FIG. 2.

Estimated probabilities of encountering the three PFGE subtype categories during antemortem sampling. The probability of encountering an E. coli O157:H7 isolate belonging to the dominant subtype, a subtype differing from the dominant subtype by 1 to 3 bands, or a subtype differing from the dominant subtype by 4 to >7 bands is indicated on the y axis, while the collections are indicated on the x axis. Isolates possibly related (4- to 6-band difference) to the dominant subtype and isolates divergent (>7-band difference) from the dominant subtype were collapsed to form the 4- to >7-band difference subtype category due to low numbers. The low frequency of the 4- to >7-band difference subtype category prevented inclusion of this category in the analysis, which prevented calculation of errors. The error bars indicate the standard error calculated for each estimated probability. For each independent collection, the estimated probability of encountering the dominant subtype was greater (P < 0.05) than the probability of encountering strains that differed by one to three bands (indicated by an asterisk). Also, for an individual collection, a dagger indicates that the probability of encountering the dominant subtype and subtypes with 1 to 3 band differences was greater (P < 0.05) than the probability of encountering subtypes with 4- to >7-band differences. Dif., differences.