Delayed Lactose Utilisation among Escherichia coli serogroup O121

In this study, the prevalence of this delayed utilisation phenotype was investigated by examining 32 STEC O121 and 37 non-O121 strains for their Beta-galactosidase and Beta-glucuronidase activity. The delayed Beta-galactosidase phenotype was observed in 12 of the 32 O121 strains. Out of the 12 delayed lactose phenotype strains, all but one strain had normal Beta-glucuronidase activity on MacConkey Agar (MAC). For the O121 strains with a normal lactose fermentation, 15 out of 19 had suppressed Beta-glucuronidase activity on MAC. Whole Genome Sequencing was used to examine the lac operon of three strains of the different phenotypes and it was discovered that strains with the delayed lactose utilisation phenotype have a transposable element inserted in the lac operon before the lacY gene, which encodes Beta-galactoside permease, which is required for substrate uptake. The transposable element may cause a reduction in the level of expression of Beta-galactoside permease, which could cause the delayed phenotype as cells without permease in there membrane would be insensitive to the presence of the substrate.本研究通过检测32株STEC O121菌株与37株非O121菌株的β-半乳糖苷酶（Beta-galactosidase）及β-葡萄糖醛酸苷酶（Beta-glucuronidase）活性，对该延迟利用表型（delayed utilisation phenotype）的检出率展开探究。32株O121菌株中，共有12株呈现延迟型β-半乳糖苷酶表型（delayed Beta-galactosidase phenotype）。在上述12株表现出延迟乳糖利用表型的菌株中，仅1株例外，其余所有菌株在麦康凯琼脂（MacConkey Agar, MAC）上均表现出正常的β-葡萄糖醛酸苷酶活性。对于乳糖发酵表型正常的19株O121菌株，其中15株在麦康凯琼脂上的β-葡萄糖醛酸苷酶活性受到抑制。本研究采用全基因组测序（Whole Genome Sequencing）对3株不同表型菌株的乳糖操纵子（lac operon）进行分析，结果显示：携带延迟乳糖利用表型的菌株，其乳糖操纵子内编码β-半乳糖苷透性酶（Beta-galactoside permease）的lacY基因上游插入了一段转座因子（transposable element）——该透性酶是底物摄取所必需的功能蛋白。该转座因子可降低β-半乳糖苷透性酶的表达水平，进而引发延迟利用表型：细胞膜缺乏该透性酶的细胞无法感知底物的存在。