Diversity of Genetic Lineages Among CTX-M-15 and CTX-M-14 Producing Escherichia coli Strains in a Tunisian Hospital

Fourteen broad-spectrum-cephalosporin-resistant Escherichia coli isolates were recovered between June and December 2007 in a Tunisian hospital. Genes encoding extended-spectrum-beta-lactamases (ESBL) and other resistance genes were characterized by PCR and sequencing. The following ESBL genes were identified: bla(CTX-M-15) (12 isolates), bla(CTX-M-14a) (one isolate), and bla(CTX-M-14b) (one isolate). The bla(OXA-1) gene was detected in 13 bla(CTX-M)-producing strains and a bla(TEM-1) gene in 6 of them. The ISEcp1 sequence was found upstream of bla(CTX-M) genes in 8 of 14 strains, and orf477 or IS903 downstream of this gene in 13 strains. Nine of the strains carried class 1 integrons and five different gene cassette arrangements were detected, dfrA17-aadA5 being the most common. One of the strains (bla(CTX-M-14a)-positive) harbored three class 1 integrons, and one of them was non-previously described containing as gene cassettes new variants of aac(6')-Ib and cmlA1 genes and it was linked to the bla(CTX-M-14a) gene flanked by a truncated ISEcp1 sequence (included in GenBank with accession number JF701188). CTX-M-15-producing strains were ascribed to phylogroup B2 (six isolates) and D (six isolates). Multilocus-sequence-typing revealed ten different sequence-types (STs) among ESBL-positive E. coli strains with prevalence of ST405 (four strains of phylogroup D) and ST131 types (two strains of phylogroup B2 and serogroup O25b). A high clonal diversity was also observed among studied strains by pulsed-field-gel-electrophoresis (11 unrelated profiles). CTX-M-15 is an emergent mechanism of resistance in the studied hospital and the world-disseminated 0:25b-ST131B2 and ST405-D clones have been identified among CTX-M-15-producing isolates.