Identification and biological characterization of a lactic acid bacterial strain isolated from the gut of Bombyx mori

Abstract:

[Background] Lactic acid bacteria as probiotics can be developed as microecological preparations for animals. The lactic acid bacteria in the gut of Bombyx mori, a model insect of Lepidoptera, remain to be studied. [Objective] To isolate the lactic acid bacteria with good biological properties from the gut of B. mori for developing microecological preparations. [Methods] The lactic acid bacteria isolated from the gut of B. mori were screened based on morphological, physiological, and biochemical characteristics and identified by 16S rRNA gene sequencing. The safety of the strain screened out was evaluated by the hemolysis test, and the growth characteristics and acid production of the strain were determined by growth curve and acid production curve. The stress tolerance of the strain was determined by temperature and pH sensitivity tests. The adhesion of the strain was evaluated based on self-aggregation, hydrophobicity, and coagglutination with pathogens. Furthermore, the drug resistance of the strain was examined by the drug sensitivity test, and the drug resistance genes carried by the strain were detected. [Results] Among the 56 strains isolated, one strain A2 was screened out and identified as Enterococcus farcalis. The strain did not present hemolysis, experienced rapid growth and acid production 3–9 h after inoculation. It could survive at a high temperature of 80 ℃. The survival rate of the strain decreased in the case of below pH 4.0 and was over 78% at pH 10.0, which indicated that the strain had strong alkaline tolerance. The strain demonstrated strong adhesion, with the self-aggregation of 18%, the hydrophobicity of 70%, and the coagglutination of 32%, 34%, and 40% with Escherichia coli, Salmonella, and Staphylococcus aureus, respectively. It was resistant to ampicillin, erythromycin, and vancomycin, moderately sensitive to gentamicin, and sensitive to sulfafurazole, chloramphenicol, tetracycline, and ofloxacin. The strain carried five drug resistance genes conferring resistance to four categories of antibiotics: β-lactam (bla_SHV and bla_TEM), chloramphenicol (cat), aminoglycosides (aph(3')-IIa), and tetracycline (tetE). [Conclusion] The lactic acid bacterial strain isolated from the gut of B. mori was safe, thermostable, and alkali-tolerant and it had strong adhesion. Therefore, it served as a candidate strain for the subsequent development of microbial fermented feed and microecological preparations for silkworm. This study provided a scientific basis and high-quality resources for the development and utilization of gut probiotics.

Key words:Bombyx mori;lactic acid bacteria;biological characteristics;drug resistance gene