A novel dihydrofolate reductase cassette inserted in an integron borne on a Tn21-like element

Abstract

In this study, a 498-bp dhfrXII gene coding for trimethoprim resistance was found inserted in a cassette-like manner in the recombinationally active locus, the integron, borne on a transposon Tn21-like element. The dhfrXII cassette is distinct from those cassettes earlier observed in integrons and was found here upstream (10,21,31,32,51

Characterisation of drug resistance gene cassettes associated with class 1 integrons in clinical isolates of Escherichia coli from Taiwan, ROC

“…Most (61%) of the class 1 integrons found in this study carried dfr12, orfF and aadA2 cassettes. This pattern was also reported in a urinary E. coli isolate from Turku, Finland [21], and in Shigella strains isolated in Finland, but originating from Asia [21]. The present study also revealed an integron carrying two newly characterised gene cassettes, dfr17 and aadA4.…”

Characterization of class I integrons in clinical strains of Salmonella enterica subsp. enterica serovars Typhimurium and Enteritidis from Norwegian hospitals

“…The dfrA12 gene encodes a variant of the dihydrofolate reductase gene and the aadA2 gene confers resistance to streptomycin. The dfrA12 gene product confers a 10-times higher level of resistance to trimethoprim than the most common trimethoprim resistance gene dfraA1 (Heikkila et al, 1993). The dfrA12-orf-aadA2 gene cluster has previously been seen in several strains, including an E. coli O157 isolate (GenBank accession no.…”

“…AF335108, unpublished), a Serratia marcescens isolate (GenBank accession no. AF284063, unpublished), a Spanish S. Typhimurium phage type U302 isolate (Guerra et al, 2001;Echeita et al, 2001), an E. coli Shiga-toxin producing O111 : H8 isolate from the USA (Zhao et al, 2001), E. coli isolates from Taiwan (Chang et al, 2000b), E. coli and Shigella isolates from Finland (Heikkila et al, 1993), an epidemic Vibrio cholerae O1 strain from Guinea-Bissau (Dalsgaard et al, 2000) and in an S. Typhimurium DT104a isolate originating from Australia (this report).…”

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

“…The number of different sized restriction fragments that showed this combination of these two genes suggest that the integrase may be incorporated into a number of different Tn21 like elements such as Tn8O56 [15] which may insert at random sites into the chromosome. It is unclear why the integrase gene of Tn2l which is normally associated with DHFR genes on plasmids [13,15,26,31] should show a preference for integration into the chromosome.…”

“…Fifty of these isolates (82%) had Tn7 located in the attTn7 site. The type II, V, VII and XII DHFRs have all been found as cassette structures on self transmissible plasmids in the integron of Tn2l like elements [13,31]. The following other DHFRs have been observed to contain the characteristic repeats that suggest that the gene forms part of a cassette: VI [15] and the type lb of Tn 4132 [32].…”