Increase of trimethoprim resistance among Shigella species, 1975-1988: analysis of resistance mechanisms.

Trimethoprim (TMP) resistance among Shigella species isolated from Finnish travelers increased from 3.0% in 1975-1982 to 42.0%-43.8% in 1987-1988. Of the 317 TMP-resistant Shigella isolates identified during 1975-1988, 175 (55%) collected in 1985-1987 and in 1988 were tested further. Almost all (98%) were highly resistant to TMP, suggesting a plasmid-mediated origin. The type I dihydrofolate reductase (DHFR) gene was detected in 85% of the isolates studied. Twenty-three percent of the type I DHFR-positive isolates failed to hybridize with a probe detecting only Tn7-derived sequences, suggesting that the type I DHFR gene may occur independently of transposon Tn7. Four of the five Shigella species isolated from travelers to Sri Lanka hybridized with the probe for type V DHFR gene, implying a local distribution of the type V DHFR gene. The type II and type III DHFR genes were not found among the isolates studied. Only 12% of the TMP-resistant Shigella isolates failed to hybridize with any of the DHFR gene probes used.