B.W. Shaheen1; D.M. Boothe1; O.A. Oyarzabal2; T. Smaha1
Resistant E. coli is emerging as an organism that develops multidrug resistance (MDR) in the face of antimicrobial therapy, as we have documented previously in dogs and cats with spontaneous infections (ACVIM 2007). Integrons associated with plasmids may carry genes that impart MDR. Mechanisms conferred by intergrons include altered cell permeability drug efflux or an increased rate of mutation that confers other mechanisms of resistance. The purpose of this study was to investigate the potential role of class 1 integrons in the emergence of MDR in canine and feline E. coli clinical isolates collected from 4 different regions in US. A total of 377 E. coli isolates associated with spontaneous (presumed) infection were collected from dogs or cats between May and September 2005. Susceptibility to 7 drugs was determined using the E-test® according to CLSI guidelines and interpretive standards: amoxicillin, amoxicillin-clavulanic acid, cefpodoxime, doxycycline, enrofloxacin, gentamicin, trimethoprim-sulfamethoxazole. A total of 32 E. coli isolates representing 7 different phenotypes were randomly selected to determine the presence of class 1 integrons, and, in order to determine the location of the integron, plasmids. Phenotypes included those resistant to no drug (n=10), resistance to one drug (SDR; n=8) or resistance to more than one drug class (MDR; n=14, with n=12 for all 7 drugs tested). Class 1 integrons were characterized for the 5' and 3' conserved segments (CS) by polymerase chain reaction (PCR) and direct sequencing. PCR analysis revealed that 59.4% (19/32) of the isolates contained class 1 integrons. MIC90 for integron-positive isolates were more than fourfold higher than the break point for all drugs tested. This is in contrast to integron-negative isolates for which MIC were below the breakpoints for doxycycline, enrofloxacin, gentamicin, and trimethoprim-sulfamethoxazole. Integron- positive isolates were also more likely to be multi-resistant 73.7% (14/19); only 1 of the MDR isolates did not have a class I integrons. This is in contrast with integron-negative isolates for which 7.7% (1/13) were MDR. However, three isolates with no resistance also carried the integrons. The majority 94.7% (18/19) of classes 1 integrons were detected in the plasmids of the same integron-positive isolates. This association indicates that integrons may confer high MIC level, multidrug resistance. However, the presence of the integron is not necessarily associated with phenotypic manifestation of the genetically-directed resistance. Class 1 integrons might play an important role in contributing to the horizontal transfer of antimicrobial resistance.