为获得高产凝乳酶的菌株,以市售干酪为原料,从中分离得到1株具有高凝乳活力的菌株XC-1。通过生理生化实验、16S rDNA同源性序列分析,鉴定该菌株为枯草芽孢杆菌(Bacillus subtilis)。采用N+注入法对筛选出的菌株进行诱变,确定最佳诱变条件为能量20 keV、剂量2.08×1015ions/cm2,获得1株特性优良的诱变菌株XCYB-6,其凝乳活力达到2 181.82 SU/mL,比出发菌株提高65.63%,蛋白水解活力为2.53 U/mL,凝乳活力与蛋白水解活力的比值高达862.38。传代(6代)实验表明,诱变菌株具有良好的遗传稳定性。

A strain XC-1 with high milk-clotting activity was isolated from the merchant cheese.Using the physiological and biochemical identification and 16S rDNA sequencing,XC-1 was identified as Bacillus subtilis.It was the first time that N+ Ion beam implantation was applied for mutagenesis of Bacillus subtilis.Using XC-1 as parent strain,the mutation experiment showed that the dose of 2.08×1015 ions/cm2 with the power of 20 keV was optimal for mutagenesis of Bacillus subtilis and a mutant strain XCYB-6 was screened.The milk-clotting activity of XCYB-6 reached 2181.82 SU/mL and was 65.63% higher than that of the parent strain.Meanwhile the proteolytic activity of XCYB-6 was 2.53 U/mL.The ratio of milk-clotting activity to proteolytic activity reached 862.38.The pass-generation(six generations) test indicated that XCYB-6 had good genetic stability.