Department of Plant Pathology and Microbiology, Texas A&M University, 2132 TAMU, College Station, TX 77843-2132, USA
Sir,
Resistance to the antibiotic streptomycin is an interesting case study due to the length of time since the discovery and introduction of streptomycin into clinical medicine (c. 55 years), and the continuing varied usage of this antibiotic in clinical medicine, animal husbandry and on crop plants to control certain plant diseases. The linked strAstrB streptomycin resistance (SmR) genes are notable because these genes are distributed among commensal and pathogenic bacteria isolated from humans, animals and plants.1 Isolates from humans and animals typically carry strAstrB on small broad-host-range non-conjugative plasmids such as RSF1010 or pBP1, while isolates from plants typically harbour strAstrB on large conjugative plasmids encoded within the transposon Tn5393.1 Tn5393 is a 6.7 kb Tn3- family transposon, which encodes the strAstrB genes downstream of the tnpR resolvase gene; closely related variants may contain IS1133 or IS6100, insertion elements that provide promoters to express strAstrB, depending upon the species of isolation.2
Recently, several discoveries have expanded the breadth of host range of the strAstrB genes associated with Tn5393 sequences among plant-associated bacteria and clinical pathogens. A report detailing the complete sequence of pTP10, a 51.4 kb multiple antibiotic resistance (AbR) plasmid from the Gram-positive opportunistic human pathogen Corynebacterium striatum, revealed the presence of an apparently complete Tn5393 sequence which was interrupted in two places by the insertion of another transposon, Tn5715, and the insertion element IS1250.3 The Tn5393 sequences were 100% identical to the previously determined Tn5393 sequence from the plant pathogen Erwinia amylovora. Sequence remnants of Tn5393 elements linked to other AbR genes have also recently been detected in the chromosome of clinical isolates of Campylobacter jejuni and Pseudomonas aeruginosa.4,5
We analysed the strAstrB sequences currently published to determine if any sequence polymorphisms existed that could be useful in epidemiological studies to track the dissemination of this determinant. While the known strB sequences are 100% identical, a total of 5 bp of 804 bp are polymorphic among the strA sequences (Table). Since the majority of the strA sequences currently published were from non-conjugative plasmids, we cloned and sequenced strA from Tn5393 of plant pathogenic isolates of Pseudomonas syringae (A2, 7B12, 8C32) from the USA6 and Xanthomonas campestris (BV5-4a) from Argentina.2 Nucleotide sequencing was carried out using the Big Dye kit (ABI, Foster City, CA, USA) following the instructions of the manufacturer; sequence reactions were run at the Genetic Technologies Center, Texas A&M University. The individual strA nucleotide sequences generated in this study have been deposited in GenBank (accession numbers M77502, U20588, AF273681 and AF273682).
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The strAstrB sequence on RSF1010 is linked to the right inverted repeat of Tn5393,1 suggesting that the ancestral plasmid from which RSF1010 evolved obtained strAstrB via the insertion of Tn5393. Following the evolution of RSF1010 and other non-conjugative SmR plasmids, the strA sequence has diverged from that harboured on intact Tn5393 elements currently distributed among plant pathogenic bacteria. However, the recent discovery of the Tn5393 sequence on a C. striatum plasmid, and sequence remnants in C. jejuni and P. aeruginosa, indicates that the magnitude of gene transfer events of Tn5393-like elements has encompassed bacteria inhabiting diverse ecological niches, and has also included Gram-positive organisms. Along these lines, we had previously shown that the IS6100 element present in Tn5393 from the plant pathogen Xanthomonas campestris is 100% identical to an element also found in Mycobacterium fortuitum.2
The experience with Tn5393 and strAstrB-mediated streptomycin resistance is important because it offers an illustration of the wide range of transferability of an AbR determinant, of how a large ecological diversity of host organisms can contribute to the long-term stability of an AbR determinant and, most notably, how resistance development is driven by natural processes of population dynamics which are not always responding to direct selection pressure. Thus, knowledge of the ecology of individual AbR determinants can provide a depth of information on the strategic response of organisms exposed to the selective pressure of antibiotic usage, and can also provide insight into evolutionary dynamics involving relative fitness aspects at the organism, accessory element (plasmid, transposon, integron) and individual gene levels.
Notes
J Antimicrob Chemother 2000; 46: 848849
* Tel: + 1-979-862-7518; Fax: + 1-979-845-6483; E-mail: gsundin{at}acs.tamu.edu
References
1 . Sundin, G. W. & Bender, C. L. (1996). Dissemination of the strAstrB streptomycin-resistance genes among commensal and pathogenic bacteria from humans, animals, and plants. Molecular Ecology 5, 13343.[ISI][Medline]
2 . Sundin, G. W. & Bender, C. L. (1995). Expression of the strAstrB streptomycin-resistance genes in Pseudomonas syringae and Xanthomonas campestris and characterization of IS6100 in X. campestris. Applied and Environmental Microbiology 61, 28917.[Abstract]
3 . Tauch, A., Krieft, S., Kalinowski, J. & Puhler, A. (2000). The 51,409-bp R-plasmid pTP10 from the multiresistant clinical isolate Corynebacterium striatum M82B is composed of DNA segments initially identified in soil bacteria and in plant, animal, and human pathogens. Molecular and General Genetics 263, 111.[ISI][Medline]
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Gibreel, A. & Skold, O. (1998). High-level resistance to trimethoprim in clinical isolates of Campylobacter jejuni by acquisition of foreign genes (dfr1 and dfr9) expressing drug-insensitive dihydrofolate reductases. Antimicrobial Agents and Chemotherapy 42, 305964.
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Naas, T., Sougakoff, W., Casetta, A. & Nordmann, P. (1998). Molecular characterization of OXA-20, a novel class D ß-lactamase, and its integron from Pseudomonas aeruginosa. Antimicrobial Agents and Chemotherapy 42, 207483.
6 . Sundin, G. W., Demezas, D. H. & Bender, C. L. (1994). Genetic and plasmid diversity within natural populations of Pseudomonas syringae with various exposures to copper and streptomycin bactericides. Applied and Environmental Microbiology 60, 442131.[Abstract]