JAC Advance Access published online on September 1, 2003
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkg402
© 2003 by The British Society for Antimicrobial Chemotherapy
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Brief report
1 Animal Sciences Group,
Wageningen University and Research Centre, P.O. Box 65, 8200 AB
Lelystad
* Corresponding author. E-mail: michiel.vanboven{at}wur.nl.
Received 11 May 2003
; revised 3 July 2003
; accepted 4 July 2003
Objective: To determine the within-host
population dynamics of Campylobacter jejuni and Escherichia coli in chickens during and after treatment
with fluoroquinolones. Materials and methods: Total and resistant faecal
counts were determined from cloacal swabs during and after treatment
with enrofloxacin. Chickens were housed individually to avoid confounding
as a result of interaction between animals, and to be able to focus
solely on the within-host dynamics. To determine the molecular basis
of resistance, a number of isolates were checked for mutations in gyrA. Results: Treatment with enrofloxacin at doses
routinely prescribed (50 ppm) rapidly reduced the faecal counts
of E. coli below the detection limit and did not
induce resistance. In C. jejuni, on the other hand,
treatment with enrofloxacin quickly selected for high frequencies
of fluoroquinolone-resistant strains. In all phenotypically resistant
isolates, resistance was traced to mutations in the gyrA gene. Conclusions: (1) A licensed dosage (50 ppm)
of enrofloxacin in drinking water of chickens is effective (i.e. markedly
reduced faecal counts) and is safe on a short time scale in E.
coli (i.e. did not rapidly select for resistance), but is neither
safe nor effective in C. jejuni. (2) The rapid
emergence of resistance to quinolones in C. jejuni does
not necessarily result from horizontal transmission of resistant
strains among chickens, but could solely be the result of de
novo selection of resistance in individual chickens.
Keywords: enrofloxacin, antimicrobial resistance, in
vivo selection, chickens
Rapid selection of quinolone resistance in Campylobacter
jejuni but not in Escherichia coli in individually
housed broilers
2 Central Institute for Animal Disease
Control, CIDC-Lelystad, P.O. Box 2004, 8203 AA Lelystad, The Netherlands
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