Vancomycin resistance in Staphylococcus aureus.

Vancomycin resistance in enterococci, predominantly Enterococcus faecium, developed in the latter half of the 1980s, and the long anticipated development of vancomycin resistance in Staphylococcus aureus has now occurred. A number of vancomycin-intermediate strains have been described, and these strains have abnormal, thickened cell walls in the presence of vancomycin. Two mechanisms of resistance have been described in the strains: affinity trapping of vancomycin molecules by cell wall monomers and clogging of the outer layers of peptidoglycan by bound vancomycin molecules, and change in the structure or metabolism of teichoic acids. Of more serious concern has been the description in 2002 of two patients with vancomycin-resistant S aureus infections. In one instance, the patient had skin lesions coinfected with vancomycin resistant, vanA genotype, E faecalis, and the vanA resistance genes could have been transferred to the S aureus strain. Expression of resistance was high in one S aureus strain and low in the other, making detection more challenging in the latter instance. These developments are of great concern, and every effort should be made to prevent further development and spread of vancomycin resistance in staphylococci.