B.W. Shaheen1; D.M. Boothe1; O.A. Oyarzabal2; T. Smaha1
The incidence of multidrug resistant E. coli as a cause of infection in companion animals is increasing. The purpose of this study was to phenotypically and genotypically describe a sample population of E. coli associated with spontaneous disease in dogs and cats. A total of 377 isolates studied between May and September 2005. Isolates were collected from 4 different regions: West, South, Midwest, and Northeast. The isolates were phenotypically characterized based on susceptibility to 7 antimicrobialagents by E-test® according to CLSI guidelines and interpretive standards: amoxicillin (A), amoxicillin-clavulanic acid (X), cefpodoxime (P), doxycycline, enrofloxacin, gentamicin, trimethoprim-sulfamethoxazole. Isolates from each phenotype were randomly selected for genotypical characterization using pulse-field gel electrophoresis (PFGE) using XbaI restriction enzyme; dendrograms were generated using the Dice correlation coefficient and the unweighted pair group mathematical average clustering algorithm of BionumericsTM. Of the 377, 183 were susceptible to all drugs, with the proportion greatest in the west (68%; 49/79) and least in the south (37.5 %; 50/133). Isolates collected from the ear were characterized by the greatest proportion of susceptibility (70%; 19/27) whereas the skin had the least number of isolates susceptible to all drugs (41%, 11/27). Of the 194 isolates expressing resistance, 85 expressed single drug resistance (SDR), with resistance to beta-lactams (any combination of A, X or P) predominating (83%; n=71/85 [30% A, 28% X, and 24% AXP]. Only two isolates expressed SDR to E. The remaining 109 isolates expressed MDR (resistance to more than one drug class), with resistance to all 7 drugs (Z) representing the largest proportion (18.3%; 20/109). The remaining MDR were represented by 34 different phenotypes. The drugs most commonly involved in MDR (n=109) were A (96.3 %), X (85%) and E (61 %). Of isolates expressing resistance, MDR was greater in the south region (67.9%; 57/84 MDR) and least in the west, (46.6 %, 14/30 MDR). Among infection sites, isolates cultured from the skin were characterized by the highest percent of MDR (12/16; 75%). A total of 82 PFGE patterns were generated among the 90 isolates for which PFGE was determined. Dendrograms revealed 5 profiles (based on > 90% similarity), representing 14 isolates, 6 tissues, and 2 regions. These results indicating extensive genetic diversity across recovered E. coli isolates, regardless of resistance phenotype, and phenotypes and genotypes are not related. The south appears to have a greater incidence of resistance, including MDR, and the west the least for both. MDR most commonly involves amoxicillin with or without clavulanic acid, and enrofloxacin.