Breeding And Characterization Of High-Yield L-Lactic Acid Producing Bacteria By Microwave Irradiation

Lactate is one of the three major organic acids, widely used in the fields of food, pharmaceutical, chemical, leather, textiles industries etc. Biodegradable polylactide products are being of concern as a result of the serious white forth pollution caused by the white thermoplastic products. It is focused on a strain of Lactobacillus rhamnosus in this paper, by which the L-lactic acid produced has optimal purity and meets the requirements of industrial production.The mutagenesis of Lactobacillus rhamnosus was carried out by the continuous low-power microwave irradiation exploded by a power-adjustable breeding equipment, which has the cooling water cycle to offset the heat of the microwave. Under the microwave power of 400W and irradiation length of 3min, the survival rate of Lactobacillus rhamnosus was 24.1% and the mutagenic effects were good. Three mutant strains W4-3-9, W4-3-28 and W2-3-7 of genetic stability and high-yield L-lactic acid were found by screening, the L-lattic acid production of which were 115.1g/L, 114.8g/L and 114.1g/L, respectively. Comparing with the parent mutant, the improvement of L-lactic acid produced were up to 56.2%,56.6% and 55.7%.It was indicated that some different microbial morphology, physiological biochemical characters and molecular biological aspects existed after microwave radiation between the parent strains and the mutant ones, of which the individual colonies became smaller, the transparent zone was larger and the thickness of colonies became thinner. Also the mutants were easy to assemble and some oil-like substance appeared on the surface of the colonies. The differences of physiological and biochemical characteristics between the parent and mutant strains caused by the microwave radiation mainly manifested in the appearance of gram-negative bacteria and some mutants which can't utilize cellobiose and esculoside. The parent and mutant images of surface morphology and AFM of DNA were observed through the atomic force microscopy for the first time. It provided technical support and theoretical basis for further study of the effect of microwave irradiation on Lactobacillus rhamnosus and the innovation of mutation techniques. And it was firmly showed that microwave irradiation had induced mutation by comparing amplified fragment length polymorphism (AFLP) fingerprints of DNA of the parent strain and the mutant one.