Vancomycin-Resistant Clinical Strains of Enterococcus spp. and Staphylococcus aureus

Vancomycin-resistant enterococci (VRE) pose a serious public health threat due to their high antibiotic resistance, ability to cause severe infections, and increased mortality due to limited therapeutic options. Therefore, studying the molecular genetic characteristics of vancomycin-resistant strains is particularly important. Methicillin-resistant Staphylococcus aureus (MRSA) is a major multidrug-resistant nosocomial pathogen. Regarding chemotherapy against invasive MRSA infections, vancomycin still remains a gold standard, in addition to some other anti-MRSA agents, such as teicoplanin, linezolid, and daptomycin. Previously, vancomycin-resistant MRSA (VRSA) was defined as strains with an MICs I 16 µg/mL; however, the number of cases caused by these strains in patients remains limited. Clinically, infections from strains with MICs of I 1.5 µg/mL, even with albeit susceptible MICs (⇄ 2 µg/mL), respond poorly to vancomycin. Some of those bacteria have been bacteriologically characterized as vancomycin-intermediate S. aureus (VISA) and heterogeneous VISA (hVISA), generally with HA-MRSA genetic backgrounds, which are currently classified by EUCAST as vancomycin-resistant.