The distribution of the DHFR genes in trimethoprim-resistant urinary tract isolates from Taiwan

Chang, Lin‐Li; Chang, Shui-Feng; Chow, Teh-Yuan; Wu, Wen‐Jeng; Chang, Jong-Chou

doi:10.1017/s0950268800050445

Cited by 8 publications

(12 citation statements)

References 29 publications

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“…It is difficult to predict how widely distributed dfr13 may be in other parts of the world. However, since this DHFR was extremely widespread in South Africa and was detected in all the rural and urban population groups, it is thought that this DHFR may be widely distributed in other parts of the world and may account for a proportion of the 10 to 68% of DHFR genes which were not identified in a number of studies (13,23,25,30,44,49). On the basis of the similarity between the sequence and inhibitor profile of the type 12 and 13 DHFRs, it is clear that these enzymes should be regarded as a monophyletic group (family 3) since they share similar degrees of homology to that found within the type I-like and type II-like DHFR enzyme families.…”

Section: Discussionmentioning

confidence: 99%

New Gene Cassettes for Trimethoprim Resistance, dfr13 , and Streptomycin-Spectinomycin Resistance, aadA4 , Inserted on a Class 1 Integron

Adrian

Thomson

Klugman

et al. 2000

Antimicrob Agents Chemother

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In a previous survey of 357 trimethoprim-resistant isolates of aerobic gram-negative bacteria from commensal fecal flora, hybridization experiments showed that 25% (90 of 357) of the isolates failed to hybridize to specific oligonucleotide probes for dihydrofolate reductase types 1, 2b, 3, 5, 6, 7, 8, 9, 10, and 12. Subsequent cloning and sequencing of a plasmid-borne trimethoprim resistance gene from one of these isolates revealed a new dihydrofolate reductase gene, dfr13, which occurred as a cassette integrated in a site-specific manner in a class 1 integron. The gene product shared 84% amino acid identity with dfr12 and exhibited a trimethoprim inhibition profile similar to that of dfr12. Gene probing experiments with an oligonucleotide probe specific for this gene showed that 12.3% (44 of 357) of the isolates which did not hybridize to probes for other dihydrofolate reductases hybridized to this probe. Immediately downstream of dfr13, a new cassette, an aminoglycoside resistance gene of the class AADA [ANT(3؆)(9)-I], which encodes streptomycin-spectinomycin resistance, was identified. This gene shares 57% identity with the consensus aadA1 (ant(3؆)-Ia) and has been called aadA4 (ant(3؆)-Id). The 3 end of the aadA4 cassette was truncated by IS26, which was contiguous with a truncated form of Tn3. On the same plasmid, pUK2381, a second copy of IS26 was associated with sul2, which suggests that both integrase and transposase activities have played major roles in the arrangement and dissemination of antibiotic resistance genes dfr13, aadA4, bla TEM-1 , and sul2.Class 1 integrons are DNA elements that encode a sitespecific recombinase (intI1) of the family of integrases and are capable of catalyzing the insertion and recombination of mobile DNA elements (gene cassettes) at specific sites (core ite) with a consensus sequence, GTTRRRY. The integron also provides a 5Ј conserved core site, attI1, into which gene cassettes are preferentially inserted, and the promoters responsible for expression of the cassette-encoded genes (10,19). Gene cassettes are identified by a 59-base element which occurs at the 3Ј end of the cassette and which consists of an inverted imperfect repeat of between 50 and 150 bp which has an inverse core site at the 5Ј end of the inverted repeat and a core site at the 3Ј end. The insertion of a gene cassette into the attI1 site results in the formation of a secondary site (attC) downstream of the cassette. The attI1 site differs from attC sites in that the inverse core site and secondary structure are missing 5Ј of the attI1 core site (21). The majority of published cassettes contain genes for resistance to various antimicrobial agents including trimethoprim and aminoglycosides (41). Trimethoprim selectively inhibits the bacterial dihydrofolate reductase (DHFR), thus preventing reduction of dihydrofolate to tetrahydrofolate (11). The most common mechanism of resistance to trimethoprim in enterobacteria is the production of an additional plasmid-mediated DHFR which, unlike the chromosomal enzyme, i...

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Section: Discussionmentioning

confidence: 99%

New Gene Cassettes for Trimethoprim Resistance, dfr13 , and Streptomycin-Spectinomycin Resistance, aadA4 , Inserted on a Class 1 Integron

Adrian

Thomson

Klugman

et al. 2000

Antimicrob Agents Chemother

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“…Radioactively labeled fragment probes were also used to detect DHFR type I, II, III, and V DNA in 374 enterobacteria from persons with urinary tract infections in Taiwan (45). DHFR type I was found in 45.4% of the isolates, DHFR type V was found in 10.4% of the isolates, whereas no hybridization was observed with the probes specific for DHFR type II and III.…”

Section: Resistance To Trimethoprimmentioning

confidence: 99%

“…All plasmids carrying DFHR type Ia also hybridized with the Tn7-specific probe, whereas plasmids harboring DHFR type Ib or V hybridized with a probe specific for Tn21-like transpsons. A Taiwanese study observed that 7.8% of 374 trimethoprinresistant isolates also carried Tn7, but no link with the DFHR types studied was made (45). It is not known whether these isolates harbor integrons.…”

Section: Multidrug Resistancementioning

confidence: 99%

Molecular Detection of Antimicrobial Resistance

2001

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The determination of antimicrobial susceptibility of a clinical isolate, especially with increasing resistance, is often crucial for the optimal antimicrobial therapy of infected patients. Nucleic acid-based assays for the detection of resistance may offer advantages over phenotypic assays. Examples are the detection of the methicillin resistance-encoding mecA gene in staphylococci, rifampin resistance in Mycobacterium tuberculosis, and the spread of resistance determinants across the globe. However, molecular assays for the detection of resistance have a number of limitations. New resistance mechanisms may be missed, and in some cases the number of different genes makes generating an assay too costly to compete with phenotypic assays. In addition, proper quality control for molecular assays poses a problem for many laboratories, and this results in questionable results at best. The development of new molecular techniques, e.g., PCR using molecular beacons and DNA chips, expands the possibilities for monitoring resistance. Although molecular techniques for the detection of antimicrobial resistance clearly are winning a place in routine diagnostics, phenotypic assays are still the method of choice for most resistance determinations. In this review, we describe the applications of molecular techniques for the detection of antimicrobial resistance and the current state of the art

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“…The most widespread accessory DHFR gene among gram-negative bacteria seems to be dhfrI, which is commonly attributed to the successful spread of its carrier transposon, Tn7 (20,63,64,66,76,127,149). This spread could be due to the capability of Tn7 for high-frequency insertion into a preferred site in the glmS terminator on the E. coli chromosome (28,92).…”

Section: Mechanisms Of Resistancementioning

confidence: 99%