Emergence of Fosfomycin-Resistant Isolates of Shiga-Like Toxin-Producing Escherichia coli O26

Abstract

We evaluated the susceptibilities of 129 Shiga-like toxin-producingEscherichia coli (STEC) isolates to various antibiotics. The numbers of isolates for which MICs were high (≧128 μg/ml) were as follows: 5 for fosfomycin, 14 for ampicillin, 1 for cefaclor, 6 for kanamycin, 22 for tetracycline, and 2 for doxycycline. For two isolates of STEC O26 MICs of fosfomycin were high (1,024 and 512 μg/ml, respectively). Conjugation experiments and glutathioneS-transferase assays suggested that the fosfomycin resistance in these isolates was determined not by a plasmid but chromosomally. The amount of active intracellular fosfomycin in these STEC isolates was 100- to 200-fold less than that in E. coli C600 harboring pREFTT47B408 in the presence of eitherl-α-glycerophosphate or glucose-6-phosphate. Cloning, sequencing, and Northern blot analysis demonstrated that the transcriptional level of the murA gene encoding UDP-N-acetylglucosamine enolpyruvoyl transferase in these isolates was greater than that in E. coli C600. Our results suggest that the fosfomycin resistance in these STEC isolates is due to concurrent effects of alteration of the glpT and/oruhp transport systems and of the enhanced transcription of the murA gene.

Molecular mechanisms of fosfomycin resistance in clinical isolates of Escherichia coli

“…In some natural fosfomycin-resistant organisms, such as Mycobacterium tuberculosis and Chlamydia trachomatis, Asp-115 at the active site of MurA has been identified as conferring resistance [26,27]. In E. coli, only an amino acid substitution of Lys11Glu in MurA has been reported from a fosfomycin-resistant clinical isolate of Shiga-like toxin-producing E. coli O26 strain (NGY60) [10]. However, no association between this substitution and resistance to fosfomycin has been described so far.…”

“…Only a few reports have identified mutations in the genes encoding a transporter and the overexpression of MurA in fosfomycin-resistant E. coli strains [9,10].…”

“…One such mechanism involves reduced uptake of fosfomycin owing to a defect in one of the two transporters caused by mutations in the structural gene [8,9]. Overexpression of the target enzyme MurA as well as acquisition of plasmid-mediated fosfomycin-modifying enzymes, such as FosA, have also been shown to confer resistance [10,11]. Moreover, mutations in the uhpA, ptsI and cyaA genes can cause resistance by reducing the expression of UhpT or GlpT [9,12,13].…”

Medium compositions and culture conditions for the assay of fosfomycin susceptibility by Etest

“…In these strains, the results of Etests on MH agar agreed with those for MH agar supplemented with G6P, as was also shown in the susceptible strains. In our previous study, 13 it was shown that the fosfomycin resistance in these resistant strains was due to the concurrent effects of alterations in the glpT and/or uhp transport systems, and to the enhanced transcription of the murA gene encoding UDP-N-acetylglucosamine enolpyruvoyl transferase. Therefore, the Etest MICs for the resistant strains were highly affected by alterations in fosfomycin incorporation because of the effects of medium composition and culture conditions.…”

Characterization of antimicrobial resistance patterns and class 1 integrons in Escherichia coli O26 isolated from humans and animals

“…The impact of antimicrobial-resistant bacteria on treatment options and outcomes in human medicine has become both a national and international concern. Recently, antimicrobial susceptibility profiles of E. coli isolates of different serotypes from non-hospital sources, especially animals [3,4,8,9] and 0924-8579/$ -see front matter doi:10.1016/j.ijantimicag.2006.08.040 humans [6,10,11], have been studied. However, reports on antimicrobial resistance profiles and the presence of class 1 integrons in E. coli O26 isolates of human and animal origin are scarce.…”