Monitoring of plasmid-encoded, trimethoprim-resistant dihydrofolate reductase genes: detection of a new resistant enzyme

From a binding module to essential catalytic activity: how nature stumbled on a good thing

Enzymes are complex macromolecules capable of catalyzing a wide variety of chemical reactions with high efficiency. Nonetheless, biological catalysis can be rudimentary. Here, we describe an enzyme that is built from a simple protein fold. This short protein sequence - almost a peptide - belongs to the ancient SH3 family of binding modules. Surprisingly, this binding module catalyzes the specific reduction of dihydrofolate using NADPH as a reducing cofactor, making this a dihydrofolate reductase. Too small to provide all the required binding and catalytic machinery on its own, it homotetramerizes, thus creating a large, central active site environment. Remarkably, none of the active site residues is essential to the catalytic function. Instead, backbone interactions juxtapose the reducing cofactor proximal to the target imine of the folate substrate, and a specific motion of the substrate promotes formation of the transition state. In this feature article, we describe the features that make this small protein a functional enzyme capable of catalyzing a metabolically essential reaction, highlighting the characteristics that make it a model for the evolution of primitive enzymes from binding modules.

Molecular detection of antimicrobial resistance

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.

In vivo R-plasmid transfer in a patient with a mixed infection of shigella dysentery

Transfer of shigella R-plasmids in vivo has seldom been demonstrated. Strains of Shigella dysenteriae type 1 and Shigella flexneri type 5b were isolated from a Bulgarian traveller who visited Vietnam and developed dysentery, which was treated with trimethoprim/sulfamethoxazole (TMP/SMZ) for a short time. Both species of shigellae are unusual in Bulgaria where strains of S. sonnei predominate. Both shigella strains were multiresistant to the same antimicrobial agents. Each strain contained a 48-kilobase plasmid that conferred the entire resistance phenotype to a susceptible Escherichia coli. Restriction endonuclease patterns of plasmid DNA from the respective strains were identical. Transmissible plasmids of the same resistance phenotypes and restriction patterns were isolated from the patient's colonic E. coli. Transconjugants hybridized to a dihydrofolate reductase type I-DNA probe. These studies support the hypothesis that R-plasmid transfer may occur between non-pathogenic, faecal strains and pathogenic shigellae, a process that may have been facilitated by inadequate treatment with TMP/SMZ at the onset of the illness.

Identification by DNA sequence analysis of a new plasmid-encoded trimethoprim resistance gene in fecal Escherichia coli isolates from children in day-care centers

In our ongoing studies of trimethoprim resistance (Tmpr) in day-care centers (DCC), we have shown a high rate of fecal colonization with Tmpr Escherichia coli and, using total plasmid content analysis, have shown that this is due to a diversity of strains. In the present study, we analyzed 367 highly Tmpr (MIC, greater than or equal to 2,000 micrograms/ml) isolates of E. coli from 72 children over a 5-month period and found at least 83 distinct plasmid patterns, indicating that at least 83 strains were involved. Several strains were particularly common in a given DCC, including one found in 61% of children with Tmpr E. coli; these common strains usually persisted within a DCC for several months. Colony lysates were hybridized with gene probes for dihydrofolate reductases (DHFR) types I, II, III, V, and VII; 21% hybridized under stringent conditions, and all of these were with type I (17%) or type V (4%) probes. Tmpr was cloned from a probe-negative Tmpr transconjugant, and an intragenic probe was prepared from this clone. Approximately 21% of the Tmpr E. coli strains (76 isolates) in the DCC were found to have this new gene, 74 of which were in one DCC. The DNA sequence of this gene was determined, and the predicted amino acid sequence was shown to have between 32% and 39% identity with the amino acid sequences for types I, III, V, VI, and VII and the partial sequence of type IV and approximately 26% identity with types IX and X DHFR. This confirms the uniqueness of this gene, which has tentatively been named dhfrxii, and its translation product, DHFR type XII.

Molecular epidemiology of trimethoprim-resistant Shigella boydii serotype 2 strains from Bulgaria

In 1990 an increased number of strains of Shigella boydii serotype 2 were isolated from different regions of Bulgaria. Strains were reported as sporadic, although they showed identical phenotypic characteristics, including resistance to ampicillin, carbenicillin, streptomycin, sulfonamide, tetracycline, ticarcillin, and trimethoprim. The objective of this study was to determine the genetic relatedness of the strains and the mechanism of their antimicrobial resistance. Plasmid fingerprinting showed an identical pattern for 23 of 25 of the selected strains. All 25 strains tested transferred their resistances en bloc to an Escherichia coli recipient. Transconjugants contained a 112-kb R plasmid which carried all the resistance genes, including that conferring type I dihydrofolate reductase-mediated trimethoprim resistance (MIC greater than 2,000 micrograms/ml). Riboprobe analysis showed identical restriction length fragment polymorphisms, suggesting a highly conserved genome. All findings indicate that strains of S. boydii serotype 2 isolated in 1990 from different regions of Bulgaria were highly related genetically and can be considered representatives of a single bacterial clone. The presence of an R plasmid and selection pressure because of the usage of antimicrobial agents, particularly trimethoprim, have likely facilitated the spread of the clone throughout the country.

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

Among a collection of clinical Escherichia coli isolates, the type I dihydrofolate reductase (DHFR) mediating trimethoprim resistance was generally observed to be chromosomally determined. Only a minority of isolates carried the type I DHFR gene simultaneously on a plasmid. The majority of E. coli isolates studied also hybridized with a probe specific for the transposition gene tnsC of transposon Tn7; and in most of these isolates, Tn7 was found to be inserted into a preferred site in the E. coli chromosome. A minority of isolates that harbored the type I DHFR gene in the chromosome lacked a complete Tn7. Some of these harbored the type I DHFR gene inserted in a structure similar to that containing the gene for streptomycin resistance in Tn21. In the other isolates that were negative for a complete Tn7, the sequences upstream of the type I DHFR gene were demonstrated to be homologous to those flanking the type I DHFR gene in Tn7. This could indicate that the antibiotic resistance region of Tn7 may occur independently of this transposon. In two isolates, no sequences resembling Tn7 or Tn21 were found adjacent to the type I DHFR gene.

Trimethoprim resistance in Haemophilus influenzae is due to altered dihydrofolate reductase(s)

We characterized a highly purified preparation of the chromosomally encoded dihydrofolate reductase (DHFR) from a trimethoprim-susceptible (Tmp8; strain MAP) and two trimethoprim-resistant (TmpR) strains (MAP/47 and MAP/42) of Haemophilus influenzae. The enzymes were purified between 650- and 3000-fold by gel-filtration and dye-ligand chromatography. The apparent molecular mass of the three proteins was 18400 Da by PAGE under denaturing and nondenaturing conditions. Total enzyme activity was greater in all fractions from the TmpR strains compared with the Tmp8 isolate. The three enzymes had a similar Km for dihydrofolate (7, 9 and 5 microM) and NADPH (2, 5 and 6 microM). However, the Tmp IC50 (the concentration necessary for 50% inhibition of DHFR activity) for the Tmp8 strain MAP was 0.001 microM, whereas DHFR from the TmpR strains MAP/47 and MAP/42 had values of 0.1 microM and 0.3 microM respectively. The methotrexate IC50 of the MAP/42 DHFR was 0.06 microM in comparison with the enzyme from MAP (0.008 microM) and MAP/47 (0.007 microM). Isoelectric focusing indicated that the DHFR from MAP/42 had a different isoelectric point (pI 7.6) compared with the enzymes from MAP and MAP/47 (pI 7.3). Peptide mapping after digestion with trypsin revealed one major peptide fragment (7.9 kDa) in the DHFR of MAP and MAP/47 and three major tryptic fragments (7.9, 9.6 and 12.5 kDa) in DHFR from MAP/42. We conclude that trimethoprim resistance in H. influenzae results from overproduction of structurally altered DHFR(s).

Molecular characterization of trimethoprim resistance in Shigella sonnei in Sicily

During the 3-year period 1985-7, all strains of Shigella sonnei isolated in Catania, Sicily, showed a high level of resistance to trimethoprim (Tp) which was invariably associated with resistance to other antibiotics. Plasmid analysis showed 18 different electropherotypes: 35 of 37 strains harboured a plasmid of 70 Megadaltons (MDa), and 29 of 37 strains a plasmid of 130 MDa. Restriction endonuclease fingerprinting of purified 70 MDa plasmid DNA from different strains demonstrated that these plasmids were similar but not identical. In some strains with transferable Tp resistance, DNA hybridization analysis demonstrated the presence of gene coding for the production of dihydrofolate reductase (DHFR) type V. In contrast, there was no detectable hybridization with DNA probes specific for genes coding for DHFR types I, II and IV. This is the first report of the DHFR type V gene outside Sri Lanka.

Trimethoprim resistance gene in Shigella dysenteriae 1 isolates obtained from widely scattered locations of Asia

Trimethoprim-resistance genes of Shigella dysenteriae 1 strains, isolated from a different location of six different countries of Asia over a 5-year period were characterized by using three different dihydrofolate reductase (DHFR) gene probes. The trimethoprim-resistant (TMPR) strains hybridized only with the type I DHFR gene probe by colony hybridization. None of the strains hybridized with types II and III DHFR gene probes. Southern blot experiments using plasmid DNA extracted from these resistant strains indicated that the type I DHFR genes were either on a 20 MDa plasmid or might be located on the chromosome. None of the other plasmids present in S. dysenteriae 1 strains hybridized with the probe. This indicates that the TMP resistance in these S. dysenteriae 1 strains are mediated by type I DHFR enzyme, and there may be transposition of this type I DHFR gene occurs between the 20 MDa plasmid and the chromosome in this serotype of shigella.

Spreading of streptomycin antibiotic resistance gene in Escherichia coli plasmids demonstrated by Southern blot analysis

Plasmids from E. coli strains of 38 donors were transconjugated to common recipient SY663 Escherichia coli K12. The restriction patterns of the isolated plasmids were highly heterogenous. However, the streptomycin (Sm) resistance genes of the plasmids were identical or closely homologous in 29 of the 33 plasmids conferring Sm resistance. These data were based on Southern blot analysis, using the Sm resistance gene (encoding aminoglycoside phosphoryl transferase) as probe cut out from pBP1 plasmid. Our data suggest an extensive spreading of streptomycin resistance gene of this type.

Cloning, purification, and properties of Candida albicans thymidylate synthase

The thymidylate synthase (TS) gene was isolated from a genomic Candida albicans library by functional complementation of a Saccharomyces cerevisiae strain deficient in TS. The gene was localized on a 4-kilobase HindIII DNA fragment and was shown to be expressed in a Thy- strain of Escherichia coli. The nucleotide sequence of the TS gene predicted a protein of 315 amino acids with a molecular weight of 36,027. The gene was cloned into a T7 expression vector in E. coli, allowing purification of large amounts of C. albicans TS. It was also purified from a wild-type C. albicans strain. Comparison of several enzyme properties including analysis of amino-terminal amino acid sequences showed the native and cloned C. albicans TS to be the same.

High-level resistance to trimethoprim in Shigella sonnei associated with plasmid-encoded dihydrofolate reductase type I

By DNA hybridization, the gene encoding dihydrofolate reductase type I was found in 58 of 59 highly trimethoprim-resistant clinical isolates of Shigella sonnei obtained from 1981 through 1987 in Madrid, Spain. No strain harbored the type II gene. In selected strains, the type I gene was demonstrated to be in a plasmid.

Molecular cloning and mechanism of trimethoprim resistance in Haemophilus influenzae

We studied 10 trimethoprim-resistant (Tmpr) Haemophilus influenzae isolates for which agar dilution MICs were 10 to greater than 200 micrograms/ml. Trimethoprim resistance was transferred from two Tmpr H. influenzae isolates to a Tmps strain by conjugation or transformation. Wild-type Tmpr strains and Tmpr transcipients did not contain detectable plasmid DNA. The trimethoprim resistance gene was cloned into a cosmid vector, and recombinant plasmids were transduced into Escherichia coli. A 0.50-kilobase intragenic probe derived from a 12.9-kilobase fragment which encoded trimethoprim resistance hybridized with whole-cell DNA from Tmps and Tmpr strains. Southern blot analysis of restricted DNA from isogenic Tmps and Tmpr H. influenzae indicated that acquisition of trimethoprim resistance involved a rearrangement or change in nucleotide sequence. Hybridization was not seen with DNA derived from Tmpr E. coli containing dihydrofolate reductase I, II, and III genes or with Tmpr Neisseria meningitidis, Neisseria gonorrhoeae, and Pseudomonas cepacia. Southern hybridization with 12 multiply resistant encapsulated H. influenzae strains confirmed that the trimethoprim resistance gene was chromosomally mediated. Dihydrofolate reductase activity was significantly greater in cell sonicate supernatants of Tmpr strains in comparison with isogenic Tmps recipients. Differences were not found in the trimethoprim inhibition profile of dihydrofolate reductase activity in Tmps and Tmpr strains. We conclude that the mechanism of trimethoprim resistance in H. influenzae is overproduction of chromosomally located dihydrofolate reductase.

Nucleotide sequence of the dihydrofolate reductase gene of Saccharomyces cerevisiae

The nucleotide sequence of a DNA fragment that contained the Saccharomyces cerevisiae gene DFR coding for dihydrofolate reductase (DHFR) was determined. The DHFR was encoded by a 633-bp open reading frame, which specified an Mr24264 protein. The polypeptide was significantly related to the DHFRs of chicken liver and Escherichia coli. The yeast enzyme shared 60 amino acid (aa) residues with the avian enzyme and 51 aa residues with the bacterial enzyme. DHFR was overproduced about 40-fold in S. cerevisiae when the cloned gene was present in the vector YEp24. As isolated from the Saccharomyces library, the DFR gene was not expressed in E. coli. When the gene was present on a 1.8-kb BamHI-SalI fragment subcloned into the E. coli vector, pUC18, weak expression in E. coli was observed.

Characterization of plasmid pAZ1 and the type III dihydrofolate reductase gene

The plasmid pAZ1, which determines trimethoprim and sulfonamide resistance, was characterized by restriction endonuclease mapping. The restriction map was identical to that of the incQ plasmid RSF1010 over a 5.1-kbp region. The type III dihydrofolate reductase gene was cloned, and the DNA sequence was determined. The predicted protein had 162 amino acid residues, and it was more closely related to the gram-negative bacterial chromosomal dihydrofolate reductases than to other plasmid or vertebrate dihydrofolate reductases. Sequence identity was 51% with the Escherichia coli enzyme and 44% with the Neisseria gonorrhoeae enzyme.

Molecular epidemiology of trimethoprim resistance among coagulase-negative staphylococci

A 42% (70 of 167 isolates) incidence of resistance to 20 micrograms of trimethoprim per ml was found among clinical isolates of coagulase-negative staphylococci from two hospitals. A specific trimethoprim resistance gene probe from a conjugative Staphylococcus aereus plasmid was used to investigate the location of the trimethoprim resistance gene among 29 isolates. In 14 trimethoprim-resistant isolates, the probe hybridized with only chromosomal DNA, in 9 it hybridized with only plasmid DNA, and in 1 isolate both plasmid and chromosomal sequences showed hybridization. In five isolates there was no hybridization of the probe with either chromosomal or plasmid DNA. Four of these five nonhybridizing isolates were Staphylococcus haemolyticus. In contrast, all 22 Staphylococcus epidermidis isolates tested hybridized with the probe. The presence of the trimethoprim resistance gene in a chromosomal location was correlated with a lower MIC (median, 80 micrograms/ml) than when it was plasmid encoded (median, 1,250 micrograms/ml). Restriction endonuclease mapping as well as DNA hybridization of cloned plasmid and chromosomal DNA showed that there were 2.7 kilobases of common DNA in the two loci. This included the 500 base pairs of DNA mediating trimethoprim resistance and a total of 2.2 kilobases of 3'- and 5'-flanking sequences. The presence of the same gene and flanking sequences in chromosomal and plasmid locations suggests that the trimethoprim resistance determinant is translocated among different genetic loci.

A plasmid of group Q which confers resistance to trimethoprim and sulfonamides

A 5.2-Mdal plasmid, determining resistance to trimethoprim and sulfonamides, is a member of incompatibility group Q.

Mapping of trimethoprim resistance genes from epidemiologically related plasmids

Trimethoprim resistance dihydrofolate reductase genes from plasmids known to be exchanging between human and animal populations were mapped. The dihydrofolate reductase gene has been highly conserved in all plasmids, but differences in the flanking regions provide evidence that the most recent exchange of plasmids between the two ecosystems has been from animals to humans.

Characterization of a staphylococcal trimethoprim resistance gene and its product

Resistance to trimethoprim (Tp) is mediated by a plasmid-encoded gene in staphylococci. The gene is responsible for high-level resistance (MIC, greater than 1,000 micrograms/ml) in both its native host and when cloned on high-copy-number vectors in Escherichia coli. Analysis of subclones of the staphylococcal Tp gene on E. coli expression vectors and estimation of the size of full and truncated proteins produced in E. coli minicells generated an approximate size limit of 505 base pairs for the gene and 18,500 daltons for the gene product. Crude extracts of E. coli containing the cloned gene had dihydrofolate reductase (DHFR) specific activity that was more than 100 times greater than that of control cells and more than 1,000 times more resistant to trimethoprim inhibition. The amount of trimethoprim required for a 50% reduction in the specific activity of staphylococcal DHFR differed from those of cells containing DHFR types I, II, or III, enzymes mediating Tp in members of the family Enterobacteriaceae. In addition, the size of the monomeric staphylococcal DHFR protein was larger than that of any of the gram-negative DHFRs both compared with published sequence data and as observed by direct comparison on polyacrylamide gels. Finally, there was no homology between a DNA fragment containing the cloned staphylococcal gene and DNA encoding any of the gram-negative DHFRs. Thus, the staphylococcal Tp gene codes for a single protein with DHFR activity that appears to be unrelated to DHFR genes that mediate Tp in members of the Enterobacteriaceae.

Plasmid-mediated trimethoprim-resistance in Staphylococcus aureus. Characterization of the first gram-positive plasmid dihydrofolate reductase (type S1)

The trimethoprim-resistance gene located on plasmid pSK1, originally identified in a multi-resistant Staphylococcus aureus from Australia, encodes the production of a dihydrofolate reductase (type S1), which confers a high degree of resistance to its host and is quite unlike any plasmid-encoded dihydrofolate reductase hitherto described. It has a low Mr (19,700) and has a higher specific activity than the constitutive Gram-negative plasmid dihydrofolate reductases. The type S1 enzyme is heat-stable and has a relatively low affinity for the substrate, dihydrofolate (Km 10.8 microM). It is moderately resistant to trimethoprim, and is competitively inhibited by this drug with an inhibitor-binding constant of 11.6 microM. This is the first identification and characterization of a plasmid-encoded trimethoprim-resistant dihydrofolate reductase derived from a Gram-positive species.

Novel type of plasmid-borne resistance to trimethoprim

A novel trait for transferable resistance to high concentrations of trimethoprim was found to dominate among enterobacteria collected from different parts of Sri Lanka. Drug resistance was a result of the production of dihydrofolate reductase with a decreased sensitivity to antifolates. By characterization of the partially purified enzyme and by restriction enzyme digestion analysis, the newly found gene was shown to be distinct from the earlier known plasmid-borne resistance genes which express dihydrofolate reductases of types I, II, and III. Cloning of fragments containing the resistance gene and further restriction enzyme digestion analysis showed that this gene was inserted very close to a sulfonamide resistance gene. Evolution of trimethoprim resistance in Sri Lanka thus seems to have taken a different route from that taken in the industrialized world, where transposon Tn7 seems to dominate. The close combination of the new trimethoprim resistance gene with sulfonamide resistance on the plasmids studied would effect an efficient spread of these genes, since trimethoprim has most often been used in combination with a sulfonamide.

Nucleic acid probes in clinical microbiology

The infectious disease applications of nucleic acid probe have been described. In addition, the basic procedures of nucleic acid probe technology have been discussed, as have the factors affecting implementation of probe technology in diagnostic laboratories. Despite the questions raised, nucleic acid probes will become part of the diagnostic laboratory in the near future. Commercial interests are developing and marketing new probes, reagents, and kits which will expedite the employment of this technology. High-volume reference laboratories will first use probes as part of a battery of tests which will include ELISA and monoclonal antibody methods. In all probability, probes will replace methods: that have proven to be ineffective, difficult, or costly such as culturing for some enteric pathogens and Legionella, that require long incubation periods, such as mycobacteria, or that have high costs and low yields, such as virology.

The genetics of bacterial trimethoprim resistance in tropical areas

Resistance to trimethoprim in Gram-negative bacteria is largely manifested by two trimethoprim resistant dihydrofolate reductases (types I and II) encoded by genes originally located on resistance plasmids. Although trimethoprim resistance increased markedly after the clinical introduction of trimethoprim in the West, its spread has slowed and, in Edinburgh at least, has actually been declining. This reduction has been accompanied by the migration of a transposon, encoding the type I plasmid resistance gene, into the bacterial chromosome. In tropical areas, the incidence of trimethoprim resistance is very much higher. In Tanzania, it has spilled over into other bacteria outside the Enterobacteriaceae, but it was in India where the major problem existed. The majority (64%) of the Indian Enterobacteriaceae studied were resistant to the drug and most of the resistance genes were located on very large plasmids which also conferred resistance to many other antibacterial drugs. Some Indian plasmids carried a new trimethoprim resistance gene which is not detectable by conventional sensitivity tests and may be spreading unnoticed elsewhere. The proportion of trimethoprim resistance has been related to the volume of antibacterial drugs used.

Unusual expression of new low-level-trimethoprim-resistance plasmids

In a survey, 42% of trimethoprim-resistant clinical members of the family Enterobacteriaceae were able to transfer trimethoprim resistance to standard Escherichia coli strains when selection was made on complex bacteriological media. When transfer experiments were performed with minimal medium, another 16% of the clinical strains were shown to have transferred trimethoprim resistance. Twelve transconjugants produced negligible trimethoprim resistance in complex media but were resistant in minimal medium. The methionine, glycine, and purine components of complex media appeared to be responsible for the reduced expression of trimethoprim resistance in these strains.

Plasmid-encoded trimethoprim resistance in staphylococci

High-level (greater than 1,000 micrograms/ml) resistance to the antimicrobial agent trimethoprim was found in 17 of 101 (17%) coagulase-negative staphylococci and 5 of 51 (10%) Staphylococcus aureus from a number of different hospitals in the United States. Resistance was plasmid encoded and could be transferred by conjugation in 4 of the 17 (24%) Tpr coagulase-negative staphylococci and 3 of the 5 (60%) Tpr S. aureus. A 1.2-kilobase segment of plasmid DNA from one of the plasmids (pG01) was cloned on a high-copy-number vector in Escherichia coli and expressed high-level Tpr (MIC, 1,025 micrograms/ml) in the gram-negative host. In situ filter hybridization demonstrated homology between the cloned Tpr gene probe and plasmid DNA from each conjugative Tpr plasmid, a single nonconjugative plasmid from a United States Staphylococcus epidermidis isolate, a nonconjugative plasmid from an Australian methicillin-resistant S. aureus isolate, and chromosomal DNA from three Tpr S. epidermidis isolates that did not contain any plasmid DNA that was homologous with the probe. No homology was seen between the probe and staphylococcal plasmids not mediating Tpr, plasmid DNA from 12 Tpr S. epidermidis isolates not transferring Tpr by conjugation, or plasmid-encoded Tpr genes derived from gram-negative bacteria. Plasmid-encoded Tpr appears to be a relatively new gene in staphylococci and, because it can be transferred by conjugation, could become more prevalent in nonsocomial isolates.

Molecular epidemiology of resistance to trimethoprim in enterobacteria isolated in a Parisian hospital

Between January, 1981 and December, 1984, 419 strains of enterobacteria isolated from patients at the Hôpital Saint-Joseph were studied for (1) the level of resistance to trimethoprim (Tp) by determination of minimal inhibitory concentration (MIC), (2) transferability of this resistance by conjugation into Escherichia coli, (3) plasmid content of wild-type strains and transconjugants by agarose gel electrophoresis of crude bacterial lysates and by incompatibility grouping, and (4) type of dihydrofolate reductase (DHFR) by colony hybridization with probes specific for DHFR types I and II. Tp resistance was defined as MIC greater than or equal to 4 micrograms/ml and high-level resistance by MIC greater than or equal to 1000 micrograms/ml. Amongst the strains studied, 90% were resistant to high levels of Tp, while 10% had low-level resistance to Tp was detected in 180 strains corresponding to 185 plasmids. In the vast majority of the plasmids, resistance to Tp was associated with resistance to sulphonamide (94%), streptomycin (75-90%), ampicillin (75-90%) and chloramphenicol (65-80%). Plasmids conferring resistance to Tp were often large, most (84%) ranging in size from 90 to 175 Kb. They belonged to six different incompatibility groups and Inc6-C was the most prevalent (34 to 75%). The study of the distribution of the dfr genes by colony hybridization in 183 transconjugants and 89 strains with non-transferable Tp resistance revealed the presence of dfrI genes in most of these strains (48 and 53%, respectively). DHFR of types I and II were found in only 3% of the transconjugants, but in 15% of the strains with non-transferable resistance. DHFR of other types were found equally (15%) in strains with transferable and non-transferable resistance. The high incidence of the type I enzyme among the Tp-resistant strains probably results from the integration of transposon Tn7 into the chromosome or into a non-transferable plasmid.

Characterization of transferable plasmids from Shigella flexneri 2a that confer resistance to trimethoprim, streptomycin, and sulfonamides

A set of plasmids conferring resistance to several antibiotics, including the combination of trimethoprim and sulfamethoxazole, has been isolated from Escherichia coli following conjugative cotransfer from a clinical isolate of Shigella flexneri 2a. One of the plasmids, pCN1, was shown by subcloning and DNA sequencing to carry a gene encoding a trimethoprim-insensitive dihydrofolate reductase identical to that found in E. coli transposon 7. This plasmid was also shown to confer resistance to both streptomycin and spectinomycin by production of an adenylyltransferase that inactivated the drugs and the gene encoding this enzyme has also been sequenced. A second plasmid from the set, pCN2, was shown to inactivate streptomycin by a phosphotransferase mechanism and also to confer resistance to sulfonamides. The third plasmid from the set could not be correlated with a drug-resistance phenotype, but does appear to play a crucial role in plasmid mobilization.

Genetic basis of trimethoprim and O/129 resistance in Vibrio cholerae

Because of its important consequences on prophylaxis and therapy of cholera and on bacterial identification, we have studied the genetic basis of cross-resistance to trimethoprim and O/129 of strains of Vibrio cholerae O1 independently isolated in Africa. Two classes of bacteria were found. In the first class, the strains were also resistant to ampicillin and kanamycin and to high levels of streptomycin by synthesis of a 3"- or 6-aminoglycoside phosphotransferase. The strains hybridized weakly with a Tn7 probe and all the resistance characters were transferable en bloc to Escherichia coli. The second class included strains which, in addition to trimethoprim and O/129, were resistant to moderate levels of streptomycin and spectinomycin by production of a 3",9-aminoglycoside-aminocyclitol adenylyltransferase. The resistance characters were not self-transferable to E. coli and the host strain hybridized strongly with Tn7. It therefore appears, that both plasmids and transposons are responsible for the dissemination of resistance to trimethoprim and O/129 in Vibrio.

Variability of IncHI1 plasmids from Salmonella typhi with special reference to Peruvian plasmids encoding resistance to trimethoprim and other antibiotics

In spite of extensive DNA homology among IncHI1 plasmids, ApaI and XbaI restriction digests of plasmids from Peruvian Salmonella typhi varied considerably from other IncHI1 plasmids isolated previously. IncHI1 plasmids appear to be undergoing a process of modular evolution, probably by sequential acquisition of resistance determinants.

Construction of a gentamicin resistance gene probe for epidemiological studies

A 7.7-kilobase BamHI fragment was cloned from the transconjugant of a clinical isolate of Escherichia coli containing a 120-kilobase multiresistance IncC plasmid. The recombinant plasmid conferred resistance to kanamycin, gentamicin, tobramycin, sulfamethoxazole, and trimethoprim. This clone was used to generate a series of subclones from which a 2.0-kilobase BamHI-HindIII probe containing a gentamicin 2''-O-adenylyltransferase [AAD(2'')] gene was obtained. This probe hybridized specifically in both colony and Southern hybridizations with the AAD(2'') gene but not with other resistance genes, including other aminoglycoside-modifying genes, or with a reference IncC plasmid lacking the AAD(2'') gene. The AAD(2'') gene may be part of a transposon, since hybridization occurred with both nonconjugative plasmids and the chromosome in some isolates.

Plasmid-borne or chromosomally mediated resistance by Tn7 is the most common response to ubiquitous use of trimethoprim

The folic acid analog trimethoprim has been in clinical use for more than 10 years. The use of it in Sweden has doubled in the last 6 to 7 years, and from the distribution statistics it can be calculated that during 1 year 4 to 5% of the population in Sweden are given this drug. The bacterial resistance mechanisms to be found in response to such a selection pressure were investigated in a relatively isolated population in northern Sweden (the county of Jämtland), in which one centrally located bacteriological laboratory serves the area. Trimethoprim-resistant strains were collected during an 8-month period from consecutive specimens of bacteria from the urinary tracts of patients. Among the highly resistant strains of enteric bacteria, trimethoprim resistance mediated by transposon-borne dihydrofolate reductase of type I was found to dominate. The corresponding Tn7-like transposon was found to be localized both on the chromosome of isolated Escherichia coli strains and also on a 50-kilobase IncI transferable plasmid which was found in several different serotypes of E. coli. In two enterobacterial strains, resistance to more than 10(3) micrograms of trimethoprim per ml was furthermore found to be caused by a ca. 80-fold increase in the formation of chromosomal dihydrofolate reductase.

Characterization of trimethoprim resistance by use of probes specific for transposon Tn7

Transposon Tn7 codes for resistance to trimethoprim and streptomycin. For detection of Tn7 by DNA-DNA hybridization, two recombinant plasmids were constructed. The former contained a 1-kilobase BamHI fragment and the latter contained a 4.3-kilobase EcoRI-BamHI fragment of Tn7. These DNA fragments, which did not include the drug resistance genes, were used as probes for detecting Tn7-like sequences in bacterial strains by colony hybridization. They hybridized strongly to bacterial DNA known to carry Tn7 but not to DNA known to carry transposons other than Tn7. These probes were used to study the occurrence of Tn7 in bacterial strains isolated in the Turku City Hospital in Finland. Transposon Tn7 was present in 47.2% of 199 trimethoprim-resistant enterobacteria (MIC greater than or equal to 8 micrograms/ml). Among the 69 Proteus mirabilis strains studied, 75% contained Tn7, although none of these strains transferred trimethoprim resistance in conjugation tests. The reliability of colony hybridization was further confirmed by Southern hybridization to detect the Tn7-specific 2.6-kilobase HindIII restriction fragment. Colony hybridization proved to be a sensitive and rapid method for detecting Tn7-determined sequences.

Nucleotide sequence of the dihydrofolate-reductase gene borne by the plasmid R67 and conferring methotrexate resistance

The complete nucleotide sequence of the methotrexate-resistant dihydrofolate reductase (DHFR) gene borne by the plasmid R67 was determined. The gene is 234 bp long and codes for 78 amino acids. The polypeptide deduced from the DNA sequence is in perfect agreement with the previously published amino acid sequence. Comparison of the nucleotide sequence with the one determined for the R388-encoded DHFR indicates that 75% of the nucleotides are conserved in the two genes. The 3' end of the R67 gene can be modified without altering significantly the activity of the enzyme.

The nucleotide sequence of the trimethoprim-resistant dihydrofolate reductase gene harbored by Tn7

The complete nucleotide sequence of the type I dihydrofolate reductase gene from Tn7 was determined. The structural gene coded for a polypeptide of 157 amino acid residues. The polypeptide deduced from the DNA sequence had a molecular weight of 17,577 which was in good agreement with that estimated by mobility in SDS-polyacrylamide gels. Sequences were identified proximal to the coding region which were similar to those found in the consensus E. coli promoter region and for the initiation of protein synthesis. Features consistent with the termination of RNA transcription were present distal to the structural gene. No homology was apparent when the DNA sequence of the type I gene was compared to the sequence of the type II plasmid DHFR genes, but sequence homology was evident when the type I and E. coli chromosomal enzymes were compared. Homology was greatest in the regions coding for amino acids which in the E. coli chromosomal enzyme are associated with substrate, cofactor and inhibitor binding.