Purpose: To 1) determine whether magnetic beads available for five Escherichia coli O groups (O157, O26, O145, O111, and O103) can be combined in a single IMS assay, and 2) evaluate if O group differentiation is possible using a chromogenic agar.
Methods: A cocktail of the five O groups was prepared by incubating each strain in tryptic soy broth for ca. 18 h at 37°C, combining the strains in equal aliquots and performing serial dilutions to achieve final culture concentrations of ca. 3 and 4 log CFU/ml. Each concentration of cocktail was subjected to IMS using two levels (20 and 10ml of magnetic beads with beads of the five O groups combined for a single IMS. Recovered cells were plated on modified Rainbow agar and incubated for ca. 24 h at 37°C. Colonies were confirmed using latex agglutination and counted to evaluate the ability to recover all O groups simultaneously on one plate.
Results: Statistical analysis revealed no significant difference (P > 0.005) between the two volumes of beads at any of the bacterial concentrations tested. Difference between the cells recovered by each microorganism was found (P< 0.05); however, serogroups O157, O26, O145, and O103 were recovered as expected based on IMS detection limits, and O111 did not grow at expected concentrations.
Significance: This study revealed that a simultaneous IMS could be conducted to recover E. coli O157, O26, O145, and O103 at once using a selective media that allows for O group differentiation. Because the amount of beads can be reduced and less time is required this could be considered as a cost and time efficient alternative to isolation of STECs one strain at a time.