Abstract
Some VanM-type vancomycin-resistant Enterococcus faecium isolates from China are also resistant to fosfomycin. To investigate the mechanism of fosfomycin resistance in these clinical isolates, antimicrobial susceptibility testing, filter-mating, Illumina/Solexa sequencing, inverse PCR and fosfomycin resistance gene cloning were performed. Three E. faecium clinical isolates were highly resistant to fosfomycin and vancomycin with minimal inhibitory concentrations (MICs) >1024 µg/ml and >256 µg/ml, respectively. The fosfomycin and vancomycin resistance of these strains could be co-transferred by conjugation. They carried a fosfomycin resistance gene fosB encoding a protein differing by one or two amino acids from FosB, which is encoded on staphylococcal plasmids. Accordingly, the gene was designated fosB3. The fosB3 gene was cloned into pMD19-T, and transformed into E. coli DH5α. The fosfomycin MIC for transformants with fosB3 was 750-fold higher than transformants without fosB3. The fosB3 gene could be transferred by an extrachromosomal circular intermediate. The results indicate that the fosB3 gene is transferable, can mediate high level fosfomycin resistance in both Gram-positive and Gram-negative bacteria, and can be located on a circular intermediate.
Highlights
Fosfomycin is a small molecule that inhibits the first step in bacterial cell wall synthesis by acting as an analogue of phosphoenolpyruvate [1]
The three E. faecium isolates were resistant to fosfomycin, vancomycin, erythromycin, ciprofloxacin, ampicillin and gentamicin, but susceptible to linezolid
The vancomycin and fosfomycin resistance of these isolates could be co-transferred by conjugation to E. faecium BM4105RF, but the resistance to other drugs could not (Table 1)
Summary
Fosfomycin is a small molecule that inhibits the first step in bacterial cell wall synthesis by acting as an analogue of phosphoenolpyruvate [1]. It is active against both Gram-positive and Gram-negative bacteria, and has been extensively used therapeutically in many countries [2,3,4]. Fosfomycin resistance is caused by plasmid-mediated fosfomycin genes in many bacterial species and by chromosomal mutations [1, 5, 6]. Plasmid-mediated fosfomycin resistance determinants, fosA, fosB and fosC, have been discovered and are related to fosfomycin resistance in Escherichia coli (fosA, fosC) [7], Enterobacter cloacae (fosA) [8], Klebsiella pneumoniae (fosA) [9], and Staphylococcus spp. Overexpression of the target protein (MurA) is involved in fosfomycin resistance as well [10]
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