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Journal of Clinical Microbiology, December 2000, p. 4593-4598, Vol. 38, No. 12
North Dakota State University, Fargo, North
Dakota1; University of
Georgia3 and the USDA Agricultural
Research Service RRC,4 Athens, Georgia; and the
Center for Veterinary Medicine, U.S. Food and Drug
Administration, Laurel, Maryland2
Received 14 June 2000/Returned for modification 20 August
2000/Accepted 13 September 2000
Florfenicol, a veterinary fluorinated analog of thiamphenicol, is
approved for treatment of bovine respiratory pathogens in the United
States. However, florfenicol resistance has recently emerged among
veterinary Escherichia coli isolates incriminated in bovine
diarrhea. The flo gene, which confers resistance to florfenicol and chloramphenicol, has previously been identified in
Photobacterium piscicida and Salmonella
enterica serovar Typhimurium DT104. The flo gene
product is closely related to the CmlA protein identified in
Pseudomonas aeruginosa. The cmlA gene confers
nonenzymatic chloramphenicol resistance via an efflux mechanism.
Forty-eight E. coli isolates recovered from calves with
diarrhea, including 41 that were both chloramphenicol and florfenicol
resistant, were assayed for the presence of both flo and
cmlA genes. Forty-two of the 44 isolates for which
florfenicol MICs were
0095-1137/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Characterization of Chloramphenicol and Florfenicol
Resistance in Escherichia coli Associated with Bovine
Diarrhea
16 µg/ml were positive via PCR for the
flo gene. All E. coli isolates for which florfenicol MICs were
8 µg/ml were negative for the flo
gene (n = 4). Twelve E. coli isolates were
positive for cmlA, and chloramphenicol MICs for all 12 were
32 µg/ml. Additionally, eight isolates were positive for both
flo and cmlA, and both florfenicol and
chloramphenicol MICs for these isolates were
64 µg/ml. DNA sequence
analysis of the E. coli flo gene demonstrated 98% identity
to the published GenBank sequences of both serovar Typhimurium
floSt and P. piscicida pp-flo. The
flo gene was identified on high-molecular-weight plasmids of approximately 225 kb among the majority of florfenicol-resistant E. coli isolates. However, not all of the
florfenicol-resistant E. coli isolates tested contained the
large flo-positive plasmids. This suggests that several of
the E. coli isolates may possess a chromosomal
flo gene. The E. coli flo gene specifies
nonenzymatic cross-resistance to both florfenicol and chloramphenicol,
and its presence among bovine E. coli isolates of diverse
genetic backgrounds indicates a distribution much wider than previously thought.
*
Corresponding author. Mailing address: Office of
Research, Center for Veterinary Medicine, U.S. Food and Drug
Administration, Laurel, MD 20708. Phone: (301) 827-8037. Fax: (301)
827-8127. E-mail: dwhite{at}cvm.fda.gov.
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