McKay L L, Baldwin K A, Zottola E A
Appl Microbiol. 1972 Jun;23(6):1090-6. doi: 10.1128/am.23.6.1090-1096.1972.
Lactose-negative mutants occurred spontaneously in broth cultures of Streptococcus lactis C(2)F. Instability of lactose metabolism was noted in other strains of S. lactis, in strains of S. cremoris, and in S. diacetilactis. Colonies of S. lactis C(2)F grown with lactose as the carbohydrate source also possessed lac(-) cells. Treatment of lactic streptococci with the mutagen acriflavine (AF) increased the number of non-lactose-fermenting variants. The effect of AF on growth and on loss of lactose-fermenting ability in S. lactis C(2)F was consequently further examined. The presence of AF appears to favor competitively the growth of spontaneously occurring lactose-negative cells and appears to act in the conversion of lactose-positive to non-lactose-fermenting cells. The lactose-negative mutants partially revert to lactose-positive variants which remain defective in lactose metabolism and remain unable to coagulate milk. The lactose-negative cells become dominant in continuous culture growth and provide evidence that alterations in the characteristics of starter strains can be produced by continuous culture, in this case, the complete loss in ability to ferment lactose.
乳糖阴性突变体在乳酸乳球菌C(2)F的肉汤培养物中自发出现。在其他乳酸乳球菌菌株、乳脂乳球菌菌株和双乙酰乳球菌中也发现了乳糖代谢的不稳定性。以乳糖作为碳水化合物来源培养的乳酸乳球菌C(2)F菌落中也存在lac(-)细胞。用诱变剂吖啶黄素（AF）处理乳酸链球菌会增加非乳糖发酵变体的数量。因此，进一步研究了AF对乳酸乳球菌C(2)F生长和乳糖发酵能力丧失的影响。AF的存在似乎在竞争上有利于自发产生的乳糖阴性细胞的生长，并且似乎在乳糖阳性细胞向非乳糖发酵细胞的转化中起作用。乳糖阴性突变体部分回复为乳糖阳性变体，但这些变体在乳糖代谢方面仍然存在缺陷，并且仍然无法使牛奶凝固。在连续培养生长中，乳糖阴性细胞占主导地位，这表明连续培养可以导致发酵剂菌株特性的改变，在这种情况下，就是完全丧失发酵乳糖的能力。
