Escherichia coli usually cause transient intramammary infections in dairy cows, but
persistent intramammary infections have been observed. The objective ofthe study was to
compare antimicrobial resistance and virulence genes found in persistent and transient E.
coli isolated from clinical mastitis cases in a cohort of 91 Canadian dairy herds monitored
over a 2-year period. Antimicrobial susceptibility was determined by broth microdilution
and the presence of 27 virulence genes associated with extra-intestinal E. coli infections
was determined by colony hybridization. Proportion of resistance in persistent E. coli
ranged from 0.0% (enrofloxacin) to 27.8% (ampicillin and tetracycline). Proportion of
resistance in transient E. coli ranged from 0.0% (enrofloxacin) to 16.8% (tetracycline). Odds
of being classified as a persistent isolate increased by a factor of 1.6 (95% CI: 1.1, 2.4) for
each aditional resistance observed (e.g. isolates resistant to four antimicrobial agents had
1.6 times higher odds of belonging to the persistent groups compared to isolates
demonstrating resistance to three agents). Persistency was associated with higher odds of
resistance to ampicillin (OR: 9.8, P < 0.01) or cephalothin (OR: 7.6, P = 0.02). Persistent
isolates had 5.4 times higher odds (95% CI: 1.2, 24.0) of harboring virulence gene iroN.
Similarly, persistent isolates had 8.6 times higher odds (95% CI: 2.8, 27.1) ofpossessing the
virulence gene sitA. In conclusion, this study confirmed that persistency ofintramammary
E. coli isolates is associated with certain traits. Findings concerning iron-acquisition shed
new light on the mechanisms of intramammary survival.