First detection of an invasive Staphylococcus aureus strain (D958) with reduced susceptibility to glycopeptides in Saudi Arabia

Abstract

Strain D958, a methicillin-resistant Staphylococcus aureus strain with reduced susceptibility to vancomycin, was isolated from a 69-year-old Saudi male patient presenting with severe sepsis immediately after admission. Despite high serum levels of vancomycin, the same S. aureus strain was isolated from five blood culture sets during 1 week. Treatment failure under therapeutic levels of vancomycin prompted us to investigate the resistance profile of this strain in further detail. The MIC values for vancomycin as determined by Etest and microdilution were 3.0 and 2.0 mg/liter, respectively, and remained unchanged during the treatment course. The macro-Etest method showed a MIC of 4 mg/liter. The strain showed liquid vancomycin and lysostaphin MBCs of 2.0 and 5.0 mg/liter, respectively. The isolates were confirmed as heterogeneously vancomycin-intermediate S. aureus (hVISA) by vancomycin population analysis profile. The areas under these curves were similar for Mu3 and D958 for vancomycin and teicoplanin (ratio values were 1 and 1.1 for vancomycin and teicoplanin, respectively). Extensive genotyping and molecular characterization demonstrated that the strain harbored a staphylococcal cassette chromosome mec element (SCCmec) type III cassette and was of sequence type ST241, a single-locus variant of the successful multiresistant clone ST239. Microarray results demonstrated that D958 contained numerous resistance determinants (generally plasmid or phage encoded). These results suggest that this strain is constitutively expressing an altered susceptibility to vancomycin. Further studies are warranted to assess the clonal distribution of such strains displaying reduced susceptibility to vancomycin prior to any antimicrobial therapy.

FIG. 1.

Population analysis of MRSA strain D958 in the presence of vancomycin or teicoplanin. Growth of subpopulations of D958 was still observed at 8.0 mg/liter of either vancomycin or teicoplanin. For comparison purposes, strains SA5454, Mu3 and Mu50, representative of fully susceptible, hVISA, and VISA phenotypes, respectively, are shown in the figure.

FIG. 2.

Genotyping and relatedness of D958 with control isolates. MLVA (A), MLST (B), and microarray comparative genomic hybridization (CGH) (C) trees of strain D958 compared to reference isolates. For reproducibility purposes, a duplicate from two independent DNA extractions of strain D958 is shown on the MLVA tree. For microarray analysis, background-subtracted data were expressed as log₁₀ ratios and analyzed by two-way clustering. Probes yielding positive and negative signals are shown in blue and yellow, respectively. Strain D958 shows relatedness using all three methods with isolates already described as GISA, such as ST239 (26, 51).