Analysis of genetic localization of the type I trimethoprim resistance gene from Escherichia coli isolated in Finland

Revitalizing antifolates through understanding mechanisms that govern susceptibility and resistance

In prokaryotes and eukaryotes, folate (vitamin B9) is an essential metabolic cofactor required for all actively growing cells. Specifically, folate serves as a one-carbon carrier in the synthesis of amino acids (such as methionine, serine, and glycine), N-formylmethionyl-tRNA, coenzyme A, purines and thymidine. Many microbes are unable to acquire folates from their environment and rely on de novo folate biosynthesis. In contrast, mammals lack the de novo folate biosynthesis pathway and must obtain folate from commensal microbiota or the environment using proton-coupled folate transporters. The essentiality and dichotomy between mammalian and bacterial folate biosynthesis and utilization pathways make it an ideal drug target for the development of antimicrobial agents and cancer chemotherapeutics. In this minireview, we discuss general aspects of folate biosynthesis and the underlying mechanisms that govern susceptibility and resistance of organisms to antifolate drugs.

Origin and Dissemination of Antimicrobial Resistance among Uropathogenic Escherichia coli

Antimicrobial agents of various types have important bearing on the outcomes of microbial infections. These agents may be bacteriostatic or –cidal, exert their impact via various means, originate from a living organism or a laboratory, and appropriately be used in or on living tissue or not. Though the primary focus of this chapter is on resistance to the antimicrobial agents used to treat uropathogenic Escherichia coli (UPEC)-caused urinary tract infections (UTIs), some attention will be given to UPEC’s resistance to silver-containing antiseptics, which may be incorporated into catheters to prevent foreign body-associated UTIs.

Regional dissemination of a trimethoprim-resistance gene cassette via a successful transposable element

Background

Antimicrobial resistance is a growing international problem. We observed a 50% increase in the prevalence of trimethoprim resistance among fecal Escherichia coli from healthy Nigerian students between 1998 and 2005, a trend to increase that continued in 2009.

Methods and Findings

A PCR-based screen revealed that 131 (43.1%) of isolates obtained in Nigeria in 2005 and 2009 carried integron-borne dfrA cassettes. In the case of 67 (51.1%) of these isolates, the cassette was a class 1-integron-borne dfrA7 gene, which has been reported at high prevalence from E. coli isolates from other parts of Africa. Complete sequencing of a 27 Kb dfrA7-bearing plasmid from one isolate located the dfrA7 gene within a Tn21-type transposon. The transposon also contained an IS26-derived bla/sul/str element, encoding resistance to β-lactams, sulphonamides and streptomycin, and mercury resistance genes. Although the plasmid backbone was only found in 12 (5.8%) of trimethoprim-resistant isolates, dfrA7 and other transposon-borne genes were detected in 14 (16.3%) and 32 (26.3%) of trimethoprim resistant isolates collected in Nigeria in 2005 and 2009, respectively. Additionally, 37 (19.3%) of trimethoprim-resistant E. coli isolates collected between 2006 and 2008 from Ghana were positive for the dfrA7 and a transposon marker, but only 4 (2.1%) harbored the plasmid backbone.

Conclusions

Our data point to transposition as a principal mechanism for disseminating dfrA7 among E. coli from Nigeria and Ghana. On-going intensive use of the affordable broad-spectrum antibacterials is likely to promote selective success of a highly prevalent transposable element in West Africa.

The integron/gene cassette system: an active player in bacterial adaptation

The integron includes a site-specific recombination system capable of integrating and expressing genes contained in structures called mobile gene cassettes. Integrons were originally identified on mobile elements from pathogenic bacteria and were found to be a major reservoir of antibiotic-resistance genes. Integrons are now known to be ancient structures that are phylogenetically diverse and, to date, have been found in approximately 9% of sequenced bacterial genomes. Overall, gene diversity in cassettes is extraordinarily high, suggesting that the integron/gene cassette system has a broad role in adaptation rather than being confined to simply conferring resistance to antibiotics. In this chapter, we provide a review of the integron/gene cassette system highlighting characteristics associated with this system, diversity of elements contained within it, and their importance in driving bacterial evolution and consequently adaptation. Ideas on the evolution of gene cassettes and gene cassette arrays are discussed.

Urinary tract infections in a South American population: dynamic spread of class 1 integrons and multidrug resistance by homologous and site-specific recombination

One hundred four bacterial strains mediating urinary tract infections in separate individuals from a Uruguayan community were isolated. Forty-six strains conferred a multidrug resistance phenotype. All 104 strains were examined for the presence of class 1, 2, and 3 integrons. Class 1 integrons were found in 21 isolates across four distinct bacterial genera. A large class 1 integron in a Klebsiella pneumoniae strain was fully sequenced and was 29,093 bp in length. This integron probably arose by homologous recombination since it was embedded in a hybrid Tn21-like transposon backbone which comprised a Tn5036-like tnp transposition module at the IRi integron end and a Tn21 mer module at the IRt integron end. The parent integron/transposon that contributed the Tn5036 module was not related to Tn1696 since the integron insertion points in the transposon backbones were 16 bases apart. Examination of the other 20 class 1 integron-containing strains revealed further evidence of genetic exchange. This included a strain that possessed a Tn5036 module at the IRt end but not at the IRi end and another that possessed a tnp module beyond IRi that was a hybrid of Tn21 and Tn5051 and that is presumed to have arisen by site-specific recombination. This study highlights the ability of different genetic elements to act cooperatively to spread and rearrange antibiotic resistance in a community.

The role of horizontal gene transfer in the spread of trimethoprim–sulfamethoxazole resistance among uropathogenic Escherichia coli in Europe and Canada

OBJECTIVES

To describe the distribution of trimethoprim-sulfamethoxazole resistance genes and the role of horizontal gene transfer and clonal expansion in recent increases of antibiotic resistance rates among uropathogenic Escherichia coli in Europe and Canada.

METHODS

We identified antibiotic resistance alleles sul1, sul2, sul3 and dfr along with type 1 and type 2 integrons among 350 uropathogenic E. coli isolates from a cross-sectional study of acute, uncomplicated, community-acquired urinary tract infections in 16 western European countries and Canada (ECOSENS).

RESULTS

Trimethoprim resistance gene distributions showed no regional dependency (P = 0.84). The most common trimethoprim resistance gene was dfrA1, which occurred in 37.9% of dfr containing isolates. Similarly, the sulfamethoxazole resistance gene distributions did not vary significantly by region (P = 0.20). sul2, the most common sulfamethoxazole resistance gene, was found in 77.9% of sulfamethoxazole-resistant isolates. The distribution of type 1 and type 2 integrons varied slightly by region (P = 0.04) with type 1 integrons being the more common (85.9%). We observed 34 combinations of the sul genes, dfr genes and integron types; the most common combinations were broadly disseminated across every region examined.

CONCLUSIONS

Horizontal gene transfer plays a larger role than clonal expansion in the increase of trimethoprim-sulfamethoxazole resistance levels in Europe and Canada.

Multiple-drug resistance in D-tartrate-positive Salmonella enterica serovar paratyphi B isolates from poultry is mediated by class 2 integrons inserted into the bacterial chromosome

The presence of integrons in 85 multiresistant German isolates of the predominating Salmonella enterica subsp. enterica serovar Paratyphi B dT(+) clone was investigated. All isolates possessed a chromosomally located Tn7-like class 2 integron carrying the same dfrA1-sat1-aadA1 array of gene cassettes. Only four isolates (4.7%) revealed an additional class 1 integron with two strains each containing the aadA1 or dfrA1-aadA1 gene cassettes.

IntI2 integron integrase in Tn7

Integrons can insert and excise antibiotic resistance genes on plasmids in bacteria by site-specific recombination. Class 1 integrons code for an integrase, IntI1 (337 amino acids in length), and are generally borne on elements derived from Tn5090, such as that found in the central part of Tn21. A second class of integron is found on transposon Tn7 and its relatives. We have completed the sequence of the Tn7 integrase gene, intI2, which contains an internal stop codon. This codon was found to be conserved among intI2 genes on three other Tn7-like transposons harboring different cassettes. The predicted peptide sequence (IntI2*) is 325 amino acids long and is 46% identical to IntI1. In order to detect recombination activity, the internal stop codon at position 179 in the parental allele was changed to a triplet coding for glutamic acid. The sequences flanking the cassette arrays in the class 1 and 2 integrons are not closely related, but a common pool of mobile cassettes is used by the different integron classes; two of the three antibiotic resistance cassettes on Tn7 and its close relatives are also found in various class 1 integrons. We also observed a fourth excisable cassette downstream of those described previously in Tn7. The fourth cassette encodes a 165-amino-acid protein of unknown function with 6.5 contiguous repeats of a sequence coding for 7 amino acids. IntI2*179E promoted site-specific excision of each of the cassettes in Tn7 at different frequencies. The integrases from Tn21 and Tn7 showed limited cross-specificity in that IntI1 could excise all cassettes from both Tn21 and Tn7. However, we did not observe a corresponding excision of the aadA1 cassette from Tn21 by IntI2*179E.

The prevalence of trimethoprim-resistance-conferring dihydrofolate reductase genes in urinary isolates of Escherichia coli in Korea

One-hundred and twenty-two urinary isolates of Escherichia coli were studied for trimethoprim resistance. Seventy-seven (63.1%) of the 122 isolates were found to be resistant to trimethoprim. Of the 77 trimethoprim-resistant isolates, 75 dfr genes were detected in 72 isolates as follows: the dfrA17 gene was the most prevalent, being found in 27 isolates, followed by dfrA12 in 26, dfrA1 in 15, dfrA5 in four and dfrA7 in three. Southern blot and PCR mapping analysis revealed that all of the dfrA17, dfrA12, dfrA5 and dfrA7 genes were located on class 1 integrons. The dfrA1 gene inserted as a gene cassette in class 1 integrons was found in 10 of 15 isolates, and the intI2 gene of Tn7 was detected in two out of five isolates. In conjugation experiments, the dfr genes inserted in class 1 integrons were transferred to a recipient E. coli in 32 (42.7%) of the 75 dfr genes. In conclusion, the dfrA17 and dfrA12 genes were the most prevalent genes responsible for trimethoprim resistance in urinary tract isolates of E. coli from Korea and the dfr genes inserted in integrons are more widespread than those that are not related to gene cassettes.

An integron cassette carrying dfr1 with 90-bp repeat sequences located on the chromosome of trimethoprim-resistant isolates of Campylobacter jejuni

The frequent occurrence of high-level trimethoprim resistance in clinical isolates of Campylobacter jejuni was shown to be related to the acquisition of foreign resistance genes (dfrl or dfr9 or both) coding for resistant variants of the enzyme dihydrofolate reductase, the target of trimethoprim. The dfr1 gene detected on the chromosome of 40 different clinical strains of C. jejuni was studied further regarding structure and genetic organization. Most of the dfr1 genes were found as integron cassettes inserted in the chromosome. In 36% of the examined isolated, the dfr1 gene showed identity to that previously characterized in trimethoprim-resistant Escherichia coli. In 40% of the cases, however, a variant of the dfr1 gene containing a 90-bp direct repeat was detected, and in 5% of the isolates, the repeat-containing dfr1 variant was found to occur in the form of two cassettes in tandem in an integron context. The existence of the 90-bp repeat within the coding sequence of the dfr1gene was found to play a role in the adaptation of C. jejuni to ambient concentrations of trimethoprim.

New gene cassettes for trimethoprim resistance, dfr13, and Streptomycin-spectinomycin resistance, aadA4, inserted on a class 1 integron

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 I integrons containing a dhfrI trimethoprim resistance gene cassette in aquatic Acinetobacter spp

The presence of antibiotic resistance gene cassettes in class I integrons was investigated in 24 sulfamethoxazole-resistant and -sensitive Acinetobacter isolates derived from two Danish freshwater trout farms. Integrons were detected in five isolates from one of the fish farms, and their inserts were characterised by DNA sequencing. Each isolate contained a dhfrI gene cassette encoding resistance to trimethoprim and an open reading frame orfC of unknown function identical to the content of an integron previously found in a clinical enterobacterial isolate. Among the five isolates, at least two different strains were differentiated based on phenotypic tests and randomly amplified polymorphic DNA analysis. To our knowledge, this is the first report and characterisation of an integron in environmental bacteria.

Detection of integrons in worldwide nosocomial isolates of Acinetobacter spp

OBJECTIVE: To examine the distribution of integrons in genotypically unrelated worldwide multiresistant clinical isolates of Acinetobacter spp. METHODS: The presence and genetic location of class 1, 2 and 3 integrons were examined in a genotypically heterogeneous collection of 25 nosocomial isolates of Acinetobacter spp., from 15 locations in 11 different countries worldwide, by hybridization and PCR-based methods. Class 1 integron structures were characterized genetically by a PCR mapping technique. RESULTS: Class 1 integrons were detected in 17 of the 25 Acinetobacter isolates examined. Only one isolate contained a class 2 integron. No class 3 integrons were detected. The integrons varied in size and in the number of inserted cassettes, but similar integrons were found in genotypically distinct isolates from different locations worldwide. These structures were integrated into the chromosome in all isolates where they were detected, although some integrons were capable of subsequent transfer or mobilization. Genes coding for aminoglycoside-modifying enzymes formed the predominant cassettes identified within the integrons. CONCLUSIONS: Clinical isolates of Acinetobacter spp. from diverse locations seem to share resistance mechanisms acquired from other genera by a variety of mechanisms, including dissemination of integrons. Once integrons are incorporated into the bacterial genome, Acinetobacter spp. are potentially able to act as a reservoir of resistance genes for other species and genera.

Molecular analysis of and identification of antibiotic resistance genes in clinical isolates of Salmonella typhi from India

A representative sample of 21 Salmonella typhi strains isolated from cultures of blood from patients at the Christian Medical College and Hospital, Vellore, India, were tested for their susceptibilities to various antimicrobial agents. Eleven of the S. typhi strains possessed resistance to chloramphenicol (256 mg/liter), trimethoprim (64 mg/liter), and amoxicillin (>128 mg/liter), while four of the isolates were resistant to each of these agents except for amoxicillin. Six of the isolates were completely sensitive to all of the antimicrobial agents tested. All the S. typhi isolates were susceptible to cephalosporin agents, gentamicin, amoxicillin plus clavulanic acid, and imipenem. The antibiotic resistance determinants in each S. typhi isolate were encoded by one of four plasmid types. Plasmid-mediated antibiotic resistance genes were identified with specific probes in hybridization experiments; the genes responsible for chloramphenicol, trimethoprim, and ampicillin resistance were chloramphenicol acetyltransferase type I, dihydrofolate reductase type VII, and TEM-1 beta-lactamase, respectively. Pulsed-field gel electrophoresis analysis of XbaI-generated genomic restriction fragments identified a single distinct profile (18 DNA fragments) for all of the resistant isolates. In comparison, six profiles, different from each other and from the resistance profile, were recognized among the sensitive isolates. It appears that a single strain containing a plasmid conferring multidrug-resistance has emerged within the S. typhi bacterial population in Vellore and has been able to adapt to and survive the challenge of antibiotics as they are introduced into clinical medicine.

Presence of integrons in isolates of different biotypes of Acinetobacter baumannii from Chilean hospitals

Class 1 and class 2 integrons were investigated by hybridisation in 100 isolates of multiresistant biotypes of Acinetobacter baumannii from Chilean hospitals. Most isolates of A. baumannii biotype 9, the prevalent biotype, harboured integrons of class 2 (Tn7-like) whereas no integrons were detected in infrequent biotypes. Integron-carrying isolates exhibited broader antibiotic resistance patterns as well as higher resistance levels to various antimicrobials.

Antimicrobial and mercury resistance in aerobic gram-negative bacilli in fecal flora among persons with and without dental amalgam fillings

Antimicrobial resistance is more widespread than can be accounted for as being a consequence of the selection pressure caused by the use of antibiotics alone. In this study, we tested the hypothesis that a high mercury content in feces might select for mercury-resistant bacteria and thus for antimicrobial resistance linked to mercury resistance. Three subject groups with different exposures to dental amalgam fillings were compared. None of the subjects had taken antimicrobial agents during the three preceding months or longer. The group exposed to dental amalgam (n = 92) had 13 times more mercury in feces than the group that had never been exposed to amalgam (n = 43) and the group whose amalgam fillings had been removed (n = 56). No significant differences in either mercury resistance or antibiotic resistance in the fecal aerobic gram-negative flora of these subject groups were seen. The following antimicrobial resistance frequencies were detected with a replica plating method: > or = 1% resistance was seen in 40% of the subjects for ampicillin, 14% of the subjects for cefuroxime, 6% of the subjects for nalidixic acid, 14% of the subjects for trimethoprim, 19% of the subjects for sulfamethoxazole, and 25% of the subjects for tetracycline. The amount of mercury in feces derived from amalgam was not selective for any resistance factors in aerobic gram-negative bacteria, but antimicrobial resistance was widespread even among healthy subjects with no recent exposure to antibiotics.

Prevalence and genetic location of non-transferable trimethoprim resistant dihydrofolate reductase genes in South African commensal faecal isolates

In a recent survey of trimethoprim resistance, 357 Gram-negative aerobic organisms were isolated from healthy volunteers from rural and urban populations in South Africa. Trimethoprim resistance did not transfer to an Escherichia coli J62-2 recipient strain by conjugation in a liquid mating in 161 (45.1%) of the isolates. These isolates which did not transfer their resistance were probed with intragenic oligonucleotide probes for the types Ia, Ib, IIIa, V, VI, VII, VIII, IX, X and XII dihydrofolate reductase genes. Contrary to all previous data, the most prevalent dihydrofolate reductase gene in this group of non-transferable isolates which hybridized, was the type VII (38%) followed by the type Ia (25%), Ib (12%), V (1.7%) and VIII (1.2%). None of the strains hybridized to the types IIIa, VI, XI, X and the XII dihydrofolate reductase probes. Southern blots of plasmid and chromosomal DNA from selective isolates revealed that the type VII dihydrofolate reductase genes were located on the chromosome and were associated with the integrase gene of Tn21. However, the type Ib and V dihydrofolate reductase genes were all found on plasmids which could not be mobilized. The type Ia dihydrofolate reductase genes were found on both non-transferable plasmids and on the chromosome. The nature of the genetic structures associated with a dihydrofolate reductase gene strongly affects the means of spread of the gene in a population.

A new dhfrVIII trimethoprim-resistance gene, flanked by IS26, whose product is remote from other dihydrofolate reductases in parsimony analysis

A new plasmid-borne gene, dhfrVIII, encoding high-level trimethoprim resistance (TpR) was found in an intestinal Escherichia coli. It seems to be a widely occurring mediator of TpR. Among 973 examined TpR E. coli, the new resistance gene was found in 13 (1.3%) isolates from Sweden, Finland and Nigeria. The new gene was sequenced and found to code for a dihydrofolate reductase (DHFR) of 169 amino acids (M(r) 19005). The dhfrVIII gene on the studied plasmid pLMO226 was observed to be flanked by IS26 elements. The dhfrVIII gene and a 3' unidentified open reading frame (ORF) seem to be borne on a compound transposon with IS26 at its ends, since the configuration of two IS26 flanking dhfrVIII and the unidentified ORF was conserved among the isolates that were probe-positive for the gene. Phylogeny parsimony analysis showed the dhfrVIII-encoded enzyme to be only remotely related to other known plasmid-mediated, drug-resistant DHFR. Only a few of the latter form well-supported monophyletic groups.

Sequence identity with type VIII and association with IS176 of type IIIc dihydrofolate reductase from Shigella sonnei

An uncommon dihydrofolate reductase (DHFR), type IIIc, was coded for by Shigella sonnei that harbors plasmid pBH700 and that was isolated in North Carolina. The trimethoprim resistance gene carried on pBH700 was subcloned and sequenced. The nucleotide sequence of the gene encoding type IIIc DHFR was identical to the gene encoding type VIII DHFR. The type IIIc amino acid sequence was approximately 50% similar to those of DHFRs commonly found in enteric bacteria. Furthermore, this gene was flanked by IS176 (IS26), an insertion sequence usually associated with those of aminoglycoside resistance genes. The gene for type IIIc DHFR was located by hybridization within a 1,993-bp PstI fragment in each of eight conjugative plasmids from geographically diverse strains of S. sonnei. Each plasmid also conferred resistance to ampicillin, streptomycin, and sulfamethoxazole and belonged to incompatibility group M. Plasmids carrying this new trimethoprim resistance gene, which is uniquely associated with IS176, have disseminated throughout the United States.

Trimethoprim resistant dihydrofolate reductases in normal faecal flora isolated in India

A high incidence of resistance to trimethoprim has been shown in the normal faecal flora in a population in south India. The dihydrofolate reductase (dhfr) genes mediating transferable resistance to trimethoprim have been identified. Unusually, in this study, the dhfrV was shown to be the predominant resistance gene (dhfrV 50% of transconjugants, dhfrIa 30%), the dhfrIb was also detected being distinguished from the dhfrV by an oligo-probe. However, when non-transferable resistance was considered, the dhfrIa was the most prevalent of the dhfrs identified. All those plasmids harbouring the dhfrIa were shown to possess Tn7. All the plasmids that probed positive for the dhfrV and the dhfrIb were shown to be associated with the integrase of the Tn21-like transposons, but 8 of the dhfrV genes were not associated with the Tn21 resolvase. The dhfrIV was shown to be present in all seven plasmids that produced low level trimethoprim-resistance. The dhfrV, first characterized in Sri Lanka, would seem to have a local distribution in this region of Asia but is distinguishable from the dhfrIb only by the use of an oligo-probe.

Characterization of transposon Tn5086, carrying the site-specifically inserted gene dhfrVII mediating trimethoprim resistance

Two different enteric plasmids of widely separate origins were observed to carry a new 15.3-kb trimethoprim resistance transposon, Tn5086, also mediating resistance to mercuric ions and to a low level of sulfonamide. The trimethoprim resistance gene characterized from Tn5086 was found to be distinct from those found earlier and was designated type VII. Molecular analysis demonstrated that Tn5086 is closely related to Tn21. The internal part of Tn21 and Tn5086, the element referred to as the integron, was found to be different. First, the integron of Tn5086 contains a 0.62-kb cassette formed by the trimethoprim resistance gene dhfrVII and its immediate surroundings instead of the 0.86-kb aadA1 cassette of Tn21. Second, the integron of Tn5086 lacks a 4.2-kb segment 3' of sulI in Tn21. The dhfrVII gene commences with a UUG codon but was otherwise seen to be markedly related to the cassette genes dhfrI, dhfrV, and dhfrVI. The four related dihydrofolate reductases of 157 amino acids encoded by these genes contain a glutamate instead of the aspartic acid residue found at position 27 of the active center of the chromosomal enzyme from Escherichia coli.

Spread of a newly found trimethoprim resistance gene, dhfrIX, among porcine isolates and human pathogens

A plasmid-borne gene mediating trimethoprim resistance, dhfrIX, newly found among porcine strains of Escherichia coli, was observed at a frequency of 11% among trimethoprim-resistant veterinary isolates. This rather high frequency of dhfrIX could be due to the extensive use of trimethoprim in veterinary practice in Sweden. After searching several hundred clinical isolates, one human E. coli strain was also found to harbor the dhfrIX gene. Thus, the dhfrIX gene seems to have spread from porcine bacteria to human pathogens. Furthermore, the occurrence of other genes coding for resistant dihydrofolate reductase enzymes (dhfrI, dhfrII, dhfrV, dhfrVII, and dhfrVIII) among the porcine isolates was investigated. In addition, association of dhfr genes with the integraselike open reading frames of transposons Tn7 and Tn21 was studied. In colony hybridization experiments, both dhfrI and dhfrII were found associated with these integrase genes. The most common combination was dhfrI and int-Tn7, indicating a high prevalence of Tn7.

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

Between July 1987 and June 1989, 1054 urinary isolates of enterobacteria from Kaohsiung, Taiwan were studied for their trimethoprim resistance. Trimethoprim resistance was defined as MIC greater than 4 micrograms/ml and high-level resistance by MIC greater than 1000 micrograms/ml. The incidence of trimethoprim resistance increased from 33.6% in 1987 to 42.1% in 1989. Among the resistant strains studied, 90% were resistant to high levels of trimethoprim. An increase in the proportion of resistant strains (33.9-46.3%) exhibiting high-level non-transferable trimethoprim resistance was noted. The distribution of the dihydrofolate reductase (DHFR) genes by colony hybridization in 374 trimethoprim-resistant isolates revealed the presence of type I and type V DHFR genes in most of these isolates (45.4% and 10.4% respectively). Type I was predominant in Escherichia coli whereas type V was frequently seen in Enterobacter spp. None showed homology with the type II and type III DHFR probe DNA. In addition, transposon Tn7 was present in 7.8% of 374 trimethoprim-resistant enterobacteria.