Fc. Tenover et al., CHARACTERIZATION OF STAPHYLOCOCCI WITH REDUCED SUSCEPTIBILITIES TO VANCOMYCIN AND OTHER GLYCOPEPTIDES, Journal of clinical microbiology, 36(4), 1998, pp. 1020-1027
During the last several years a series of staphylococcal isolates that
demonstrated reduced susceptibility to vancomycin or other glycopepti
des have been reported, We selected 12 isolates of staphylococci for w
hich the vancomycin MICs were greater than or equal to 4 mu g/ml or fo
r which the teicoplanin MICs were greater than or equal to 8 mu g/ml a
nd 24 control strains for which the vancomycin MICs were less than or
equal to 2 mu g/ml or for which the teicoplanin MICs were less than or
equal to 4 mu g/ml to determine the ability of commercial susceptibil
ity testing procedures and vancomycin agar screening methods to detect
isolates with reduced glycopeptide susceptibility. By PCR analysis, n
one of the isolates with decreased glycopeptide susceptibility contain
ed known vancomycin resistance genes, Broth microdilution tests held a
full 24 h were best at detecting strains with reduced glycopeptide su
sceptibility, Disk diffusion did not differentiate the strains inhibit
ed by 8 mu g of vancomycin per mi from more susceptible isolates, Most
of the isolates with reduced glycopeptide susceptibility were recogni
zed by MicroScan conventional panels and Etest vancomycin strips, Sens
ititre panels read visually were more variable, although with some of
the panels MICs of 8 mu g/ml were noted for these isolates, Vitek resu
lts were 4 mu g/ml for all strains for which the vancomycin MICs were
greater than or equal to 4 mu g/ml. Vancomycin MICs on Rapid MicroScan
panels were not predictive, giving MICs of either less than or equal
to 2 or greater than or equal to 16 mu g/ml for these isolates, Commer
cial brain heart infusion vancomycin agar screening plates containing
6 mu g of vancomycin per mi consistently differentiated those strains
inhibited by 8 mu g/ml from more susceptible strains, Vancomycin-conta
ining media prepared in-house showed occasional growth of susceptible
strains, Staphylococcus aureus ATCC 29213, and on occasion, Enterococc
us faecalis ATCC 29212, Thus, strains of staphylococci with reduced su
sceptibility to glycopeptides, such as vancomycin, are best detected i
n the laboratory by nonautomated quantitative tests incubated for a fu
ll 24 h. Furthermore, it appears that commercial vancomycin agar scree
ning plates can be used to detect these isolates.