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.