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Characterization of gentamicin-susceptible strains of methicillin-resistant Staphylococcus aureus involved in
N Lemaître1, W Sougakoff, A Masmoudi
1Laboratoire de Bactériologie-Hygiène, Groupe Hospitalier Pitié-Salpêtrière, Paris, France. sougakof@lmcp.jusssieu.fr
Abstract:
We report an outbreak of epidemic Staphylococcus aureus strains characterized by an unusual heterogeneous resistance to methicillin and resistance to tobramycin but susceptibility to gentamicin (gentamicin-susceptible methicillin-resistant S. aureus [GS-MRSA]), contrasting with gentamicin-resistant homogeneous MRSA (GR-MRSA) that have been endemic in our hospital since the 1970s. A total of 97 GS-MRSA strains, which were shown by DNA hybridization to carry the mecA and ant(4')-Ia genes, were studied. The 40 GS-MRSA strains isolated at the beginning of the outbreak (January 1992 to June 1993) were typed by using resistance patterns, phage typing, serotyping, and pulsed-field gel electrophoresis and were compared with GR-MRSA and methicillin-susceptible S. aureus (MSSA) strains isolated during the same period. Two dominant clones, A::1 and B::3, and one minor clone, C::5, were identified among the 40 GS-MRSA strains, according to pulsotypes (A to C) and their resistance patterns (1, 3, and 5), which were distinguishable from those of GR-MRSA and MSSA strains. A selection of 57 GS-MRSA strains, isolated from 1994 to 1996, were clustered in the same three clones. However, their distribution had changed in comparison with that in the 1992 to 1993 period: clone A::1 remained dominant (47 versus 42.5%), whereas clone B::3 progressively declined (5 versus 35%) and clone C::5, the most susceptible to antibiotics, spread (44 versus 2.5%). Epidemiological investigations revealed that some clones had been introduced via patients transferred from other hospitals and that cross-infection occurred within and between wards. Major changes in the use of antibiotics, especially aminoglycosides, cyclines, and macrolides, likely played a role in the emergence and spread of GS-MRSA strains.
Insights
A new epidemic strain of gentamicin-susceptible methicillin-resistant Staphylococcus aureus (GS-MRSA) emerged, differing from endemic gentamicin-resistant MRSA. This strain spread through hospital transfers and cross-infections, influenced by antibiotic usage changes.
Area of Science:
- Microbiology
- Infectious Diseases
- Epidemiology
Background:
- Methicillin-resistant Staphylococcus aureus (MRSA) poses a significant healthcare challenge.
- Endemic gentamicin-resistant MRSA (GR-MRSA) has been prevalent since the 1970s.
- A novel gentamicin-susceptible MRSA (GS-MRSA) strain emerged, exhibiting distinct resistance profiles.
Purpose of the Study:
- To characterize the emergence and spread of a new epidemic GS-MRSA strain.
- To compare GS-MRSA with endemic GR-MRSA and methicillin-susceptible S. aureus (MSSA) strains.
- To investigate the clonal evolution and transmission dynamics of GS-MRSA.
Main Methods:
- DNA hybridization to confirm mecA and ant(4')-Ia genes in 97 GS-MRSA strains.
- Phenotypic and genotypic typing: resistance patterns, phage typing, serotyping, and pulsed-field gel electrophoresis (PFGE).
- Comparison of GS-MRSA strains from different time periods (1992-1993 vs. 1994-1996) and with GR-MRSA and MSSA.
Main Results:
- Two dominant clones (A::1, B::3) and one minor clone (C::5) identified among initial GS-MRSA isolates.
- Over time, clone A::1 remained dominant, clone B::3 declined, and clone C::5 (most antibiotic-susceptible) increased significantly.
- Epidemiological data indicated introduction via patient transfers and cross-infection within and between hospital wards.
Conclusions:
- The emergence and spread of GS-MRSA were driven by distinct clones with changing distribution patterns.
- Hospital-acquired infections and inter-hospital transfers facilitated GS-MRSA dissemination.
- Altered antibiotic usage, particularly of aminoglycosides, cyclines, and macrolides, likely contributed to the rise of GS-MRSA.