabolites such as biogenic amines, nitroso compounds and so on. Therefore, the safety assesment of LAB are of great significance to improve the quality and safety of LAB products and ensure people ’s health. In this study, 9 strains including 5 Streptococcus thermophilus strains, 2 lactobacillus acidophilus strains, 1 Lactobacillus delbrueckii subsp. Bulgaricus strain and 1 Lactobacillus plantaram strain from Key Lab of Dairy Science(KLDS) were assessed, assement were consist of antibiotic resistance assay, mucin degradation assay and harmful metabolites assay.The study on antibiotic resistance of 9 strains of LAB was carried out through the following three parts, the Kirby-Bauer disc diffusion method, the detection of resistance genes and detection of plasmid. The antibiotic sensitivity of 9 strains to 15 kinds of antibiotics was tested by the Kirby-Bauer disc diffusion method, the results indicated that all the strains tested were resistant to two or more antibiotics. The rate of resistance to gentamicin and kanamycin were 100%, to penicillin G, ampicillin, amoxicillin, streptomycin, ciprofloxacin and ofloxacin, the strains showed different levels of antibiotic resistance, the rates were respectively 33.33%、55.56%、22.22%、22.22%、55.56%、44.44%. All the strains were sensitive or intermediately sensitive to piperacillin, roxithromycin, erythromycin, rifampin, vancomycin, tetracycline and chloramphenicol. The genes(tetM, tet K, tetL and tetS) related to tetracycline resistance, genes(erm A, erm B, msr A/B) related to erythromycin resistance and gene(cat) related to chloramphenicol resistance were detected by PCR, however, the genes were not found in the genomic DNA of strains above. Plasmid extraction test indicated that only one Lactobacillus plantaram had plasmid, while the other strains had no plasmid, and the PCR results showed the resistance genes were not found in the plasmid.The ability to degrade mucin in vitro of 9 strains of LAB was examined using the following three tests: growth in liquid medium, SDS-PAGE analysis of degraded mucin residues, and degradation assay in Petri dish. In this study, fecal sample and the autoclaved fecal sample were used as positive and negative control, respectively. The tested strains failed to grow in the liquidmedium containing mucin, while fecal sample grew well. In the SDS-PAGE analyses of mucin residues and observation of mucinolytic zone in agar plate, all the tested strains also showed no mucin degradation activity, while fecal sample yielded positive results. The results above showed that all the testing strains had no ability to degrade gastrointestinal mucin in vitro.Besides, harmful metabolites production were detected by indole test, aminodecarboxylase test, nitratereductase test and the activity of hemolysis test, the results demonstrated that all the strains didn’t produce tryptophanas, nitratereductase, azoreductase, amino decarboxylase; negative results were observed in hemolysis test and indole test. |
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