Molecular characterization of tetracycline-resistant genes and integrons from avirulent strains of Escherichia coli isolated from catfish.
A study was undertaken to investigate the occurrence of tetracycline-resistant genes and to characterize the integrons present in Escherichia coli isolated from catfish. Sixty-three tetracycline-resistant E. coli strains were isolated from the intestinal contents of 407 farm-raised catfish. All strains were resistant to multiple antibiotics. A polymerase chain reaction (PCR) assay detected tetA in the DNA of 15 of 63 (25.0%) isolates by amplifying a PCR amplicon measuring 957 bp. Oligonucleotide primers targeting a 436-bp region of tetB successfully amplified a PCR amplicon from 47 of 63 (77.0%) isolates, indicating that tetB was predominant. Oligonucleotide primers specific for tetC amplified a 589-bp PCR amplicon from 3 of 63 (5%) isolates. Eleven (17.0%) of the isolates contained both tetA and tetB genes. Class I integrons amplified from the genomic DNA of 14 of 63 (22.0%) isolates measured 1.6 and 1.8 kb. Sequence analysis of the 1.6 kb integrons indicated the presence of three different gene cassettes: a dfrA12, conferring resistance to trimethoprim; an open reading frame, orfF, a hypothetical protein of unknown function; and aadA2, conferring resistance to aminoglycosides. Sequence analysis of the 1.8-kb integron indicated the presence of dfrA17 and aadA5. PCR assays for the detection of the six predominant virulence genes failed to amplify any genes from the genomic DNA. Pulsed-field gel electrophoresis using XbaI identified 16 distinct macro restriction patterns among the 63 isolates. The dendrogram analysis indicated that the DNA from 4 of 16 isolates had a similarity index of 90.0%. Our results indicate that the use of oxytetracycline and Romet 30 (sulfadimethoxine and ormetoprim) in farm-raised catfish may select for multiple antibiotic-resistant E. coli that could serve as a reservoir of tetracycline, trimethoprim, and aminoglycoside resistance genes.
Nawaz
Mohamed
M
Division of Microbiology, National Center for Toxicological Research, Jefferson, Arkansas 72079, USA. mohamed.nawaz@fda.hhs.gov
Khan
Ashraf A
AA
Khan
Saeed
S
Sung
Kidon
K
Kerdahi
Khalil
K
Steele
Roger
R
D000818:Animals; D002397:Catfishes / Q000382:microbiology*; D004269:DNA, Bacterial / Q000032:analysis; D004926:Escherichia coli / Q000187:drug effects / Q000235:genetics* / Q000302:isolation & purification; D032023:Integrons / Q000235:genetics*; D007422:Intestines / Q000382:microbiology; D016133:Polymerase Chain Reaction; D013753:Tetracycline Resistance / Q000235:genetics*
D004269:DNA, Bacterial