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

The dhfrI trimethoprim resistance gene of Tn7 can be found at specific sites in other genetic surroundings

The dhfrI gene, mediating high-level trimethoprim resistance, was earlier found only on Tn7. Evidence is given here for an alternative location of this gene at a site identical to sites observed earlier for dhfrII on plasmid R388, dhfrV on pLMO20, and aadA on Tn21. All these genes and dhfrI are precisely inserted as discrete GTTA-flanked elements at distinct loci in very conserved surrounding sequences. One of these dhfrI insertions was observed to occur in association with a similarly inserted aadA nucleotidyltransferase gene, which mediates streptomycin and spectinomycin resistance. Close to the insertion site, there is an open reading frame translating into a 337-amino-acid peptide which shows striking similarities to recombinases of the integrase family, sulI, the sulfonamide resistance gene, is very often found close to the insertion point forming a genetic surrounding, originally observed as a part of Tn21-like transposons. The alleged integration mechanism thus provides a recombination pathway for the genetic linkage of sulfonamide and other antibiotic resistance genes, including the most frequently encountered gene for trimethoprim resistance, dhfrI. Furthermore, the newly observed location of dhfrI could shed light on the evolution of the antibiotic resistance region of Tn7, which could be able to take up genes by the same mechanism as that of Tn21-like transposons.

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.

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.

Microplate phosphocellulose binding assay for aminoglycoside-modifying enzymes

We modified the phosphocellulose binding assay for aminoglycoside-modifying enzymes (AMEs) by use of microdilution plates and a multichannel micropipette. Batteries of aminoglycoside substrates for screening organisms for the presence of AMEs as well as for subclassifying enzymes were prepared and stored in microdilution plates. When tested in parallel with the conventional tube reaction assay, the microplate assay yielded comparable radioactive counts and therefore equally correct identifications of AMEs in 32 isolates representing nine bacterial species. Other modifications, such as multichannel dispensing of crude enzyme preparations and radioisotopic precursors, provided a more rapid, convenient, and less expensive means of examining large collections of organisms for AMEs.

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.

Emergence of trimethoprim resistance in relation to drug consumption in a Finnish hospital from 1971 through 1984

Emergence of trimethoprim resistance among urinary tract Escherichia coli strains, isolated mostly from long-term patients in the Turku City Hospital, Turku, Finland, was studied from 1971 through 1984. Emergence of resistance to trimethoprim was associated with changes in the consumption of both trimethoprim-sulfamethoxazole and trimethoprim, with occurrence of high-level trimethoprim resistance and sequences homologous to trimethoprim resistance transposon Tn7. Since 1971, resistance of E. coli to trimethoprim-sulfamethoxazole increased from 8 to between 32 and 35% in 1983 and 1984; resistance to sulfamethoxazole varied from 39 to between 40 and 44%. The frequency of DNA sequence homology with our Tn7 probe among trimethoprim-resistant E. coli strains was 42% from 1980 to 1981 and 64% in 1983 (P less than 0.005). Fourteen years after the introduction of trimethoprim therapy in this hospital, resistance has reached the level of resistance to sulfonamide.

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.