Distribution and molecular characterization of tetracycline resistance in Laribacter hongkongensis

Laribacter hongkongensis is a newly discovered bacterium associated with gastroenteritis and found in freshwater fish. Although isolates resistant to tetracycline have been described, their resistance mechanisms have not been studied.

We describe the distribution and molecular characterization of tetracycline resistance in 48 L. hongkongensis isolates from humans and fish.

Three human isolates and one fish isolate were resistant to tetracycline (MIC 128 mg/L) and doxycycline (MIC 8-16 mg/L) and had reduced susceptibility to minocycline (MIC 1-4 mg/L). A 3566 bp gene cluster, which contains tetR and tetA, was cloned from one of the tetracycline-resistant strains, HLHK5. While the flanking regions and 3' end of the tetA of HLHK5 were identical to the corresponding regions of a tetC island in Chlamydia suis, the tetA gene was almost identical to that of transposon Tn1721 and plasmids of gram-negative bacteria, suggesting that the tetA/tetR of HLHK5 may have arisen from illegitimate recombination. PCR and DNA sequencing showed the presence of tetA in the other three tetracycline-resistant L. hongkongensis strains. Sequencing and characterization of a 15,665 bp plasmid, pHLHK22, from strain HLHK22 revealed the presence of a similar tetA/tetR gene cluster. This novel plasmid also confers tetracycline resistance when transformed to Escherichia coli and other L. hongkongensis isolates.

Horizontal transfer of genes, especially through Tn1721 and related plasmids, is likely an important mechanism for acquisition and dissemination of tetracycline resistance in L. hongkongensis. The present study is the first report on identification of tetA genes in bacteria of the Neisseriaceae family.