Presence of a group II intron in a multiresistant Serratia marcescens strain that harbors three integrons and a novel gene fusion

We analyzed the role of integrons in the dissemination of antibiotic resistance in a recent multiresistant clinical isolate, Serratia marcescens SCH88050909 (SCH909). This isolate harbors three integrons, all on a 60-kb conjugative plasmid. By PCR, hybridization, and sequencing analyses, we found that integron 1 has the dfrA1 and ant(3)-Ia cassettes. The first cassette in integron 2 contains the ant(2)-Ia gene, separated from its attC site (59-base element) by a 1,971-bp insert containing a group II intron; this intron codes for a putative maturase-reverse transcriptase on the complementary strand and is the first such intron to be found associated with an integron. The attC site is followed by a novel aminoglycoside resistance gene, ant(3)-Ii-aac(6')-IId, which has been characterized for its bifunctional ANT(3)-I and AAC(6')-II activities. DNA sequence analysis of this fused cassette suggests that insertion and excision due to the integrase activity could have an important role in the evolution of aminoglycoside resistance genes. This gene is followed by an unknown open reading frame with a typical attC site and a partial cassette composed of the beginning of the bla(OXA-10) cassette interrupted by IS1. The sequence downstream of IS1 revealed that the bla(OXA-10) cassette is incomplete and that the 3' conserved segment of this integron is absent. Integron 3 is in a Tn1696-like transposon with the aac(3)-Ia cassette followed by three unknown cassettes and ant(3)-Ia. The presence of the group II intron and the relationship of group II introns in eubacteria with mobile elements suggest a possible role of this element in events such as cassette formation and/or plasmid evolution.