Glycopeptide resistance in coagulase-negative staphylococci

Coagulase-negative staphylococci (CNS) were the first organisms in which ac quired glycopeptide resistance was recognized. Ever since the early reports . it has been apparent that resistance to teicoplanin is more common than t hat to vancomycin and that resistance occurs mostly in species such as Stap hylococcus haemolyticus and Staphylococcus epidermidis. The minimum inhibit ory concentrations (MICs) of teicoplanin for CNS usually fall over a wide r ange, and, especially in some methicillin-resistant isolates of the two abo ve-mentioned species, they can reach and even exceed the resistance breakpo int. whereas vancomycin MICs tend to remain more stable over a narrower ran ge within the limits of susceptibility. CNS strains intermediately suscepti ble and even resistant not only to teicoplanin but also to vancomycin have, however, been isolated, most frequently from patients subjected to prolong ed glycopeptide treatment. Laboratory detection of glycopeptide-resistant C NS may be problematic, mainly because susceptibility tests, particularly th ose for teicoplanin, are influenced by various technical factors, and agar diffusion tests may yield false susceptibility data. In studies with experi mental glycopeptides, some molecules have exhibited improved in vitro activ ity compared with teicoplanin and vancomycin, but these encouraging microbi ological findings have not usually been followed by in vivo trials. Stepwis e and single-step exposure to teicoplanin and vancomycin has allowed stable clones for which glycopeptide MICs are increased to be obtained from susce ptible CNS strains, particularly strains of Staphylococcus haemolyticus and Staphylococcus epidermidis. In these studies, resistance to teicoplanin wa s generally easier to obtain than resistance to vancomycin, and the levels of teicoplanin resistance were higher. Population studies have demonstrated the usually heterogeneous nature of glycopeptide resistance in CNS. Althou gh glycopeptide-resistant CNS have been shown to differ in several features from their glycopeptide-susceptible counterparts, the exact mechanism of s taphylococcal glycopeptide resistance remains unknown.