Two outbreaks of Shiga toxin-producing Escherichia coli O121:H19 associated with wheat flour, in the United States of America and Canada, involved strains with an unusual phenotype, delayed lactose utilization (DLU). These strains do not ferment lactose when initially cultured on MacConkey agar (MAC), but lactose fermentation occurs following subculture to a second plate of MAC. The prevalence of DLU was determined by examining the β-galactosidase activity of 49 strains of E. coli O121, and of 37 other strains of E. coli. Twenty four of forty three O121:H19 and one O121:NM displayed DLU. Two strains (O121:NM and O145:H34) did not have detectable β-galactosidase activity. β-glucuronidase activity of O121 strains was also determined. All but six DLU strains had normal β-glucuronidase activity. β-glucuronidase activity was suppressed on MAC for 17 of 23 O121 non-DLU strains. Genomic analysis found that DLU strains possessed an insertion sequence, IS600 (1267 bp), between lacZ (β-galactosidase) and lacY (β-galactoside permease), that was not present in strains exhibiting normal lactose utilization. The insert might reduce the expression of β-galactoside permease, delaying import of lactose, resulting in the DLU phenotype. The high probability of DLU should be considered when using lactose-containing media for the isolation of STEC O121.

在美利坚合众国和加拿大爆发了两次与小麦粉相关的产志贺毒素大肠杆菌O121:H19，涉及具有不寻常表型、乳糖利用延迟 (DLU) 的菌株。这些菌株最初在 MacConkey 琼脂 (MAC) 上培养时不发酵乳糖，但在继代培养到第二块 MAC 后会发生乳糖发酵。DLU的患病率通过检查的49株的β半乳糖苷酶活测定大肠杆菌O121，和37的其它菌株大肠杆菌. 43 个 O121:H19 和一个 O121:NM 中的 24 个显示 DLU。两种菌株（O121:NM 和 O145:H34）没有可检测的 β-半乳糖苷酶活性。还测定了 O121 菌株的 β-葡萄糖醛酸酶活性。除了六个 DLU 菌株外，所有菌株都具有正常的 β-葡萄糖醛酸酶活性。对于 23 个 O121 非 DLU 菌株中的 17 个，MAC 上的 β-葡萄糖醛酸酶活性受到抑制。基因组分析发现 DLU 菌株在lacZ（β-半乳糖苷酶）和lacY之间具有插入序列 IS 600（1267 bp）（β-半乳糖苷渗透酶），这在表现出正常乳糖利用的菌株中不存在。插入片段可能会降低 β-半乳糖苷渗透酶的表达，延迟乳糖的输入，导致 DLU 表型。使用含乳糖培养基分离 STEC O121 时，应考虑 DLU 的高概率。