TetAB46, a predicted heterodimeric ABC transporter conferring tetracycline resistance in Streptococcus australis isolated from the oral cavity

Abstract

Objectives: To identify the genes responsible for tetracycline resistance in a strain of Streptococcus australis isolated from pooled saliva from healthy volunteers in France. S. australis is a viridans Streptococcus, originally isolated from the oral cavity of children in Australia, and subsequently reported in the lungs of cystic fibrosis patients and as a cause of invasive disease in an elderly patient.

Methods: Agar containing 2 mg/L tetracycline was used for the isolation of tetracycline-resistant organisms. A genomic library in Escherichia coli was used to isolate the tetracycline resistance determinant. In-frame deletions and chromosomal repair were used to confirm function. Antibiotic susceptibility was determined by agar dilution and disc diffusion assay.

Results: The tetracycline resistance determinant from S. australis FRStet12 was isolated from a genomic library in E. coli and DNA sequencing showed two open reading frames predicted to encode proteins with similarity to multidrug resistance-type ABC transporters. Both genes were required for tetracycline resistance (to both the naturally occurring and semi-synthetic tetracyclines) and they were designated tetAB(46).

Conclusions: This is the first report of a predicted ABC transporter conferring tetracycline resistance in a member of the oral microbiota.

Figure 1.

Diagram of the cloned genomic DNA fragment from S. australis FRStet12, indicated by the black line. Five putative orfs are indicated in grey [tetA(46); tetB(46); orf3, encodes a putative metalloprotease; orf4, encodes a putative diacylglycerol kinase; and orf5, encodes a putative GTP-binding protein—accession number HQ652506]. The vertical broken black lines indicate the point of the in-frame deletions in tetA(46) and tetB(46) and the sequence of the deletion is given beneath.