Evolution of multiple antimicrobial drug resistance conservation of genes encoding streptomycin, sulfonamide and tetracycline resistance among Escherichia coli with increasing multi-drug resistance /

by Joseph, Renu.

by Renu Joseph, MS Washington State University December 2007 Chair: Thomas E. Besser Commensal E. coli may serve as a reservoir of resistance genes transferable to pathogens and may also at as foci for “assembly” of multi-drug resistance (MDR). MDR to streptomycin, sulfonamide and tetracycline (R-type SSuT) is particularly frequent among E. coli isolated from cattle, either alone (3-drug resistance) or in combination with additional resistance phenotypes (SSuT+). We hypothesized that SSuT+ strains are assembled from a backbone of conserved SSuT resistance determinants and therefore compared the locations (chromosomal vs. plasmid) of different resistance genes in E. coli isolated from clinically normal cattle. Streptomycin resistance conferring genes (strA, strB) and a sulfonamide conferring gene (sul2) were largely conserved across the range of MDR commensal E. coli. Consistent with the hypothesis, an additional sulfonamide resistance gene, sul1, was detected more frequently in isolates with higher level MDR but iv the distribution of tetracycline resistance genes was inconsistent with the hypothesis as the frequency of tet(B) decreased and the frequency of tet(A) increased in R-type SSuT8+ isolates; relatively few high-level MDR isolates retained tet(B). The locations of the resistance genes were also unconserved, being universally non-plasmid encoded in the R-type SSuT but increasingly plasmid-encoded in isolates with increasing MDR. Together, these observations suggest that R-type SSuT+ isolates do not evolve directly from R-type SSuT isolates and that some other process must account for the conservation of the SSuT phenotype in SSuT+ isolates. v