ANTIMICROBIAL AGENTS
AND
CHEMOTHERAPY, Oct. 1990,
p.
2013-2015
0066-4804/90/102013-03$02.00/0 Copyright C 1990, American Society for Microbiology
Vol. 34, No. 10
Trimethoprim Resistance in Escherichia coli Isolates from a Geriatric Unit ELINA HEIKKILA,l* LARS SUNDSTROM,2 AND PENTTI HUOVINEN' Department of Medical Microbiology, Turku University, 20520 Turku, Finland,' and Department of Pharmaceutical Microbiology, University of Uppsala, 5-751 23 Uppsala, Sweden2 Received 19 April 1990/Accepted 31 July 1990
The frequency of trimethoprim resistance among Escherichia coli isolates from urine samples collected at Turku City Hospital, Turku, Finland, remained at 40% during 1984 to 1988. The proportion of highly resistant (MIC, .1,024 ,ug/ml) isolates increased, however, and most of these harbored the type I dihydrofolate reductase gene. Only a few isolates possessed type II or VII genes. The widespread use of trimethoprim (TMP) has promoted emergence of TMP-resistant pathogens, especially in hospitals (7, 11-13, 15, 21). The most important mechanism for TMP resistance in bacteria is production of drug-resistant dihydrofolate reductase (DHFR) (5, 10). Type I DHFR, which confers high-level resistance to TMP, is most frequently encountered and is determined by transposons such as Tn7. Type II and V DHFRs have also been reported to occur in some areas (15, 19). The occurrence of other types of DHFR among clinical bacteria has not been confirmed. In Finland, TMP has been used to treat urinary tract infections in combination with sulfonamides since 1969 and alone since 1973. TMP resistance at Turku City Hospital, Turku, Finland, has been monitored since 1978. This hospital is a 1,200-bed facility where most of the patients require extended stays. This duration favors the emergence of antibiotic resistance among bacterial pathogens. Between 1978 and 1979, TMP resistance among Escherichia coli isolates from urine samples increased from 10 to 30%, thereafter reaching a plateau of around 30% during 1980 to 1982 (11). From 1982 to 1984, TMP resistance again increased to 41%. The increase from 10% in 1978 to 30% in 1979 was likely due to markedly increased consumption of TMP in that unit in 1977-13 kg/year, including TMP both alone and in combination with sulfonamides (11). Thereafter, TMP consumption decreased to half of that used in 1977, varying from 2 to 8 kg/year during 1978 to 1988. The present study was a follow-up survey of TMP resistance at Turku City Hospital from 1984 to 1988. To reveal which types of DHFR genes were responsible for the spread of TMP resistance, we conducted DNA hybridization studies of TMP-resistant urinary E. coli isolates collected in 1980 to 1987. For resistance pattern studies, 200 consecutive E. coli isolates from urine samples were collected in Turku City Hospital annually in January and July from 1984 to 1988. Duplicate samples were excluded. Susceptibilities to TMP, sulfamethoxazole, and ampicillin were defined by the disk diffusion method (AB Biodisk, Solna, Sweden; 11). For hybridization studies, 399 TMP-resistant E. coli isolates (94 consecutive isolates in 1980, 123 in 1983, and 182 in 1987) were collected. MICs of TMP were determined by a MIC plate dilution method (12). Breakpoints for TMP resistance were 8 and 1,024 ,ug/ml (high-level resistance). Colony *
hybridization procedures and DNA probes for detection of sequences homologous to DHFR gene types I, Ila, Ilb, lIc, III, and V, as well as to a part of Tn7 that controls its transposition, were described previously (9). For detection of the type VII DHFR gene (dhfrVlI; 16), a 314-base-pair EcoRV fragment from plasmid pLMO27 was used (19). TMP resistance in Turku City Hospital increased from 10% in 1978 to 41% in 1984 (11; Fig. 1). From 1984 to 1988, TMP resistance remained at approximately 40%, varying from 34 to 41% (Fig. 1). The proportion of TMP-resistant E. coli isolates that were highly resistant increased from 79% in 1980 to 89% in 1983 and 97% in 1987 (Table 1). A major proportion of the TMP-resistant E. coli isolates were simultaneously resistant to sulfamethoxazole (72 to 90%) or ampicillin (53 to 84%), and a TMP-sulfamethoxazole-ampicillin resistance pattern was present in 48 to 80% of the TMP-resistant isolates. Of the 399 TMP-resistant E. coli isolates studied by DNA hybridization, 70% hybridized with the probe specific for the type I DHFR gene, i.e., 62% of the isolates collected in 1980, 71% of those collected in 1983, and 73% of those collected in 1987 (Table 1). Of the isolates collected in 1980, 2% hybridized with the type II probe, whereas 2% of those collected in 1987 hybridized with the type VII probe. None of the isolates hybridized with the type III or V DHFR probe. Hybridization with the probe specific for transposon Tn7 occurred with 47 to 59% of the isolates collected in 1980 to 1987 (Table 1). Of the isolates that hybridized with the type I DHFR probe, 76 to 82% also hybridized with the Tn7 probe. In addition, one isolate collected in 1983 hybridized with the Tn7 probe but not with the type I DHFR probe or with any other DHFR probes. All of the isolates that hybridized with any of the probes were highly resistant to TMP. Only a few long-term follow-up studies on TMP resistance among inpatients have been published. In a London hospital, TMP resistance increased from 19% in 1975 to 28 to 32% in 1977 to 1983 among urinary pathogenic E. coli isolates (3). In the Nottingham area in the United Kingdom, TMP resistance among hospitalized patients increased from 2% in 1978 to 12 to 14% in 1984 to 1985 (21). On the other hand, in an Edenhall Hospital in the United Kingdom, TMP resistance among urinary bacteria declined from 18% in 1982 to 12% in 1984 (1). However, in both London and Edenhall, the frequencies of high-level resistance to TMP ultimately increased (1, 3), a finding similar to ours. In our study, the type I DHFR gene was observed in most
Corresponding author. 2013
ANTIMICROB. AGENTS CHEMOTHER.
NOTES
2014
-60 50
w
z
Ln
40
30
0~
w20
a-
10
MH 1978 79 80 81 82 83 84 85 86 87 88 YEAR FIG. 1. Emergence of TMP resistance among urinary E. coli isolates collected at Turku City Hospital during 1978 to 1988.
of the TMP-resistant urinary pathogenic E. coli isolates. The type I DHFR gene is most commonly found not only among TMP-resistant E. coli isolates but also in some other clinically important pathogens, such as shigellae (2, 4, 9). Only a few type II DHFR genes were detected in our study, even though different probes for the three known type II genes were used. Similar results were obtained when we studied TMP-resistant E. coli isolates from outpatients throughout Finland (8). In contrast, Goldstein et al. (7) and Mayer et al. (15) found the type II DHFR genes commonly among TMP-resistant urinary pathogens in Europe and the United States, respectively. The observed differences might be due to the use of different probes; our probes were smaller and contained no extragenic sequences that might have influenced the hybridization results (19). However, it is also possible that TMP resistance has evolved differently in various parts of the world. The ubiquitous occurrence of type I DHFR has been explained by the location of its gene on transposon Tn7 (6). Transposon Tn7 can efficiently transpose itself into a single site of the E. coli chromosome and, at a lower rate, into random sites in the chromosome or in other replicons (14). About 80% of the isolates in our material carrying the type I DHFR gene also hybridized with the probe specific for transposon Tn7. Another commonly observed DNA segment that has the ability for site-specific recombination and is also recognized as a part of transposon Tn2J has recently been shown to represent a second kind of location for the type I DHFR gene (18). This recombination site has been TABLE 1. Distribution of DHFR genes among TMP-resistant urinary E. coli isolates % of TMP-resistant isolates' Probe(s) with which
hybridization occurreda
1980 1983 1987 All (n = 94) (n = 123) (n = 182) (n = 399)
DHFR I DHFR II DHFR VII None
62 2 0 36
71 0 0 29
73 0 2 25
70 0.5 1 29
Tn7
47 76
59 82
55 76
54 78
DHFR I and Tn7-DHFR I
aNone of the isolates hydridized with the type III or V DHFR probe. b The percentages of isolates that were highly resistant in 1980, 1983, and 1987, respectively, were 79, 89, and 97%; the overall percentage was 90%o.
shown earlier to harbor type II and V DHFR genes (17) and could be where the type I DHFR genes are located in the non-Tn7-containing bacteria studied here. Moreover, in our study, one isolate that hybridized with the Tn7 probe did not hybridize with the type I DHFR probe. New variants of Tn7 lacking the type I DHFR gene are known to occur, representing a potential dissemination vehicle for various sitespecifically inserted resistance genes (17, 20, 22). It has been proposed that reduced use of TMP could decrease the frequency of clinically observed TMP-resistant bacteria. However, TMP consumption decreased by 50% in Turku City Hospital since 1977, while the frequency of high-level resistance, as well as the frequency of the type I DHFR gene, increased in the monitored facility. This study shows that the mechanisms for the persistence of the type I DHFR gene need further research. We thank Mary Fling for control strains and oligonucleotide probes, Ann-Sofie Hakulinen and Tarja Laine for technical assistance, and Jeri Hill for language revision. This study was supported by the Research and Science Foundation of Farmos (E.H.), the Turku University Foundation (The Valto Takala Fund) (E.H.), and the Sigrid Juselius Foundation (E.H. and P.H.). LITERATURE CITED 1. Amyes, S. G. B., C. J. Doherty, and H.-K. Young. 1986. High-level trimethoprim resistance in urinary bacteria. Eur. J. Clin. Microbiol. 5:287-291. 2. Bratoeva, M. P., and J. F. John. 1989. Dissemination of trimethoprim-resistant clones of Shigella sonnei in Bulgaria. J. Infect. Dis. 159:648-653. 3. Chirnside, E. D., A. M. Emmerson, and J. T. Smith. 1985. A follow-up survey of transferable, plasmid-encoded trimethoprim resistance in a general hospital (1975-1983). J. Antimicrob. Chemother. 16:419-434. 4. Delgado, R., and J. R. Otero. 1988. High-level resistance to trimethoprim in Shigella sonnei associated with plasmid-encoded dihydrofolate reductase type I. Antimicrob. Agents Chemother. 32:1598-1599. 5. Elwell, L. P., and M. E. Fling. 1989. Resistance to trimethoprim, p. 249-290. In L. E. Bryan (ed.), Handbook of experimental pharmacology, vol. 91. Springer-Verlag KG, Berlin. 6. Fling, M. E., and C. Richards. 1983. The nucleotide sequence of the trimethoprim-resistance dihydrofolate reductase gene harbored by Tn7. Nucleic Acids Res. 11:5147-5158. 7. Goldstein, F. W., B. Papadopoulou, and J. F. Acar. 1986. The changing pattern of trimethoprim resistance in Paris, with a review of worldwide experience. Rev. Infect. Dis. 8:725-737. 8. Heikkila, E., 0. V. Renkonen, R. Sunila, P. Uurasmaa, and P. Huovinen. 1990. The emergence and mechanisms of trimethoprim resistance in Escherichia coli isolated from outpatients in Finland. J. Antimicrob. Chemother. 25:275-283. 9. Heikkila, E., A. Siitonen, M. Jahkola, M. Fling, L. Sundstrom, and P. Huovinen. 1990. Increase of trimethoprim resistance among Shigella spp., 1975-1988; analysis of resistance mechanisms. J. Infect. Dis. 161:1242-1248. 10. Huovinen, P. 1987. Trimethoprim resistance. Antimicrob. Agents Chemother. 31:1451-1456. 11. Huovinen, P., L. Pulkkinen, H. L. Helin, M. Mlkilli, and P. Toivanen. 1986. Emergence of trimethoprim resistance in relation to drug consumption in a Finnish hospital from 1971 through 1984. Antimicrob. Agents Chemother. 29:73-76. 12. Huovinen, P., 0. V. Renkonen, L. Pulkkinen, R. Sunila, P. Gronroos, M. L. Klossner, S. Virtanen, and P. Toivanen. 1985. Trimethoprim resistance of Escherichia coli in outpatients in Finland after ten years' use of plain trimethoprim. J. Antimicrob. Chemother. 16:435-441. 13. Kraft, C. A., D. J. Platt, and M. C. Timbury. 1984. Distribution and transferability of plasmids in trimethoprim-resistant urinary strains of Escherichia coli: a comparative study of hospital
NOTES
VOL. 34, 1990 isolates. J. Med. Microbiol. 18:95-105. 14. Lichtenstein, C., and S. Brenner. 1982. Unique insertion site of Tn7 in the E. coli chromosome. Nature (London) 297:601-603. 15. Mayer, K. H., M. E. Fling, J. D. Hopkins, and T. F. O'Brien. 1985. Trimethoprim resistance in multiple genera of Enterobacteriaceae at a U.S. hospital: spread of the type II dihydrofolate reductase gene by a single plasmid. J. Infect. Dis. 151:783-789. 16. Sundstrom, L. 1989. The genes of trimethoprim resistance and their recombinational dissemination. Acta Pharm. Nord. 1:375. 17. Sundstrom, L., P. RAdstr6m, G. Swedberg, and 0. Skold. 1988. Site-specific recombination promotes linkage between trimethoprim and sulfonamide resistance genes. Sequence characterization of dhfrV and sulI and a recombination active locus of Tn2l. Mol. Gen. Genet. 213:191-201. 18. Sundstrom, L., and 0. Skold. 1990. The dhfrl trimethoprim resistance gene of Tn7 can be found at specific sites in other
19. 20.
21.
22.
2015
genetic surroundings. Antimicrob. Agents Chemother. 34:642650. Sundstrom, L., T. Vinayagamoorthy, and 0. Skold. 1987. Novel type of plasmid-borne resistance to trimethoprim. Antimicrob. Agents Chemother. 31:60-66. Tietze, E., J. Brevet, and H. Tschipe. 1987. Relationships among the streptothricin resistance transposons Tn1825 and Tn1826 and the trimethoprim resistance transposon Tn7. Plasmid 18: 246-249. Towner, K. J., and R. C. B. Slack. 1986. Effect of changing selection pressures on trimethoprim resistance in Enterobacteriaceae. Eur. J. Clin. Microbiol. 5:502-506. Wiedemann, B., J. F. Meyer, and M. T. Zuhlsdorf. 1987. Insertions of resistance genes into Tn21-like transposons. J. Antimicrob. Chemother. 18(Suppl. C):85-92.
AND
CHEMOTHERAPY, Oct. 1990,
p.
2013-2015
0066-4804/90/102013-03$02.00/0 Copyright C 1990, American Society for Microbiology
Vol. 34, No. 10
Trimethoprim Resistance in Escherichia coli Isolates from a Geriatric Unit ELINA HEIKKILA,l* LARS SUNDSTROM,2 AND PENTTI HUOVINEN' Department of Medical Microbiology, Turku University, 20520 Turku, Finland,' and Department of Pharmaceutical Microbiology, University of Uppsala, 5-751 23 Uppsala, Sweden2 Received 19 April 1990/Accepted 31 July 1990
The frequency of trimethoprim resistance among Escherichia coli isolates from urine samples collected at Turku City Hospital, Turku, Finland, remained at 40% during 1984 to 1988. The proportion of highly resistant (MIC, .1,024 ,ug/ml) isolates increased, however, and most of these harbored the type I dihydrofolate reductase gene. Only a few isolates possessed type II or VII genes. The widespread use of trimethoprim (TMP) has promoted emergence of TMP-resistant pathogens, especially in hospitals (7, 11-13, 15, 21). The most important mechanism for TMP resistance in bacteria is production of drug-resistant dihydrofolate reductase (DHFR) (5, 10). Type I DHFR, which confers high-level resistance to TMP, is most frequently encountered and is determined by transposons such as Tn7. Type II and V DHFRs have also been reported to occur in some areas (15, 19). The occurrence of other types of DHFR among clinical bacteria has not been confirmed. In Finland, TMP has been used to treat urinary tract infections in combination with sulfonamides since 1969 and alone since 1973. TMP resistance at Turku City Hospital, Turku, Finland, has been monitored since 1978. This hospital is a 1,200-bed facility where most of the patients require extended stays. This duration favors the emergence of antibiotic resistance among bacterial pathogens. Between 1978 and 1979, TMP resistance among Escherichia coli isolates from urine samples increased from 10 to 30%, thereafter reaching a plateau of around 30% during 1980 to 1982 (11). From 1982 to 1984, TMP resistance again increased to 41%. The increase from 10% in 1978 to 30% in 1979 was likely due to markedly increased consumption of TMP in that unit in 1977-13 kg/year, including TMP both alone and in combination with sulfonamides (11). Thereafter, TMP consumption decreased to half of that used in 1977, varying from 2 to 8 kg/year during 1978 to 1988. The present study was a follow-up survey of TMP resistance at Turku City Hospital from 1984 to 1988. To reveal which types of DHFR genes were responsible for the spread of TMP resistance, we conducted DNA hybridization studies of TMP-resistant urinary E. coli isolates collected in 1980 to 1987. For resistance pattern studies, 200 consecutive E. coli isolates from urine samples were collected in Turku City Hospital annually in January and July from 1984 to 1988. Duplicate samples were excluded. Susceptibilities to TMP, sulfamethoxazole, and ampicillin were defined by the disk diffusion method (AB Biodisk, Solna, Sweden; 11). For hybridization studies, 399 TMP-resistant E. coli isolates (94 consecutive isolates in 1980, 123 in 1983, and 182 in 1987) were collected. MICs of TMP were determined by a MIC plate dilution method (12). Breakpoints for TMP resistance were 8 and 1,024 ,ug/ml (high-level resistance). Colony *
hybridization procedures and DNA probes for detection of sequences homologous to DHFR gene types I, Ila, Ilb, lIc, III, and V, as well as to a part of Tn7 that controls its transposition, were described previously (9). For detection of the type VII DHFR gene (dhfrVlI; 16), a 314-base-pair EcoRV fragment from plasmid pLMO27 was used (19). TMP resistance in Turku City Hospital increased from 10% in 1978 to 41% in 1984 (11; Fig. 1). From 1984 to 1988, TMP resistance remained at approximately 40%, varying from 34 to 41% (Fig. 1). The proportion of TMP-resistant E. coli isolates that were highly resistant increased from 79% in 1980 to 89% in 1983 and 97% in 1987 (Table 1). A major proportion of the TMP-resistant E. coli isolates were simultaneously resistant to sulfamethoxazole (72 to 90%) or ampicillin (53 to 84%), and a TMP-sulfamethoxazole-ampicillin resistance pattern was present in 48 to 80% of the TMP-resistant isolates. Of the 399 TMP-resistant E. coli isolates studied by DNA hybridization, 70% hybridized with the probe specific for the type I DHFR gene, i.e., 62% of the isolates collected in 1980, 71% of those collected in 1983, and 73% of those collected in 1987 (Table 1). Of the isolates collected in 1980, 2% hybridized with the type II probe, whereas 2% of those collected in 1987 hybridized with the type VII probe. None of the isolates hybridized with the type III or V DHFR probe. Hybridization with the probe specific for transposon Tn7 occurred with 47 to 59% of the isolates collected in 1980 to 1987 (Table 1). Of the isolates that hybridized with the type I DHFR probe, 76 to 82% also hybridized with the Tn7 probe. In addition, one isolate collected in 1983 hybridized with the Tn7 probe but not with the type I DHFR probe or with any other DHFR probes. All of the isolates that hybridized with any of the probes were highly resistant to TMP. Only a few long-term follow-up studies on TMP resistance among inpatients have been published. In a London hospital, TMP resistance increased from 19% in 1975 to 28 to 32% in 1977 to 1983 among urinary pathogenic E. coli isolates (3). In the Nottingham area in the United Kingdom, TMP resistance among hospitalized patients increased from 2% in 1978 to 12 to 14% in 1984 to 1985 (21). On the other hand, in an Edenhall Hospital in the United Kingdom, TMP resistance among urinary bacteria declined from 18% in 1982 to 12% in 1984 (1). However, in both London and Edenhall, the frequencies of high-level resistance to TMP ultimately increased (1, 3), a finding similar to ours. In our study, the type I DHFR gene was observed in most
Corresponding author. 2013
ANTIMICROB. AGENTS CHEMOTHER.
NOTES
2014
-60 50
w
z
Ln
40
30
0~
w20
a-
10
MH 1978 79 80 81 82 83 84 85 86 87 88 YEAR FIG. 1. Emergence of TMP resistance among urinary E. coli isolates collected at Turku City Hospital during 1978 to 1988.
of the TMP-resistant urinary pathogenic E. coli isolates. The type I DHFR gene is most commonly found not only among TMP-resistant E. coli isolates but also in some other clinically important pathogens, such as shigellae (2, 4, 9). Only a few type II DHFR genes were detected in our study, even though different probes for the three known type II genes were used. Similar results were obtained when we studied TMP-resistant E. coli isolates from outpatients throughout Finland (8). In contrast, Goldstein et al. (7) and Mayer et al. (15) found the type II DHFR genes commonly among TMP-resistant urinary pathogens in Europe and the United States, respectively. The observed differences might be due to the use of different probes; our probes were smaller and contained no extragenic sequences that might have influenced the hybridization results (19). However, it is also possible that TMP resistance has evolved differently in various parts of the world. The ubiquitous occurrence of type I DHFR has been explained by the location of its gene on transposon Tn7 (6). Transposon Tn7 can efficiently transpose itself into a single site of the E. coli chromosome and, at a lower rate, into random sites in the chromosome or in other replicons (14). About 80% of the isolates in our material carrying the type I DHFR gene also hybridized with the probe specific for transposon Tn7. Another commonly observed DNA segment that has the ability for site-specific recombination and is also recognized as a part of transposon Tn2J has recently been shown to represent a second kind of location for the type I DHFR gene (18). This recombination site has been TABLE 1. Distribution of DHFR genes among TMP-resistant urinary E. coli isolates % of TMP-resistant isolates' Probe(s) with which
hybridization occurreda
1980 1983 1987 All (n = 94) (n = 123) (n = 182) (n = 399)
DHFR I DHFR II DHFR VII None
62 2 0 36
71 0 0 29
73 0 2 25
70 0.5 1 29
Tn7
47 76
59 82
55 76
54 78
DHFR I and Tn7-DHFR I
aNone of the isolates hydridized with the type III or V DHFR probe. b The percentages of isolates that were highly resistant in 1980, 1983, and 1987, respectively, were 79, 89, and 97%; the overall percentage was 90%o.
shown earlier to harbor type II and V DHFR genes (17) and could be where the type I DHFR genes are located in the non-Tn7-containing bacteria studied here. Moreover, in our study, one isolate that hybridized with the Tn7 probe did not hybridize with the type I DHFR probe. New variants of Tn7 lacking the type I DHFR gene are known to occur, representing a potential dissemination vehicle for various sitespecifically inserted resistance genes (17, 20, 22). It has been proposed that reduced use of TMP could decrease the frequency of clinically observed TMP-resistant bacteria. However, TMP consumption decreased by 50% in Turku City Hospital since 1977, while the frequency of high-level resistance, as well as the frequency of the type I DHFR gene, increased in the monitored facility. This study shows that the mechanisms for the persistence of the type I DHFR gene need further research. We thank Mary Fling for control strains and oligonucleotide probes, Ann-Sofie Hakulinen and Tarja Laine for technical assistance, and Jeri Hill for language revision. This study was supported by the Research and Science Foundation of Farmos (E.H.), the Turku University Foundation (The Valto Takala Fund) (E.H.), and the Sigrid Juselius Foundation (E.H. and P.H.). LITERATURE CITED 1. Amyes, S. G. B., C. J. Doherty, and H.-K. Young. 1986. High-level trimethoprim resistance in urinary bacteria. Eur. J. Clin. Microbiol. 5:287-291. 2. Bratoeva, M. P., and J. F. John. 1989. Dissemination of trimethoprim-resistant clones of Shigella sonnei in Bulgaria. J. Infect. Dis. 159:648-653. 3. Chirnside, E. D., A. M. Emmerson, and J. T. Smith. 1985. A follow-up survey of transferable, plasmid-encoded trimethoprim resistance in a general hospital (1975-1983). J. Antimicrob. Chemother. 16:419-434. 4. Delgado, R., and J. R. Otero. 1988. High-level resistance to trimethoprim in Shigella sonnei associated with plasmid-encoded dihydrofolate reductase type I. Antimicrob. Agents Chemother. 32:1598-1599. 5. Elwell, L. P., and M. E. Fling. 1989. Resistance to trimethoprim, p. 249-290. In L. E. Bryan (ed.), Handbook of experimental pharmacology, vol. 91. Springer-Verlag KG, Berlin. 6. Fling, M. E., and C. Richards. 1983. The nucleotide sequence of the trimethoprim-resistance dihydrofolate reductase gene harbored by Tn7. Nucleic Acids Res. 11:5147-5158. 7. Goldstein, F. W., B. Papadopoulou, and J. F. Acar. 1986. The changing pattern of trimethoprim resistance in Paris, with a review of worldwide experience. Rev. Infect. Dis. 8:725-737. 8. Heikkila, E., 0. V. Renkonen, R. Sunila, P. Uurasmaa, and P. Huovinen. 1990. The emergence and mechanisms of trimethoprim resistance in Escherichia coli isolated from outpatients in Finland. J. Antimicrob. Chemother. 25:275-283. 9. Heikkila, E., A. Siitonen, M. Jahkola, M. Fling, L. Sundstrom, and P. Huovinen. 1990. Increase of trimethoprim resistance among Shigella spp., 1975-1988; analysis of resistance mechanisms. J. Infect. Dis. 161:1242-1248. 10. Huovinen, P. 1987. Trimethoprim resistance. Antimicrob. Agents Chemother. 31:1451-1456. 11. Huovinen, P., L. Pulkkinen, H. L. Helin, M. Mlkilli, and P. Toivanen. 1986. Emergence of trimethoprim resistance in relation to drug consumption in a Finnish hospital from 1971 through 1984. Antimicrob. Agents Chemother. 29:73-76. 12. Huovinen, P., 0. V. Renkonen, L. Pulkkinen, R. Sunila, P. Gronroos, M. L. Klossner, S. Virtanen, and P. Toivanen. 1985. Trimethoprim resistance of Escherichia coli in outpatients in Finland after ten years' use of plain trimethoprim. J. Antimicrob. Chemother. 16:435-441. 13. Kraft, C. A., D. J. Platt, and M. C. Timbury. 1984. Distribution and transferability of plasmids in trimethoprim-resistant urinary strains of Escherichia coli: a comparative study of hospital
NOTES
VOL. 34, 1990 isolates. J. Med. Microbiol. 18:95-105. 14. Lichtenstein, C., and S. Brenner. 1982. Unique insertion site of Tn7 in the E. coli chromosome. Nature (London) 297:601-603. 15. Mayer, K. H., M. E. Fling, J. D. Hopkins, and T. F. O'Brien. 1985. Trimethoprim resistance in multiple genera of Enterobacteriaceae at a U.S. hospital: spread of the type II dihydrofolate reductase gene by a single plasmid. J. Infect. Dis. 151:783-789. 16. Sundstrom, L. 1989. The genes of trimethoprim resistance and their recombinational dissemination. Acta Pharm. Nord. 1:375. 17. Sundstrom, L., P. RAdstr6m, G. Swedberg, and 0. Skold. 1988. Site-specific recombination promotes linkage between trimethoprim and sulfonamide resistance genes. Sequence characterization of dhfrV and sulI and a recombination active locus of Tn2l. Mol. Gen. Genet. 213:191-201. 18. Sundstrom, L., and 0. Skold. 1990. The dhfrl trimethoprim resistance gene of Tn7 can be found at specific sites in other
19. 20.
21.
22.
2015
genetic surroundings. Antimicrob. Agents Chemother. 34:642650. Sundstrom, L., T. Vinayagamoorthy, and 0. Skold. 1987. Novel type of plasmid-borne resistance to trimethoprim. Antimicrob. Agents Chemother. 31:60-66. Tietze, E., J. Brevet, and H. Tschipe. 1987. Relationships among the streptothricin resistance transposons Tn1825 and Tn1826 and the trimethoprim resistance transposon Tn7. Plasmid 18: 246-249. Towner, K. J., and R. C. B. Slack. 1986. Effect of changing selection pressures on trimethoprim resistance in Enterobacteriaceae. Eur. J. Clin. Microbiol. 5:502-506. Wiedemann, B., J. F. Meyer, and M. T. Zuhlsdorf. 1987. Insertions of resistance genes into Tn21-like transposons. J. Antimicrob. Chemother. 18(Suppl. C):85-92.
