Since the discovery of the vancomycin-resistant Staphylococcus aureus (VRSA) strain
Mu50 (minimum inhibitory concentration [MIC] 8 mg/L), there has been concern about
the potential spread of such strains throughout Japanese hospitals. Two important
questions need to be answered: (1) what is the prevalence of VRSA, and (2) by what
mechanism does vancomycin resistance occur.
The vancomycin susceptibilities of three methicillin-resistant S aureus (MRSA) strains
(Mu50, Mu3, and H1) and the methicillin-susceptible S aureus type strain FDA209P were
compared by MIC determinations and population analysis. Mu3 (MIC 3 mg/L) was isolated
from the sputum of a patient with pneumonia after surgery who had failed vancomycin
therapy. H1 (MIC 2 mg/L), which is a representative vancomycin-susceptible MRSA strain,
was isolated from a patient with pneumonia who responded favourably to vancomycin
therapy. Subclones of Mu3 with increased resistance against vancomycin were selected
with serial concentrations of vancomycin and their MICs were determined. The prevalence
of VRSA and Mu3-like strains in Japanese hospitals was estimated by population analysis
from 1149 clinical MRSA isolates obtained from 203 hospitals throughout Japan. The
genetic traits of the Mu3 and Mu50 strains were compared with clonotypes of MRSA from
around the world.
Mu3 and Mu50 had an identical pulsed-field gel electrophoresis banding pattern. When
grown in a drug-free medium, Mu3 produced subpopulation of cells with varying degrees
of vancomycin resistance, thus demonstrating natural heterogeneity, or variability,
in susceptibility to vancomycin. In the presence of vancomycin, Mu3 produced subclones
with resistance roughly proportional to the concentrations of vancomycin used. Selection
of Mu3 with 8 mg/L or more of vancomycin gave rise to subclones with vancomycin resistance
equal to that of Mu50 (MIC 8 mg/L) at a frequency of 1/1,000,000. During screening
of Japanese MRSA strains, no strain of VRSA additional to Mu50 was found. The prevalence
of MRSA isolates heterogeneously resistant to vancomycin was 20% in Juntendo University
Hospital, 9.3% in the other seven university hospitals, and 1.3% in non-university
hospitals or clinics.
Heterogeneously resistant VRSA is a preliminary stage that allows development into
VRSA upon exposure to vancomycin. Heterogeneously resistant VRSA was found in hospitals
throughout Japan. This finding could explain, at least partly, the frequent therapeutic
failure of MRSA infection with vancomycin in Japan.