Increased mutation frequency among Enterococcus faecium belonging to clonal complex 17

Abstract number: P2043

Ruiz-Garbajosa P., Top J., Coque T.M., Cantón R., Bonten M.J., Baquero F., Willems R.J.

Objectives: Mutator phenotype might confer a selective advantage since different studies have demonstrated a link between elevated mutation frequencies (mutator phenotype), antibiotic resistance and pathogenicity. The aim of this study is to assess whether the distinct genetic Enterococcus faecium Clonal Complex 17 (CC17) subpopulation, responsible for the majority of E. faecium nosocomial infections and characterised by high antibiotic resistance and recombination levels, presents a high mutation frequency (MF) compared to non-CC17.

Methods: 46 epidemiological unrelated E. faecium strains (22 CC17 and 24 non-CC17 strains) were included. MF was determined by calculating the proportion of CFU on Mueller-Hinton agar (MHA) supplemented with 256 mg/L of fosfomycin plus 100 mg/L of glucose-6-phosphate (after 48 h of incubation) versus total number of viable cells on MHA without supplements. All strains were susceptible (MIC range, 16–64 mg/L) to this concentration of fosfomycin at the start of the experiment. Experiments were performed in triplicate for each strain and mean values were calculated. Stability of mutants was assayed in selective plates with fosfomycin. The Mann-Whitney U test was used to determine the statistical significance of MF differences between CC17 and non-CC17 (p < 0.05 was considered statistically significant).

Results: Among 46 E. faecium strains MF ranged from 7×108 to 5.2×106 (median 6.3×107). Stability was demonstrated in all fosfomycin mutants (5 colonies from each plate). The MF of the 22 CC17 strains ranged from 1.3×107 to 5.2×106 with a median of 1.5×106 and was significantly higher than that of the 24 non-CC17 strains ranged from 7×108 to 3.1×106 with a median 3.7×107 (p = 0.001). CC17 strains with an elevated MF (>1×106) were not confined to particular STs but were randomly distributed among different STs or sublineages within CC17.

Conclusions: This study shows for the first time that strains belonging to the E. faecium CC17 genetic subpopulation have an increased MF. This increased MF may have promoted the emergence of antibiotic resistance, most notably ampicillin resistance, within this subpopulation providing a selective advantage that facilitated subsequent adaptation to the hospital environment.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
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