Relationship between Phenotypic and Genotypic Florfenicol Resistance in Escherichia coli

Abstract

This study evaluated the relationship between florfenicol resistance and flo genotypes in 1,987 Escherichia coli isolates from cattle. The flo gene was detected in 164 isolates, all of which expressed resistance to florfenicol at MICs of >256 g/ml. The florfenicol MICs for all isolates that lacked flo were <16 g/ml.Florfenicol is a fluorinated analog of chloramphenicol approved by the Food and Drug Administration in 1996 for the treatment of bovine respiratory disease (BRD) pathogens. The main florfenicol targets in respiratory diseases of cattle are Pasteurella multocida, Mannheimia haemolytica, and Haemophilus somnus. Florfenicol is occasionally used extra-label in the treatment of calf diarrhea (10) and can be effective against Escherichia coli strains isolated from diarrheic calves (9). Florfenicol binds to the 50S subunit of the bacterial ribosome and disrupts protein synthesis. E. coli strains have been recovered with a gene known as flo that expresses resistance to florfenicol and chloramphenicol and is typically located on large transferable plasmids (7). Neither the enzyme chloramphenicol acetyltransferase nor the nonenzymatic chloramphenicol resistance gene cmlA confers resistance to florfenicol (10).Past studies documented florfenicol resistance in animal isolates of E. coli. White et al. (10) found that for 44 of 48 neonatal calf diarrhea E. coli isolates, there was decreased susceptibility to florfenicol, flo was present, and the florfenicol MIC range was 16 to Ն256 g/ml. For 4 of the 48 E. coli isolates, cmlA was present, flo was not present, and the florfenicol MIC range was Յ8 to 64 g/ml. Keyes et al. (6) analyzed chloramphenicol-resistant E. coli from sick chickens and found that some isolates possessed the flo gene. The florfenicol MICs for isolates harboring flo were Ն32 g/ml, whereas the florfenicol MICs for isolates without flo were Յ8 g/ml. Bischoff et al. (1) analyzed 48 chloramphenicol-resistant E. coli isolates from cases of neonatal swine diarrhea. The florfenicol MIC for one isolate, which possessed flo, was 256 g/ml. The florfenicol MICs for the remaining 47 isolates ranged from 8 to Ն16 g/ml.Currently, there are no National Committee for Clinical Laboratory Standards (NCCLS) MIC breakpoints approved to indicate florfenicol resistance in E. coli, and many studies (1, 10) use MIC breakpoints for the BRD pathogens. When these guidelines are used, an MIC of Ն8 g/ml indicates resistance, an MIC of 4 g/ml indicates intermediate susceptibility, and an MIC of Յ2 g/ml indicates susceptibility. Previous studies demonstrated that the presence of flo is associated with very high florfenicol MICs (1, 6, 10). Therefore, we hypothesized that MIC breakpoints for BRD pathogens would not be useful in correlating the presence of flo in E. coli isolates with the observed florfenicol MIC. The objective of this study was to evaluate the relationship between florfenicol resistance phenotypes and flo genotypes in commensal bovine isolates of E. coli.During an intensive, longitudinal study of antimicrobial ...

Assessing the effect of a single dose florfenicol treatment in feedlot cattle on the antimicrobial resistance patterns in faecal Escherichia coli

“…We hypothesize that this is due to selection of E. coli with a plasmid or chromosomally located flo gene conferring resistance to florfenicol and chloramphenicol [27,33]. Additionally, florfenicol treatment selected for E. coli isolates that exhibited simultaneous resistance to several other antimicrobial classes.…”

“…Florfenicol is a bacteriostatic antimicrobial with similar mode of action to chloramphenicol, binding to the 50S subunit of bacterial ribosomes. The flo gene confers resistance to chloramphenicol and florfenicol, and is most commonly located on a large transferable plasmid in bovine enteric E. coli [27,33].…”

Development of a Polymerase Chain Reaction Assay for the Detection of Antibiotic Resistance Genes in Community DNA

“…Consequently, detection of flo in a sample can indicate whether colonies are likely to be florfenicol resistant. 37 In large epidemiologic studies, many samples are evaluated for the presence of resistant organisms. In addition to screening many different bacteria for specific resistances, community DNA approaches could quickly indicate whether specific resistance genes are present within any of the bacteria in the community of the sample.…”

Antibiotic Resistance Patterns in Faecal E. coli: A Longitudinal Cohort-Control Study of Hospitalized Horses

“…However florfenicol resistance has been documented in E. coli of animal origin (Singer et al, 2004) and it has been shown that floR genetic determinants and others (i.e. cmlA, cat1, cat2) were also largely related to florfenicol resistance (Li et al, 2007).…”