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Frequencies of various virulence factor genes in E. coli strains carrying tet(A) or tet(B) and in strains susceptible to tetracycline. The white bars denote tetracycline-susceptible strains, striped bars tet(A)-positive strains, and black bars tet(B)-positive strains. **, P < 0.01, and ***, P < 0.001, compared to susceptible strains by Fisher's exact test.
