Characterization of Shiga toxin-producing Escherichia coli isolates associated with two multistate food-borne outbreaks that occurred in 2006

Abstract

Shiga toxin-producing Escherichia coli isolates from two 2006 outbreaks were compared to other O157:H7 isolates for virulence genotype, biofilm formation, and stress responses. Spinach- and lettuce-related-outbreak strains had similar pulsed-field gel electrophoresis patterns, and all carried both stx2 and stx2c variant genes. Cooperative biofilm formation involving an E. coli O157:H7 strain and a non-O157:H7 strain was also demonstrated.

FIG. 1.

Comparison of PFGE patterns of spinach- and lettuce-associated-outbreak strains. The enzyme pattern names and outbreak codes were assigned by the PulseNet team of the CDC.

FIG. 2.

Mean log₁₀ numbers of CFU/ml of strains recovered from biofilms formed on glass in LB-NS ± standard deviations. Strains 06F00475 (serotype O157:H7; spinach isolate) and 06E01767 (serotype O−:H4; spinach isolate), carrying either plasmid pGFPuv or plasmid pUC19, were tested in biofilms individually (plain bars) or as mixed strains (patterned bars). The two plasmids used in the experiment whose results are represented by the bars with broken horizontal lines are reversed from the plasmids used in the experiment whose results are represented by hatched bars. The means were calculated from three independent samples of each strain. Letters in parentheses following the strain designations represent the results of a Bonferroni least square difference means separation. Values for strains with the same letter are not statistically different from each other (P < 0.05). *, the number of CFU was below the limits of detection.