Role of coresistance in the development of resistance to chloramphenicol in Escherichia coli isolated from sick cattle and pigs.
OBJECTIVE
To determine the cause of persistent resistance to chloramphenicol (CP) after the ban on its use in food-producing animals in several countries.


METHODS
71 CP-resistant and 104 CP-susceptible Escherichia coli strains isolated from sick cattle and pigs in Japan.


METHODS
Susceptibility of all bacterial strains to thiamphenicol (TP) and florfenicol (FFC) was tested by use of an agar dilution method. The CP-resistance genes and variable region within class 1 integrons in CP-resistant strains were identified by use of a PCR assay.


RESULTS
The CP acetyltransferase gene (ie, cat1) was identified as the predominant CP-resistance gene in strains isolated from cattle, and the cat1and nonenzymatic CP-resistance gene (ie, cmlA) were the predominant CP-resistance genes in strains isolated from pigs. Additionally, strains with cat1 isolated from cattle often were resistant to ampicillin, dihydrostreptomycin (DSM), oxytetracycline, and trimethoprim (TMP), whereas strains with cat1 or cmlA isolated from pigs often were resistant to DSM and TMP. Class 1 integrons were significantly more prevalent in strains with CP-resistance genes, compared with prevalence in strains without CP-resistance genes. All gene cassettes within the integrons were involved in resistance to DSM, TMP, or both.


CONCLUSIONS
Coresistance that develops because of the use of DSM and TMP in cattle and pigs apparently contributes to the selection of CP-resistant strains of E coli. Thus, it is possible that bacterial resistance to CP in animals would persist despite a ban on the use of CP in cattle and pigs.
Harada
Kazuki
K
National Veterinary Assay Laboratory, Ministry of Agriculture, Forestry and Fisheries, 1-15-1 Tokura, Kokubunji, Tokyo 185-8511, Japan.
Asai
Tetsuo
T
Kojima
Akemi
A
Ishihara
Kanako
K
Takahashi
Toshio
T
D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D002417:Cattle; D002418:Cattle Diseases / Q000453:epidemiology / Q000382:microbiology*; D002701:Chloramphenicol / Q000494:pharmacology*; D002702:Chloramphenicol Resistance / Q000235:genetics / Q000502:physiology*; D024881:Drug Resistance, Bacterial; D004926:Escherichia coli / Q000187:drug effects* / Q000235:genetics; D004927:Escherichia coli Infections / Q000382:microbiology / Q000662:veterinary*; D032023:Integrons; D007564:Japan / Q000453:epidemiology; D007880:Legislation, Drug; D008826:Microbial Sensitivity Tests; D013552:Swine; D013553:Swine Diseases / Q000453:epidemiology / Q000382:microbiology*
D000900:Anti-Bacterial Agents; D002701:Chloramphenicol